path: root/target/linux/generic-2.6/patches-2.6.32/230-union_mounts.patch

--- /dev/null
+++ b/Documentation/filesystems/union-mounts.txt
@@ -0,0 +1,187 @@
+VFS based Union Mounts
+----------------------
+
+ 1. What are "Union Mounts"
+ 2. The Union Stack
+ 3. Whiteouts, Opaque Directories, and Fallthrus
+ 4. Copy-up
+ 5. Directory Reading
+ 6. Known Problems
+ 7. References
+
+-------------------------------------------------------------------------------
+
+1. What are "Union Mounts"
+==========================
+
+Please note: this is NOT about UnionFS and it is NOT derived work!
+
+Traditionally the mount operation is opaque, which means that the content of
+the mount point, the directory where the file system is mounted on, is hidden
+by the content of the mounted file system's root directory until the file
+system is unmounted again. Unlike the traditional UNIX mount mechanism, that
+hides the contents of the mount point, a union mount presents a view as if
+both filesystems are merged together. Although only the topmost layer of the
+mount stack can be altered, it appears as if transparent file system mounts
+allow any file to be created, modified or deleted.
+
+Most people know the concepts and features of union mounts from other
+operating systems like Sun's Translucent Filesystem, Plan9 or BSD. For an
+in-depth review of union mounts and other unioning file systems, see:
+
+http://lwn.net/Articles/324291/
+http://lwn.net/Articles/325369/
+http://lwn.net/Articles/327738/
+
+Here are the key features of this implementation:
+- completely VFS based
+- does not change the namespace stacking
+- directory listings have duplicate entries removed in the kernel
+- writable unions: only the topmost file system layer may be writable
+- writable unions: new whiteout filetype handled inside the kernel
+
+-------------------------------------------------------------------------------
+
+2. The Union Stack
+==================
+
+The mounted file systems are organized in the "file system hierarchy" (tree of
+vfsmount structures), which keeps track about the stacking of file systems
+upon each other. The per-directory view on the file system hierarchy is called
+"mount stack" and reflects the order of file systems, which are mounted on a
+specific directory.
+
+Union mounts present a single unified view of the contents of two or more file
+systems as if they are merged together. Since the information which file
+system objects are part of a unified view is not directly available from the
+file system hierarchy there is a need for a new structure. The file system
+objects, which are part of a unified view are ordered in a so-called "union
+stack". Only directories can be part of a unified view.
+
+The link between two layers of the union stack is maintained using the
+union_mount structure (#include <linux/union.h>):
+
+struct union_mount {
+       atomic_t u_count;               /* reference count */
+       struct mutex u_mutex;
+       struct list_head u_unions;      /* list head for d_unions */
+       struct hlist_node u_hash;       /* list head for searching */
+       struct hlist_node u_rhash;      /* list head for reverse searching */
+
+       struct path u_this;             /* this is me */
+       struct path u_next;             /* this is what I overlay */
+};
+
+The union_mount structure holds a reference (dget,mntget) to the next lower
+layer of the union stack. Since a dentry can be part of multiple unions
+(e.g. with bind mounts) they are tied together via the d_unions field of the
+dentry structure.
+
+All union_mount structures are cached in two hash tables, one for lookups of
+the next lower layer of the union stack and one for reverse lookups of the
+next upper layer of the union stack. The reverse lookup is necessary to
+resolve CWD relative path lookups. For calculation of the hash value, the
+(dentry,vfsmount) pair is used. The u_this field is used for the hash table
+which is used in forward lookups and the u_next field for the reverse lookups.
+
+During every new mount (or mount propagation), a new union_mount structure is
+allocated. A reference to the mountpoint's vfsmount and dentry is taken and
+stored in the u_next field.  In almost the same manner an union_mount
+structure is created during the first time lookup of a directory within a
+union mount point. In this case the lookup proceeds to all lower layers of the
+union. Therefore the complete union stack is constructed during lookups.
+
+The union_mount structures of a dentry are destroyed when the dentry itself is
+destroyed. Therefore the dentry cache is indirectly driving the union_mount
+cache like this is done for inodes too. Please note that lower layer
+union_mount structures are kept in memory until the topmost dentry is
+destroyed.
+
+-------------------------------------------------------------------------------
+
+3. Whiteouts, Opaque Directories, and Fallthrus
+===========================================================
+
+The whiteout filetype isn't new. It has been there for quite some time now
+but Linux's VFS hasn't used it yet. With the availability of union mount code
+inside the VFS the whiteout filetype is getting important to support writable
+union mounts. For read-only union mounts, support for whiteouts or
+copy-on-open is not necessary.
+
+The whiteout filetype has the same function as negative dentries: they
+describe a filename which isn't there. The creation of whiteouts needs
+lowlevel filesystem support. At the time of writing this, there is whiteout
+support for tmpfs, ext2 and ext3 available. The VFS is extended to make the
+whiteout handling transparent to all its users. The whiteouts are not
+visible to user-space.
+
+What happens when we create a directory that was previously whited-out? We
+don't want the directory entries from underlying filesystems to suddenly appear
+in the newly created directory.  So we mark the directory opaque (the file
+system must support storage of the opaque flag).
+
+Fallthrus are directory entries that override the opaque flag on a directory
+for that specific directory entry name (the lookup "falls through" to the next
+layer of the union mount).  Fallthrus are mainly useful for implementing
+readdir().
+
+-------------------------------------------------------------------------------
+
+4. Copy-up
+===========
+
+Any write to an object on any layer other than the topmost triggers a copy-up
+of the object to the topmost file system. For regular files, the copy-up
+happens when it is opened in writable mode.
+
+Directories are copied up on open, regardless of intent to write, to simplify
+copy-up of any object located below it in the namespace. Otherwise we have to
+walk the entire pathname to create intermediate directories whenever we do a
+copy-up. This is the same approach as BSD union mounts and uses a negigible
+amount of disk space.  Note that the actual directory entries themselves are
+not copied-up from the lower levels until (a) the directory is written to, or
+(b) the first readdir() of the directory (more on that later).
+
+Rename across different levels of the union is implemented as a copy-up
+operation for regular files. Rename of directories simply returns EXDEV, the
+same as if we tried to rename across different mounts. Most applications have
+to handle this case anyway. Some applications do not expect EXDEV on
+rename operations within the same directory, but these applications will also
+be broken with bind mounts.
+
+-------------------------------------------------------------------------------
+
+5. Directory Reading
+====================
+
+readdir() is somewhat difficult to implement in a unioning file system. We must
+eliminate duplicates, apply whiteouts, and start up readdir() where we left
+off, given a single f_pos value. Our solution is to copy up all the directory
+entries to the topmost directory the first time readdir() is called on a
+directory. During this copy-up, we skip duplicates and entries covered by
+whiteouts, and then create fallthru entries for each remaining visible dentry.
+Then we mark the whole directory opaque. From then on, we just use the topmost
+file system's normal readdir() operation.
+
+-------------------------------------------------------------------------------
+
+6. Known Problems
+=================
+
+- copyup() for other filetypes that reg and dir (e.g. for chown() on devices)
+- symlinks are untested
+
+-------------------------------------------------------------------------------
+
+7. References
+=============
+
+[1] http://marc.info/?l=linux-fsdevel&m=96035682927821&w=2
+[2] http://marc.info/?l=linux-fsdevel&m=117681527820133&w=2
+[3] http://marc.info/?l=linux-fsdevel&m=117913503200362&w=2
+[4] http://marc.info/?l=linux-fsdevel&m=118231827024394&w=2
+
+Authors:
+Jan Blunck <jblunck@suse.de>
+Bharata B Rao <bharata@linux.vnet.ibm.com>
+Valerie Aurora <vaurora@redhat.com>
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -130,6 +130,7 @@ struct autofs_sb_info {
 	int reghost_enabled;
 	int needs_reghost;
 	struct super_block *sb;
+	struct vfsmount *mnt;
 	struct mutex wq_mutex;
 	spinlock_t fs_lock;
 	struct autofs_wait_queue *queues; /* Wait queue pointer */
--- a/fs/autofs4/init.c
+++ b/fs/autofs4/init.c
@@ -17,7 +17,16 @@
 static int autofs_get_sb(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	return get_sb_nodev(fs_type, flags, data, autofs4_fill_super, mnt);
+	struct autofs_sb_info *sbi;
+	int ret;
+
+	ret = get_sb_nodev(fs_type, flags, data, autofs4_fill_super, mnt);
+	if (ret)
+		return ret;
+
+	sbi = autofs4_sbi(mnt->mnt_sb);
+	sbi->mnt = mnt;
+	return 0;
 }
 
 static struct file_system_type autofs_fs_type = {
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -179,6 +179,12 @@ static void *autofs4_follow_link(struct 
 	DPRINTK("dentry=%p %.*s oz_mode=%d nd->flags=%d",
 		dentry, dentry->d_name.len, dentry->d_name.name, oz_mode,
 		nd->flags);
+
+	dput(nd->path.dentry);
+	mntput(nd->path.mnt);
+	nd->path.mnt = mntget(sbi->mnt);
+	nd->path.dentry = dget(dentry);
+
 	/*
 	 * For an expire of a covered direct or offset mount we need
 	 * to break out of follow_down() at the autofs mount trigger
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -840,6 +840,9 @@ static int compat_fillonedir(void *__buf
 	struct compat_old_linux_dirent __user *dirent;
 	compat_ulong_t d_ino;
 
+	if (d_type == DT_WHT)
+		return 0;
+
 	if (buf->result)
 		return -EINVAL;
 	d_ino = ino;
@@ -911,6 +914,9 @@ static int compat_filldir(void *__buf, c
 	compat_ulong_t d_ino;
 	int reclen = ALIGN(NAME_OFFSET(dirent) + namlen + 2, sizeof(compat_long_t));
 
+	if (d_type == DT_WHT)
+		return 0;
+
 	buf->error = -EINVAL;	/* only used if we fail.. */
 	if (reclen > buf->count)
 		return -EINVAL;
@@ -1000,6 +1006,9 @@ static int compat_filldir64(void * __buf
 	int reclen = ALIGN(jj + namlen + 1, sizeof(u64));
 	u64 off;
 
+	if (d_type == DT_WHT)
+		return 0;
+
 	buf->error = -EINVAL;	/* only used if we fail.. */
 	if (reclen > buf->count)
 		return -EINVAL;
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -18,6 +18,7 @@
 #include <linux/string.h>
 #include <linux/mm.h>
 #include <linux/fs.h>
+#include <linux/union.h>
 #include <linux/fsnotify.h>
 #include <linux/slab.h>
 #include <linux/init.h>
@@ -158,14 +159,19 @@ static void dentry_lru_del_init(struct d
 }
 
 /**
- * d_kill - kill dentry and return parent
+ * __d_kill - kill dentry and return parent
  * @dentry: dentry to kill
+ * @list: kill list
+ * @greedy: return parent instead of putting it on the kill list
  *
  * The dentry must already be unhashed and removed from the LRU.
  *
- * If this is the root of the dentry tree, return NULL.
+ * If this is the root of the dentry tree, return NULL. If greedy is zero, we
+ * put the parent of this dentry on the kill list instead. The callers must
+ * make sure that __d_kill_final() is called on all dentries on the kill list.
  */
-static struct dentry *d_kill(struct dentry *dentry)
+static struct dentry *__d_kill(struct dentry *dentry, struct list_head *list,
+			       int greedy)
 	__releases(dentry->d_lock)
 	__releases(dcache_lock)
 {
@@ -173,13 +179,78 @@ static struct dentry *d_kill(struct dent
 
 	list_del(&dentry->d_u.d_child);
 	dentry_stat.nr_dentry--;	/* For d_free, below */
-	/*drops the locks, at that point nobody can reach this dentry */
+
+	/*
+	 * If we are not greedy we just put this on a list for later processing
+	 * (follow up to parent, releasing of inode and freeing dentry memory).
+	 */
+	if (!greedy) {
+		list_del_init(&dentry->d_alias);
+		/* at this point nobody can reach this dentry */
+		list_add(&dentry->d_lru, list);
+		spin_unlock(&dentry->d_lock);
+		spin_unlock(&dcache_lock);
+		__shrink_d_unions(dentry, list);
+		return NULL;
+	}
+
+	/* drops the locks, at that point nobody can reach this dentry */
 	dentry_iput(dentry);
+	/* If the dentry was in an union delete them */
+	__shrink_d_unions(dentry, list);
+	if (IS_ROOT(dentry))
+		parent = NULL;
+	else
+		parent = dentry->d_parent;
+	d_free(dentry);
+	return parent;
+}
+
+void __dput(struct dentry *, struct list_head *, int);
+
+static void __d_kill_final(struct dentry *dentry, struct list_head *list)
+{
+	struct dentry *parent;
+	struct inode *inode = dentry->d_inode;
+
+	if (inode) {
+		dentry->d_inode = NULL;
+		if (!inode->i_nlink)
+			fsnotify_inoderemove(inode);
+		if (dentry->d_op && dentry->d_op->d_iput)
+			dentry->d_op->d_iput(dentry, inode);
+		else
+			iput(inode);
+	}
+
 	if (IS_ROOT(dentry))
 		parent = NULL;
 	else
 		parent = dentry->d_parent;
 	d_free(dentry);
+	__dput(parent, list, 1);
+}
+
+/**
+ * d_kill - kill dentry and return parent
+ * @dentry: dentry to kill
+ *
+ * The dentry must already be unhashed and removed from the LRU.
+ *
+ * If this is the root of the dentry tree, return NULL.
+ */
+static struct dentry *d_kill(struct dentry *dentry)
+{
+	LIST_HEAD(mortuary);
+	struct dentry *parent;
+
+	parent = __d_kill(dentry, &mortuary, 1);
+	while (!list_empty(&mortuary)) {
+		dentry = list_entry(mortuary.next, struct dentry, d_lru);
+		list_del(&dentry->d_lru);
+		__d_kill_final(dentry, &mortuary);
+	}
+
 	return parent;
 }
 
@@ -200,19 +271,24 @@ static struct dentry *d_kill(struct dent
  * Real recursion would eat up our stack space.
  */
 
-/*
- * dput - release a dentry
- * @dentry: dentry to release 
+/**
+ * __dput - release a dentry
+ * @dentry: dentry to release
+ * @list: kill list argument for __d_kill()
+ * @greedy: greedy argument for __d_kill()
  *
  * Release a dentry. This will drop the usage count and if appropriate
  * call the dentry unlink method as well as removing it from the queues and
  * releasing its resources. If the parent dentries were scheduled for release
- * they too may now get deleted.
+ * they too may now get deleted if @greedy is not zero. Otherwise parent is
+ * added to the kill list. The callers must make sure that __d_kill_final() is
+ * called on all dentries on the kill list.
+ *
+ * You probably want to use dput() instead.
  *
  * no dcache lock, please.
  */
-
-void dput(struct dentry *dentry)
+void __dput(struct dentry *dentry, struct list_head *list, int greedy)
 {
 	if (!dentry)
 		return;
@@ -253,12 +329,35 @@ unhash_it:
 kill_it:
 	/* if dentry was on the d_lru list delete it from there */
 	dentry_lru_del(dentry);
-	dentry = d_kill(dentry);
+	dentry = __d_kill(dentry, list, greedy);
 	if (dentry)
 		goto repeat;
 }
 
 /**
+ * dput - release a dentry
+ * @dentry: dentry to release
+ *
+ * Release a dentry. This will drop the usage count and if appropriate
+ * call the dentry unlink method as well as removing it from the queues and
+ * releasing its resources. If the parent dentries were scheduled for release
+ * they too may now get deleted.
+ *
+ * no dcache lock, please.
+ */
+void dput(struct dentry *dentry)
+{
+	LIST_HEAD(mortuary);
+
+	__dput(dentry, &mortuary, 1);
+	while (!list_empty(&mortuary)) {
+		dentry = list_entry(mortuary.next, struct dentry, d_lru);
+		list_del(&dentry->d_lru);
+		__d_kill_final(dentry, &mortuary);
+	}
+}
+
+/**
  * d_invalidate - invalidate a dentry
  * @dentry: dentry to invalidate
  *
@@ -690,6 +789,7 @@ static void shrink_dcache_for_umount_sub
 					iput(inode);
 			}
 
+			shrink_d_unions(dentry);
 			d_free(dentry);
 
 			/* finished when we fall off the top of the tree,
@@ -952,6 +1052,10 @@ struct dentry *d_alloc(struct dentry * p
 	INIT_LIST_HEAD(&dentry->d_lru);
 	INIT_LIST_HEAD(&dentry->d_subdirs);
 	INIT_LIST_HEAD(&dentry->d_alias);
+#ifdef CONFIG_UNION_MOUNT
+	INIT_LIST_HEAD(&dentry->d_unions);
+	dentry->d_unionized = 0;
+#endif
 
 	if (parent) {
 		dentry->d_parent = dget(parent);
@@ -982,8 +1086,10 @@ struct dentry *d_alloc_name(struct dentr
 /* the caller must hold dcache_lock */
 static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 {
-	if (inode)
+	if (inode) {
+		dentry->d_flags &= ~(DCACHE_WHITEOUT|DCACHE_FALLTHRU);
 		list_add(&dentry->d_alias, &inode->i_dentry);
+	}
 	dentry->d_inode = inode;
 	fsnotify_d_instantiate(dentry, inode);
 }
@@ -1514,7 +1620,9 @@ void d_delete(struct dentry * dentry)
 	spin_lock(&dentry->d_lock);
 	isdir = S_ISDIR(dentry->d_inode->i_mode);
 	if (atomic_read(&dentry->d_count) == 1) {
+		__d_drop_unions(dentry);
 		dentry_iput(dentry);
+		shrink_d_unions(dentry);
 		fsnotify_nameremove(dentry, isdir);
 		return;
 	}
@@ -1525,14 +1633,14 @@ void d_delete(struct dentry * dentry)
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&dcache_lock);
 
+	shrink_d_unions(dentry);
 	fsnotify_nameremove(dentry, isdir);
 }
 
 static void __d_rehash(struct dentry * entry, struct hlist_head *list)
 {
-
- 	entry->d_flags &= ~DCACHE_UNHASHED;
- 	hlist_add_head_rcu(&entry->d_hash, list);
+	entry->d_flags &= ~DCACHE_UNHASHED;
+	hlist_add_head_rcu(&entry->d_hash, list);
 }
 
 static void _d_rehash(struct dentry * entry)
@@ -1551,6 +1659,7 @@ void d_rehash(struct dentry * entry)
 {
 	spin_lock(&dcache_lock);
 	spin_lock(&entry->d_lock);
+	BUG_ON(!d_unhashed(entry));
 	_d_rehash(entry);
 	spin_unlock(&entry->d_lock);
 	spin_unlock(&dcache_lock);
@@ -2183,7 +2292,9 @@ resume:
 		struct list_head *tmp = next;
 		struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
 		next = tmp->next;
-		if (d_unhashed(dentry)||!dentry->d_inode)
+		if (d_unhashed(dentry)||(!dentry->d_inode &&
+					 !d_is_whiteout(dentry) &&
+					 !d_is_fallthru(dentry)))
 			continue;
 		if (!list_empty(&dentry->d_subdirs)) {
 			this_parent = dentry;
--- a/fs/ext2/dir.c
+++ b/fs/ext2/dir.c
@@ -219,7 +219,8 @@ static inline int ext2_match (int len, c
 {
 	if (len != de->name_len)
 		return 0;
-	if (!de->inode)
+	if (!de->inode && ((de->file_type != EXT2_FT_WHT) &&
+			   (de->file_type != EXT2_FT_FALLTHRU)))
 		return 0;
 	return !memcmp(name, de->name, len);
 }
@@ -255,6 +256,8 @@ static unsigned char ext2_filetype_table
 	[EXT2_FT_FIFO]		= DT_FIFO,
 	[EXT2_FT_SOCK]		= DT_SOCK,
 	[EXT2_FT_SYMLINK]	= DT_LNK,
+	[EXT2_FT_WHT]		= DT_WHT,
+	[EXT2_FT_FALLTHRU]	= DT_UNKNOWN,
 };
 
 #define S_SHIFT 12
@@ -341,6 +344,18 @@ ext2_readdir (struct file * filp, void *
 					ext2_put_page(page);
 					return 0;
 				}
+			} else if (de->file_type == EXT2_FT_FALLTHRU) {
+				int over;
+				unsigned char d_type = DT_UNKNOWN;
+
+				offset = (char *)de - kaddr;
+				over = filldir(dirent, de->name, de->name_len,
+						(n<<PAGE_CACHE_SHIFT) | offset,
+						123, d_type);
+				if (over) {
+					ext2_put_page(page);
+					return 0;
+				}
 			}
 			filp->f_pos += ext2_rec_len_from_disk(de->rec_len);
 		}
@@ -448,6 +463,30 @@ ino_t ext2_inode_by_name(struct inode *d
 	return res;
 }
 
+/* Special version for filetype based whiteout support */
+ino_t ext2_inode_by_dentry(struct inode *dir, struct dentry *dentry)
+{
+	ino_t res = 0;
+	struct ext2_dir_entry_2 *de;
+	struct page *page;
+
+	de = ext2_find_entry (dir, &dentry->d_name, &page);
+	if (de) {
+		res = le32_to_cpu(de->inode);
+		if (!res && de->file_type == EXT2_FT_WHT) {
+			spin_lock(&dentry->d_lock);
+			dentry->d_flags |= DCACHE_WHITEOUT;
+			spin_unlock(&dentry->d_lock);
+		} else if(!res && de->file_type == EXT2_FT_FALLTHRU) {
+			spin_lock(&dentry->d_lock);
+			dentry->d_flags |= DCACHE_FALLTHRU;
+			spin_unlock(&dentry->d_lock);
+		}
+		ext2_put_page(page);
+	}
+	return res;
+}
+
 /* Releases the page */
 void ext2_set_link(struct inode *dir, struct ext2_dir_entry_2 *de,
 		   struct page *page, struct inode *inode, int update_times)
@@ -472,9 +511,10 @@ void ext2_set_link(struct inode *dir, st
 }
 
 /*
- *	Parent is locked.
+ * Find or append a given dentry to the parent directory
  */
-int ext2_add_link (struct dentry *dentry, struct inode *inode)
+static ext2_dirent * ext2_append_entry(struct dentry * dentry,
+				       struct page ** page)
 {
 	struct inode *dir = dentry->d_parent->d_inode;
 	const char *name = dentry->d_name.name;
@@ -482,13 +522,10 @@ int ext2_add_link (struct dentry *dentry
 	unsigned chunk_size = ext2_chunk_size(dir);
 	unsigned reclen = EXT2_DIR_REC_LEN(namelen);
 	unsigned short rec_len, name_len;
-	struct page *page = NULL;
-	ext2_dirent * de;
+	ext2_dirent * de = NULL;
 	unsigned long npages = dir_pages(dir);
 	unsigned long n;
 	char *kaddr;
-	loff_t pos;
-	int err;
 
 	/*
 	 * We take care of directory expansion in the same loop.
@@ -498,55 +535,97 @@ int ext2_add_link (struct dentry *dentry
 	for (n = 0; n <= npages; n++) {
 		char *dir_end;
 
-		page = ext2_get_page(dir, n, 0);
-		err = PTR_ERR(page);
-		if (IS_ERR(page))
+		*page = ext2_get_page(dir, n, 0);
+		de = ERR_PTR(PTR_ERR(*page));
+		if (IS_ERR(*page))
 			goto out;
-		lock_page(page);
-		kaddr = page_address(page);
+		lock_page(*page);
+		kaddr = page_address(*page);
 		dir_end = kaddr + ext2_last_byte(dir, n);
 		de = (ext2_dirent *)kaddr;
 		kaddr += PAGE_CACHE_SIZE - reclen;
 		while ((char *)de <= kaddr) {
 			if ((char *)de == dir_end) {
 				/* We hit i_size */
-				name_len = 0;
-				rec_len = chunk_size;
+				de->name_len = 0;
 				de->rec_len = ext2_rec_len_to_disk(chunk_size);
 				de->inode = 0;
+				de->file_type = 0;
 				goto got_it;
 			}
 			if (de->rec_len == 0) {
 				ext2_error(dir->i_sb, __func__,
 					"zero-length directory entry");
-				err = -EIO;
+				de = ERR_PTR(-EIO);
 				goto out_unlock;
 			}
-			err = -EEXIST;
 			if (ext2_match (namelen, name, de))
-				goto out_unlock;
+				goto got_it;
 			name_len = EXT2_DIR_REC_LEN(de->name_len);
 			rec_len = ext2_rec_len_from_disk(de->rec_len);
-			if (!de->inode && rec_len >= reclen)
+			if (!de->inode && (de->file_type != EXT2_FT_WHT) &&
+			    (de->file_type != EXT2_FT_FALLTHRU) &&
+			    (rec_len >= reclen))
 				goto got_it;
 			if (rec_len >= name_len + reclen)
 				goto got_it;
 			de = (ext2_dirent *) ((char *) de + rec_len);
 		}
-		unlock_page(page);
-		ext2_put_page(page);
+		unlock_page(*page);
+		ext2_put_page(*page);
 	}
+
 	BUG();
-	return -EINVAL;
 
 got_it:
+	return de;
+	/* OFFSET_CACHE */
+out_unlock:
+	unlock_page(*page);
+	ext2_put_page(*page);
+out:
+	return de;
+}
+
+/*
+ *	Parent is locked.
+ */
+int ext2_add_link (struct dentry *dentry, struct inode *inode)
+{
+	struct inode *dir = dentry->d_parent->d_inode;
+	const char *name = dentry->d_name.name;
+	int namelen = dentry->d_name.len;
+	unsigned short rec_len, name_len;
+	ext2_dirent * de;
+	struct page *page;
+	loff_t pos;
+	int err;
+
+	de = ext2_append_entry(dentry, &page);
+	if (IS_ERR(de))
+		return PTR_ERR(de);
+
+	err = -EEXIST;
+	if (ext2_match (namelen, name, de)) {
+		if ((de->file_type == EXT2_FT_WHT) ||
+		    (de->file_type == EXT2_FT_FALLTHRU))
+			goto got_it;
+		goto out_unlock;
+	}
+
+got_it:
+	name_len = EXT2_DIR_REC_LEN(de->name_len);
+	rec_len = ext2_rec_len_from_disk(de->rec_len);
+
 	pos = page_offset(page) +
 		(char*)de - (char*)page_address(page);
 	err = __ext2_write_begin(NULL, page->mapping, pos, rec_len, 0,
 							&page, NULL);
 	if (err)
 		goto out_unlock;
-	if (de->inode) {
+	if (de->inode || (((de->file_type == EXT2_FT_WHT) ||
+			   (de->file_type == EXT2_FT_FALLTHRU)) &&
+			  !ext2_match (namelen, name, de))) {
 		ext2_dirent *de1 = (ext2_dirent *) ((char *) de + name_len);
 		de1->rec_len = ext2_rec_len_to_disk(rec_len - name_len);
 		de->rec_len = ext2_rec_len_to_disk(name_len);
@@ -563,7 +642,60 @@ got_it:
 	/* OFFSET_CACHE */
 out_put:
 	ext2_put_page(page);
-out:
+	return err;
+out_unlock:
+	unlock_page(page);
+	goto out_put;
+}
+
+/*
+ * Create a fallthru entry.
+ */
+int ext2_fallthru_entry (struct inode *dir, struct dentry *dentry)
+{
+	const char *name = dentry->d_name.name;
+	int namelen = dentry->d_name.len;
+	unsigned short rec_len, name_len;
+	ext2_dirent * de;
+	struct page *page;
+	loff_t pos;
+	int err;
+
+	de = ext2_append_entry(dentry, &page);
+	if (IS_ERR(de))
+		return PTR_ERR(de);
+
+	err = -EEXIST;
+	if (ext2_match (namelen, name, de))
+		goto out_unlock;
+
+	name_len = EXT2_DIR_REC_LEN(de->name_len);
+	rec_len = ext2_rec_len_from_disk(de->rec_len);
+
+	pos = page_offset(page) +
+		(char*)de - (char*)page_address(page);
+	err = __ext2_write_begin(NULL, page->mapping, pos, rec_len, 0,
+							&page, NULL);
+	if (err)
+		goto out_unlock;
+	if (de->inode || (de->file_type == EXT2_FT_WHT) ||
+	    (de->file_type == EXT2_FT_FALLTHRU)) {
+		ext2_dirent *de1 = (ext2_dirent *) ((char *) de + name_len);
+		de1->rec_len = ext2_rec_len_to_disk(rec_len - name_len);
+		de->rec_len = ext2_rec_len_to_disk(name_len);
+		de = de1;
+	}
+	de->name_len = namelen;
+	memcpy(de->name, name, namelen);
+	de->inode = 0;
+	de->file_type = EXT2_FT_FALLTHRU;
+	err = ext2_commit_chunk(page, pos, rec_len);
+	dir->i_mtime = dir->i_ctime = CURRENT_TIME_SEC;
+	EXT2_I(dir)->i_flags &= ~EXT2_BTREE_FL;
+	mark_inode_dirty(dir);
+	/* OFFSET_CACHE */
+out_put:
+	ext2_put_page(page);
 	return err;
 out_unlock:
 	unlock_page(page);
@@ -616,6 +748,70 @@ out:
 	return err;
 }
 
+int ext2_whiteout_entry (struct inode * dir, struct dentry * dentry,
+			 struct ext2_dir_entry_2 * de, struct page * page)
+{
+	const char *name = dentry->d_name.name;
+	int namelen = dentry->d_name.len;
+	unsigned short rec_len, name_len;
+	loff_t pos;
+	int err;
+
+	if (!de) {
+		de = ext2_append_entry(dentry, &page);
+		BUG_ON(!de);
+	}
+
+	err = -EEXIST;
+	if (ext2_match (namelen, name, de) &&
+	    (de->file_type == EXT2_FT_WHT)) {
+		ext2_error(dir->i_sb, __func__,
+			   "entry is already a whiteout in directory #%lu",
+			   dir->i_ino);
+		goto out_unlock;
+	}
+
+	name_len = EXT2_DIR_REC_LEN(de->name_len);
+	rec_len = ext2_rec_len_from_disk(de->rec_len);
+
+	pos = page_offset(page) +
+		(char*)de - (char*)page_address(page);
+	err = __ext2_write_begin(NULL, page->mapping, pos, rec_len, 0,
+							&page, NULL);
+	if (err)
+		goto out_unlock;
+	/*
+	 * We whiteout an existing entry. Do what ext2_delete_entry() would do,
+	 * except that we don't need to merge with the previous entry since
+	 * we are going to reuse it.
+	 */
+	if (ext2_match (namelen, name, de))
+		de->inode = 0;
+	if (de->inode || (((de->file_type == EXT2_FT_WHT) ||
+			   (de->file_type == EXT2_FT_FALLTHRU)) &&
+			  !ext2_match (namelen, name, de))) {
+		ext2_dirent *de1 = (ext2_dirent *) ((char *) de + name_len);
+		de1->rec_len = ext2_rec_len_to_disk(rec_len - name_len);
+		de->rec_len = ext2_rec_len_to_disk(name_len);
+		de = de1;
+	}
+	de->name_len = namelen;
+	memcpy(de->name, name, namelen);
+	de->inode = 0;
+	de->file_type = EXT2_FT_WHT;
+	err = ext2_commit_chunk(page, pos, rec_len);
+	dir->i_mtime = dir->i_ctime = CURRENT_TIME_SEC;
+	EXT2_I(dir)->i_flags &= ~EXT2_BTREE_FL;
+	mark_inode_dirty(dir);
+	/* OFFSET_CACHE */
+out_put:
+	ext2_put_page(page);
+	return err;
+out_unlock:
+	unlock_page(page);
+	goto out_put;
+}
+
 /*
  * Set the first fragment of directory.
  */
--- a/fs/ext2/ext2.h
+++ b/fs/ext2/ext2.h
@@ -102,9 +102,13 @@ extern void ext2_rsv_window_add(struct s
 /* dir.c */
 extern int ext2_add_link (struct dentry *, struct inode *);
 extern ino_t ext2_inode_by_name(struct inode *, struct qstr *);
+extern ino_t ext2_inode_by_dentry(struct inode *, struct dentry *);
 extern int ext2_make_empty(struct inode *, struct inode *);
 extern struct ext2_dir_entry_2 * ext2_find_entry (struct inode *,struct qstr *, struct page **);
 extern int ext2_delete_entry (struct ext2_dir_entry_2 *, struct page *);
+extern int ext2_whiteout_entry (struct inode *, struct dentry *,
+				struct ext2_dir_entry_2 *, struct page *);
+extern int ext2_fallthru_entry (struct inode *, struct dentry *);
 extern int ext2_empty_dir (struct inode *);
 extern struct ext2_dir_entry_2 * ext2_dotdot (struct inode *, struct page **);
 extern void ext2_set_link(struct inode *, struct ext2_dir_entry_2 *, struct page *, struct inode *, int);
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -1178,7 +1178,8 @@ void ext2_set_inode_flags(struct inode *
 {
 	unsigned int flags = EXT2_I(inode)->i_flags;
 
-	inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
+	inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC|
+			    S_OPAQUE);
 	if (flags & EXT2_SYNC_FL)
 		inode->i_flags |= S_SYNC;
 	if (flags & EXT2_APPEND_FL)
@@ -1189,6 +1190,8 @@ void ext2_set_inode_flags(struct inode *
 		inode->i_flags |= S_NOATIME;
 	if (flags & EXT2_DIRSYNC_FL)
 		inode->i_flags |= S_DIRSYNC;
+	if (flags & EXT2_OPAQUE_FL)
+		inode->i_flags |= S_OPAQUE;
 }
 
 /* Propagate flags from i_flags to EXT2_I(inode)->i_flags */
@@ -1196,8 +1199,8 @@ void ext2_get_inode_flags(struct ext2_in
 {
 	unsigned int flags = ei->vfs_inode.i_flags;
 
-	ei->i_flags &= ~(EXT2_SYNC_FL|EXT2_APPEND_FL|
-			EXT2_IMMUTABLE_FL|EXT2_NOATIME_FL|EXT2_DIRSYNC_FL);
+	ei->i_flags &= ~(EXT2_SYNC_FL|EXT2_APPEND_FL|EXT2_IMMUTABLE_FL|
+			 EXT2_NOATIME_FL|EXT2_DIRSYNC_FL|EXT2_OPAQUE_FL);
 	if (flags & S_SYNC)
 		ei->i_flags |= EXT2_SYNC_FL;
 	if (flags & S_APPEND)
@@ -1208,6 +1211,8 @@ void ext2_get_inode_flags(struct ext2_in
 		ei->i_flags |= EXT2_NOATIME_FL;
 	if (flags & S_DIRSYNC)
 		ei->i_flags |= EXT2_DIRSYNC_FL;
+	if (flags & S_OPAQUE)
+		ei->i_flags |= EXT2_OPAQUE_FL;
 }
 
 struct inode *ext2_iget (struct super_block *sb, unsigned long ino)
--- a/fs/ext2/namei.c
+++ b/fs/ext2/namei.c
@@ -54,15 +54,16 @@ static inline int ext2_add_nondir(struct
  * Methods themselves.
  */
 
-static struct dentry *ext2_lookup(struct inode * dir, struct dentry *dentry, struct nameidata *nd)
+static struct dentry *ext2_lookup(struct inode * dir, struct dentry *dentry,
+				  struct nameidata *nd)
 {
 	struct inode * inode;
 	ino_t ino;
-	
+
 	if (dentry->d_name.len > EXT2_NAME_LEN)
 		return ERR_PTR(-ENAMETOOLONG);
 
-	ino = ext2_inode_by_name(dir, &dentry->d_name);
+	ino = ext2_inode_by_dentry(dir, dentry);
 	inode = NULL;
 	if (ino) {
 		inode = ext2_iget(dir->i_sb, ino);
@@ -230,6 +231,10 @@ static int ext2_mkdir(struct inode * dir
 	else
 		inode->i_mapping->a_ops = &ext2_aops;
 
+	/* if we call mkdir on a whiteout create an opaque directory */
+	if (dentry->d_flags & DCACHE_WHITEOUT)
+		inode->i_flags |= S_OPAQUE;
+
 	inode_inc_link_count(inode);
 
 	err = ext2_make_empty(inode, dir);
@@ -293,6 +298,78 @@ static int ext2_rmdir (struct inode * di
 	return err;
 }
 
+/*
+ * Create a whiteout for the dentry
+ */
+static int ext2_whiteout(struct inode *dir, struct dentry *dentry,
+			 struct dentry *new_dentry)
+{
+	struct inode * inode = dentry->d_inode;
+	struct ext2_dir_entry_2 * de = NULL;
+	struct page * page;
+	int err = -ENOTEMPTY;
+
+	if (!EXT2_HAS_INCOMPAT_FEATURE(dir->i_sb,
+				       EXT2_FEATURE_INCOMPAT_FILETYPE)) {
+		ext2_error (dir->i_sb, "ext2_whiteout",
+			    "can't set whiteout filetype");
+		err = -EPERM;
+		goto out;
+	}
+
+	if (inode) {
+		if (S_ISDIR(inode->i_mode) && !ext2_empty_dir(inode))
+			goto out;
+
+		err = -ENOENT;
+		de = ext2_find_entry (dir, &dentry->d_name, &page);
+		if (!de)
+			goto out;
+		lock_page(page);
+	}
+
+	err = ext2_whiteout_entry (dir, dentry, de, page);
+	if (err)
+		goto out;
+
+	spin_lock(&new_dentry->d_lock);
+	new_dentry->d_flags &= ~DCACHE_FALLTHRU;
+	new_dentry->d_flags |= DCACHE_WHITEOUT;
+	spin_unlock(&new_dentry->d_lock);
+	d_add(new_dentry, NULL);
+
+	if (inode) {
+		inode->i_ctime = dir->i_ctime;
+		inode_dec_link_count(inode);
+		if (S_ISDIR(inode->i_mode)) {
+			inode->i_size = 0;
+			inode_dec_link_count(inode);
+			inode_dec_link_count(dir);
+		}
+	}
+	err = 0;
+out:
+	return err;
+}
+
+/*
+ * Create a fallthru entry.
+ */
+static int ext2_fallthru (struct inode *dir, struct dentry *dentry)
+{
+	int err;
+
+	err = ext2_fallthru_entry(dir, dentry);
+	if (err)
+		return err;
+
+	d_instantiate(dentry, NULL);
+	spin_lock(&dentry->d_lock);
+	dentry->d_flags |= DCACHE_FALLTHRU;
+	spin_unlock(&dentry->d_lock);
+	return 0;
+}
+
 static int ext2_rename (struct inode * old_dir, struct dentry * old_dentry,
 	struct inode * new_dir,	struct dentry * new_dentry )
 {
@@ -392,6 +469,8 @@ const struct inode_operations ext2_dir_i
 	.mkdir		= ext2_mkdir,
 	.rmdir		= ext2_rmdir,
 	.mknod		= ext2_mknod,
+	.whiteout	= ext2_whiteout,
+	.fallthru	= ext2_fallthru,
 	.rename		= ext2_rename,
 #ifdef CONFIG_EXT2_FS_XATTR
 	.setxattr	= generic_setxattr,
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -1062,6 +1062,13 @@ static int ext2_fill_super(struct super_
 	if (EXT2_HAS_COMPAT_FEATURE(sb, EXT3_FEATURE_COMPAT_HAS_JOURNAL))
 		ext2_warning(sb, __func__,
 			"mounting ext3 filesystem as ext2");
+
+	/*
+	 * Whiteouts (and fallthrus) require explicit whiteout support.
+	 */
+	if (EXT2_HAS_INCOMPAT_FEATURE(sb, EXT2_FEATURE_INCOMPAT_WHITEOUT))
+		sb->s_flags |= MS_WHITEOUT;
+
 	ext2_setup_super (sb, es, sb->s_flags & MS_RDONLY);
 	return 0;
 
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -59,6 +59,14 @@ source "fs/notify/Kconfig"
 
 source "fs/quota/Kconfig"
 
+config UNION_MOUNT
+       bool "Union mount support (EXPERIMENTAL)"
+       depends on EXPERIMENTAL
+       ---help---
+         If you say Y here, you will be able to mount file systems as
+         union mount stacks. This is a VFS based implementation and
+         should work with all file systems. If unsure, say N.
+
 source "fs/autofs/Kconfig"
 source "fs/autofs4/Kconfig"
 source "fs/fuse/Kconfig"
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -133,6 +133,7 @@ int dcache_readdir(struct file * filp, v
 	struct dentry *cursor = filp->private_data;
 	struct list_head *p, *q = &cursor->d_u.d_child;
 	ino_t ino;
+	int d_type;
 	int i = filp->f_pos;
 
 	switch (i) {
@@ -158,14 +159,25 @@ int dcache_readdir(struct file * filp, v
 			for (p=q->next; p != &dentry->d_subdirs; p=p->next) {
 				struct dentry *next;
 				next = list_entry(p, struct dentry, d_u.d_child);
-				if (d_unhashed(next) || !next->d_inode)
+				if (d_unhashed(next) || (!next->d_inode && !d_is_fallthru(next)))
 					continue;
 
+				if (d_is_fallthru(next)) {
+					/* XXX Make up things we can
+					 * only get out of the inode.
+					 * Should probably really do a
+					 * lookup instead. */
+					ino = 100; /* XXX Made up number of no significance */
+					d_type = DT_UNKNOWN;
+				} else {
+					ino = next->d_inode->i_ino;
+					d_type = dt_type(next->d_inode);
+				}
+
 				spin_unlock(&dcache_lock);
 				if (filldir(dirent, next->d_name.name, 
 					    next->d_name.len, filp->f_pos, 
-					    next->d_inode->i_ino, 
-					    dt_type(next->d_inode)) < 0)
+					    ino, d_type) < 0)
 					return 0;
 				spin_lock(&dcache_lock);
 				/* next is still alive */
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -52,6 +52,7 @@ obj-$(CONFIG_NFS_COMMON)	+= nfs_common/
 obj-$(CONFIG_GENERIC_ACL)	+= generic_acl.o
 
 obj-y				+= quota/
+obj-$(CONFIG_UNION_MOUNT)	+= union.o
 
 obj-$(CONFIG_PROC_FS)		+= proc/
 obj-y				+= partitions/
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -33,6 +33,7 @@
 #include <linux/fcntl.h>
 #include <linux/device_cgroup.h>
 #include <linux/fs_struct.h>
+#include <linux/union.h>
 #include <asm/uaccess.h>
 
 #define ACC_MODE(x) ("\000\004\002\006"[(x)&O_ACCMODE])
@@ -242,16 +243,17 @@ int generic_permission(struct inode *ino
 }
 
 /**
- * inode_permission  -  check for access rights to a given inode
+ * __inode_permission  -  check for access rights to a given inode
  * @inode:	inode to check permission on
  * @mask:	right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
+ * @rofs:	check for read-only fs
  *
  * Used to check for read/write/execute permissions on an inode.
  * We use "fsuid" for this, letting us set arbitrary permissions
  * for filesystem access without changing the "normal" uids which
  * are used for other things.
  */
-int inode_permission(struct inode *inode, int mask)
+int __inode_permission(struct inode *inode, int mask, int rofs)
 {
 	int retval;
 
@@ -261,7 +263,7 @@ int inode_permission(struct inode *inode
 		/*
 		 * Nobody gets write access to a read-only fs.
 		 */
-		if (IS_RDONLY(inode) &&
+		if ((rofs & IS_RDONLY(inode)) &&
 		    (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
 			return -EROFS;
 
@@ -289,6 +291,18 @@ int inode_permission(struct inode *inode
 }
 
 /**
+ * inode_permission  -  check for access rights to a given inode
+ * @inode:	inode to check permission on
+ * @mask:	right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
+ *
+ * This version pays attention to the MS_RDONLY flag on the fs.
+ */
+int inode_permission(struct inode *inode, int mask)
+{
+	return __inode_permission(inode, mask, 1);
+}
+
+/**
  * file_permission  -  check for additional access rights to a given file
  * @file:	file to check access rights for
  * @mask:	right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
@@ -417,15 +431,10 @@ do_revalidate(struct dentry *dentry, str
  * Internal lookup() using the new generic dcache.
  * SMP-safe
  */
-static struct dentry * cached_lookup(struct dentry * parent, struct qstr * name, struct nameidata *nd)
+static struct dentry *cache_lookup(struct dentry *parent, struct qstr *name,
+				   struct nameidata *nd)
 {
-	struct dentry * dentry = __d_lookup(parent, name);
-
-	/* lockess __d_lookup may fail due to concurrent d_move() 
-	 * in some unrelated directory, so try with d_lookup
-	 */
-	if (!dentry)
-		dentry = d_lookup(parent, name);
+	struct dentry *dentry = d_lookup(parent, name);
 
 	if (dentry && dentry->d_op && dentry->d_op->d_revalidate)
 		dentry = do_revalidate(dentry, nd);
@@ -434,6 +443,208 @@ static struct dentry * cached_lookup(str
 }
 
 /*
+ * Theory of operation for opaque, whiteout, and fallthru:
+ *
+ * whiteout: Unconditionally stop lookup here - ENOENT
+ *
+ * opaque: Don't lookup in directories lower in the union stack
+ *
+ * fallthru: While looking up an entry, ignore the opaque flag for the
+ * current directory only.
+ *
+ * A union stack is a linked list of directory dentries which appear
+ * in the same place in the namespace.  When constructing the union
+ * stack, we include directories below opaque directories so that we
+ * can properly handle fallthrus.  All non-fallthru lookups have to
+ * check for the opaque flag on the parent directory and obey it.
+ *
+ * In general, the code pattern is to lookup the the topmost entry
+ * first (either the first visible non-negative dentry or a negative
+ * dentry in the topmost layer of the union), then build the union
+ * stack for the newly looked-up entry (if it is a directory).
+ */
+
+/**
+ * __cache_lookup_topmost - lookup the topmost (non-)negative dentry
+ *
+ * @nd - parent's nameidata
+ * @name - pathname part to lookup
+ * @path - found dentry for pathname part
+ *
+ * This is used for union mount lookups from dcache. The first non-negative
+ * dentry is searched on all layers of the union stack. Otherwise the topmost
+ * negative dentry is returned.
+ */
+static int __cache_lookup_topmost(struct nameidata *nd, struct qstr *name,
+				  struct path *path)
+{
+	struct dentry *dentry;
+
+	dentry = d_lookup(nd->path.dentry, name);
+	if (dentry && dentry->d_op && dentry->d_op->d_revalidate)
+		dentry = do_revalidate(dentry, nd);
+
+	/*
+	 * Remember the topmost negative dentry in case we don't find anything
+	 */
+	path->dentry = dentry;
+	path->mnt = dentry ? nd->path.mnt : NULL;
+
+	if (!dentry || (dentry->d_inode || d_is_whiteout(dentry)))
+		return !dentry;
+
+	/* Keep going through opaque directories if we found a fallthru */
+	if (IS_OPAQUE(nd->path.dentry->d_inode) && !d_is_fallthru(dentry))
+		return !dentry;
+
+	/* look for the first non-negative or whiteout dentry */
+
+	while (follow_union_down(&nd->path)) {
+		dentry = d_hash_and_lookup(nd->path.dentry, name);
+
+		/*
+		 * If parts of the union stack are not in the dcache we need
+		 * to do a real lookup
+		 */
+		if (!dentry)
+			goto out_dput;
+
+		/*
+		 * If parts of the union don't survive the revalidation we
+		 * need to do a real lookup
+		 */
+		if (dentry->d_op && dentry->d_op->d_revalidate) {
+			dentry = do_revalidate(dentry, nd);
+			if (!dentry)
+				goto out_dput;
+		}
+
+		if (dentry->d_inode || d_is_whiteout(dentry))
+			goto out_dput;
+
+		/* Stop the lookup on opaque parent and non-fallthru child */
+		if (IS_OPAQUE(nd->path.dentry->d_inode) && !d_is_fallthru(dentry))
+			goto out_dput;
+
+		dput(dentry);
+	}
+
+	return !dentry;
+
+out_dput:
+	dput(path->dentry);
+	path->dentry = dentry;
+	path->mnt = dentry ? mntget(nd->path.mnt) : NULL;
+	return !dentry;
+}
+
+/**
+ * __cache_lookup_build_union - build the union stack for this part,
+ * cached version
+ *
+ * This is called after you have the topmost dentry in @path.
+ */
+static int __cache_lookup_build_union(struct nameidata *nd, struct qstr *name,
+				      struct path *path)
+{
+	struct path last = *path;
+	struct dentry *dentry;
+
+	while (follow_union_down(&nd->path)) {
+		dentry = d_hash_and_lookup(nd->path.dentry, name);
+		if (!dentry)
+			return 1;
+
+		if (dentry->d_op && dentry->d_op->d_revalidate) {
+			dentry = do_revalidate(dentry, nd);
+			if (!dentry)
+				return 1;
+		}
+
+		if (d_is_whiteout(dentry)) {
+			dput(dentry);
+			break;
+		}
+
+		if (!dentry->d_inode) {
+			dput(dentry);
+			continue;
+		}
+
+		/* only directories can be part of a union stack */
+		if (!S_ISDIR(dentry->d_inode->i_mode)) {
+			dput(dentry);
+			break;
+		}
+
+		/* Add the newly discovered dir to the union stack */
+		append_to_union(last.mnt, last.dentry, nd->path.mnt, dentry);
+
+		if (last.dentry != path->dentry)
+			path_put(&last);
+		last.dentry = dentry;
+		last.mnt = mntget(nd->path.mnt);
+	}
+
+	if (last.dentry != path->dentry)
+		path_put(&last);
+
+	return 0;
+}
+
+/**
+ * cache_lookup_union - lookup a single pathname part from dcache
+ *
+ * This is a union mount capable version of what d_lookup() & revalidate()
+ * would do. This function returns a valid (union) dentry on success.
+ *
+ * Remember: On failure it means that parts of the union aren't cached. You
+ * should call real_lookup() afterwards to find the proper (union) dentry.
+ */
+static int cache_lookup_union(struct nameidata *nd, struct qstr *name,
+			      struct path *path)
+{
+	int res ;
+
+	if (!IS_MNT_UNION(nd->path.mnt)) {
+		path->dentry = cache_lookup(nd->path.dentry, name, nd);
+		path->mnt = path->dentry ? nd->path.mnt : NULL;
+		res = path->dentry ? 0 : 1;
+	} else {
+		struct path safe = {
+			.dentry = nd->path.dentry,
+			.mnt = nd->path.mnt
+		};
+
+		path_get(&safe);
+		res = __cache_lookup_topmost(nd, name, path);
+		if (res)
+			goto out;
+
+		/* only directories can be part of a union stack */
+		if (!path->dentry->d_inode ||
+		    !S_ISDIR(path->dentry->d_inode->i_mode))
+			goto out;
+
+		/* Build the union stack for this part */
+		res = __cache_lookup_build_union(nd, name, path);
+		if (res) {
+			dput(path->dentry);
+			if (path->mnt != safe.mnt)
+				mntput(path->mnt);
+			goto out;
+		}
+
+out:
+		path_put(&nd->path);
+		nd->path.dentry = safe.dentry;
+		nd->path.mnt = safe.mnt;
+	}
+
+	return res;
+}
+
+/*
  * Short-cut version of permission(), for calling by
  * path_walk(), when dcache lock is held.  Combines parts
  * of permission() and generic_permission(), and tests ONLY for
@@ -473,10 +684,11 @@ ok:
  * make sure that nobody added the entry to the dcache in the meantime..
  * SMP-safe
  */
-static struct dentry * real_lookup(struct dentry * parent, struct qstr * name, struct nameidata *nd)
+static int real_lookup(struct nameidata *nd, struct qstr *name,
+		       struct path *path)
 {
-	struct dentry * result;
-	struct inode *dir = parent->d_inode;
+	struct inode *dir = nd->path.dentry->d_inode;
+	int res = 0;
 
 	mutex_lock(&dir->i_mutex);
 	/*
@@ -493,27 +705,36 @@ static struct dentry * real_lookup(struc
 	 *
 	 * so doing d_lookup() (with seqlock), instead of lockfree __d_lookup
 	 */
-	result = d_lookup(parent, name);
-	if (!result) {
+	path->dentry = d_lookup(nd->path.dentry, name);
+	path->mnt = nd->path.mnt;
+	if (!path->dentry) {
 		struct dentry *dentry;
 
 		/* Don't create child dentry for a dead directory. */
-		result = ERR_PTR(-ENOENT);
-		if (IS_DEADDIR(dir))
+		if (IS_DEADDIR(dir)) {
+			res = -ENOENT;
 			goto out_unlock;
+		}
 
-		dentry = d_alloc(parent, name);
-		result = ERR_PTR(-ENOMEM);
+		dentry = d_alloc(nd->path.dentry, name);
 		if (dentry) {
-			result = dir->i_op->lookup(dir, dentry, nd);
-			if (result)
+			path->dentry = dir->i_op->lookup(dir, dentry, nd);
+			if (path->dentry) {
 				dput(dentry);
-			else
-				result = dentry;
+				if (IS_ERR(path->dentry)) {
+					res = PTR_ERR(path->dentry);
+					path->dentry = NULL;
+					path->mnt = NULL;
+				}
+			} else
+				path->dentry = dentry;
+		} else {
+			res = -ENOMEM;
+			path->mnt = NULL;
 		}
 out_unlock:
 		mutex_unlock(&dir->i_mutex);
-		return result;
+		return res;
 	}
 
 	/*
@@ -521,12 +742,170 @@ out_unlock:
 	 * we waited on the semaphore. Need to revalidate.
 	 */
 	mutex_unlock(&dir->i_mutex);
-	if (result->d_op && result->d_op->d_revalidate) {
-		result = do_revalidate(result, nd);
-		if (!result)
-			result = ERR_PTR(-ENOENT);
+	if (path->dentry->d_op && path->dentry->d_op->d_revalidate) {
+		path->dentry = do_revalidate(path->dentry, nd);
+		if (!path->dentry) {
+			res = -ENOENT;
+			path->mnt = NULL;
+		}
+		if (IS_ERR(path->dentry)) {
+			res = PTR_ERR(path->dentry);
+			path->dentry = NULL;
+			path->mnt = NULL;
+		}
 	}
-	return result;
+
+	return res;
+}
+
+/**
+ * __real_lookup_topmost - lookup topmost dentry, non-cached version
+ *
+ * If we reach a dentry with restricted access, we just stop the lookup
+ * because we shouldn't see through that dentry. Same thing for dentry
+ * type mismatch and whiteouts.
+ *
+ * FIXME:
+ * - handle union stacks in use
+ * - handle union stacks mounted upon union stacks
+ * - avoid unnecessary allocations of union locks
+ */
+static int __real_lookup_topmost(struct nameidata *nd, struct qstr *name,
+				 struct path *path)
+{
+	struct path next;
+	int err;
+
+	err = real_lookup(nd, name, path);
+	if (err)
+		return err;
+
+	if (path->dentry->d_inode || d_is_whiteout(path->dentry))
+		return 0;
+
+	if (IS_OPAQUE(nd->path.dentry->d_inode) && !d_is_fallthru(path->dentry))
+		return 0;
+
+	while (follow_union_down(&nd->path)) {
+		name->hash = full_name_hash(name->name, name->len);
+		if (nd->path.dentry->d_op && nd->path.dentry->d_op->d_hash) {
+			err = nd->path.dentry->d_op->d_hash(nd->path.dentry,
+							    name);
+			if (err < 0)
+				goto out;
+		}
+
+		err = real_lookup(nd, name, &next);
+		if (err)
+			goto out;
+
+		if (next.dentry->d_inode || d_is_whiteout(next.dentry)) {
+			dput(path->dentry);
+			mntget(next.mnt);
+			*path = next;
+			goto out;
+		}
+
+		if (IS_OPAQUE(nd->path.dentry->d_inode) && !d_is_fallthru(next.dentry))
+			goto out;
+
+		dput(next.dentry);
+	}
+out:
+	if (err)
+		dput(path->dentry);
+	return err;
+}
+
+/**
+ * __real_lookup_build_union: build the union stack for this pathname
+ * part, non-cached version
+ *
+ * Called when not all parts of the union stack are in cache
+ */
+
+static int __real_lookup_build_union(struct nameidata *nd, struct qstr *name,
+				     struct path *path)
+{
+	struct path last = *path;
+	struct path next;
+	int err = 0;
+
+	while (follow_union_down(&nd->path)) {
+		/* We need to recompute the hash for lower layer lookups */
+		name->hash = full_name_hash(name->name, name->len);
+		if (nd->path.dentry->d_op && nd->path.dentry->d_op->d_hash) {
+			err = nd->path.dentry->d_op->d_hash(nd->path.dentry,
+							    name);
+			if (err < 0)
+				goto out;
+		}
+
+		err = real_lookup(nd, name, &next);
+		if (err)
+			goto out;
+
+		if (d_is_whiteout(next.dentry)) {
+			dput(next.dentry);
+			break;
+		}
+
+		if (!next.dentry->d_inode) {
+			dput(next.dentry);
+			continue;
+		}
+
+		/* only directories can be part of a union stack */
+		if (!S_ISDIR(next.dentry->d_inode->i_mode)) {
+			dput(next.dentry);
+			break;
+		}
+
+		/* now we know we found something "real" */
+		append_to_union(last.mnt, last.dentry, next.mnt, next.dentry);
+
+		if (last.dentry != path->dentry)
+			path_put(&last);
+		last.dentry = next.dentry;
+		last.mnt = mntget(next.mnt);
+	}
+
+	if (last.dentry != path->dentry)
+		path_put(&last);
+out:
+	return err;
+}
+
+static int real_lookup_union(struct nameidata *nd, struct qstr *name,
+			     struct path *path)
+{
+	struct path safe = { .dentry = nd->path.dentry, .mnt = nd->path.mnt };
+	int res ;
+
+	path_get(&safe);
+	res = __real_lookup_topmost(nd, name, path);
+	if (res)
+		goto out;
+
+	/* only directories can be part of a union stack */
+	if (!path->dentry->d_inode ||
+	    !S_ISDIR(path->dentry->d_inode->i_mode))
+		goto out;
+
+	/* Build the union stack for this part */
+	res = __real_lookup_build_union(nd, name, path);
+	if (res) {
+		dput(path->dentry);
+		if (path->mnt != safe.mnt)
+			mntput(path->mnt);
+		goto out;
+	}
+
+out:
+	path_put(&nd->path);
+	nd->path.dentry = safe.dentry;
+	nd->path.mnt = safe.mnt;
+	return res;
 }
 
 /*
@@ -629,11 +1008,8 @@ static __always_inline int __do_follow_l
 	touch_atime(path->mnt, dentry);
 	nd_set_link(nd, NULL);
 
-	if (path->mnt != nd->path.mnt) {
-		path_to_nameidata(path, nd);
-		dget(dentry);
-	}
-	mntget(path->mnt);
+	if (path->mnt == nd->path.mnt)
+		mntget(nd->path.mnt);
 	cookie = dentry->d_inode->i_op->follow_link(dentry, nd);
 	error = PTR_ERR(cookie);
 	if (!IS_ERR(cookie)) {
@@ -721,7 +1097,7 @@ static int __follow_mount(struct path *p
 	return res;
 }
 
-static void follow_mount(struct path *path)
+void follow_mount(struct path *path)
 {
 	while (d_mountpoint(path->dentry)) {
 		struct vfsmount *mounted = lookup_mnt(path);
@@ -786,6 +1162,7 @@ static __always_inline void follow_dotdo
 		nd->path.mnt = parent;
 	}
 	follow_mount(&nd->path);
+	follow_union_mount(&nd->path);
 }
 
 /*
@@ -796,35 +1173,55 @@ static __always_inline void follow_dotdo
 static int do_lookup(struct nameidata *nd, struct qstr *name,
 		     struct path *path)
 {
-	struct vfsmount *mnt = nd->path.mnt;
-	struct dentry *dentry = __d_lookup(nd->path.dentry, name);
+	int err;
+
+	if (IS_MNT_UNION(nd->path.mnt))
+		goto need_union_lookup;
 
-	if (!dentry)
+	path->dentry = __d_lookup(nd->path.dentry, name);
+	path->mnt = nd->path.mnt;
+	if (!path->dentry)
 		goto need_lookup;
-	if (dentry->d_op && dentry->d_op->d_revalidate)
+	if (path->dentry->d_op && path->dentry->d_op->d_revalidate)
 		goto need_revalidate;
+
 done:
-	path->mnt = mnt;
-	path->dentry = dentry;
-	__follow_mount(path);
+	if (nd->path.mnt != path->mnt) {
+		nd->um_flags |= LAST_LOWLEVEL;
+		follow_mount(path);
+	} else
+		__follow_mount(path);
+	follow_union_mount(path);
 	return 0;
 
 need_lookup:
-	dentry = real_lookup(nd->path.dentry, name, nd);
-	if (IS_ERR(dentry))
+	err = real_lookup(nd, name, path);
+	if (err)
+		goto fail;
+	goto done;
+
+need_union_lookup:
+	err = cache_lookup_union(nd, name, path);
+	if (!err && path->dentry)
+		goto done;
+
+	err = real_lookup_union(nd, name, path);
+	if (err)
 		goto fail;
 	goto done;
 
 need_revalidate:
-	dentry = do_revalidate(dentry, nd);
-	if (!dentry)
+	path->dentry = do_revalidate(path->dentry, nd);
+	if (!path->dentry)
 		goto need_lookup;
-	if (IS_ERR(dentry))
+	if (IS_ERR(path->dentry)) {
+		err = PTR_ERR(path->dentry);
 		goto fail;
+	}
 	goto done;
 
 fail:
-	return PTR_ERR(dentry);
+	return err;
 }
 
 /*
@@ -851,6 +1248,8 @@ static int __link_path_walk(const char *
 	if (nd->depth)
 		lookup_flags = LOOKUP_FOLLOW | (nd->flags & LOOKUP_CONTINUE);
 
+	follow_union_mount(&nd->path);
+
 	/* At this point we know we have a real path component. */
 	for(;;) {
 		unsigned long hash;
@@ -913,6 +1312,44 @@ static int __link_path_walk(const char *
 		if (err)
 			break;
 
+		/*
+		 * We want to create this element on the top level
+		 * file system in two cases:
+		 *
+		 * - We are specifically told to - LOOKUP_TOPMOST.
+		 * - This is a directory, and it does not yet exist on
+		 *   the top level.  Various tricks only work if
+		 *   directories always exist on the top level.
+		 *
+		 * In either case, only create this element on the top
+		 * level if the last element is located on the lower
+		 * level.  If the last element is located on the top
+		 * level, then every single element in the path
+		 * already exists on the top level.
+		 *
+		 * Note that we can assume that the parent is on the
+		 * top level since we always create the directory on
+		 * the top level.
+		 */
+
+		if ((nd->um_flags & LAST_LOWLEVEL) &&
+		    ((next.dentry->d_inode &&
+		      S_ISDIR(next.dentry->d_inode->i_mode) &&
+		      (nd->path.mnt != next.mnt)) ||
+		     (nd->flags & LOOKUP_TOPMOST))) {
+			struct dentry *dentry;
+
+			dentry = union_create_topmost(nd, &this, &next);
+			if (IS_ERR(dentry)) {
+				err = PTR_ERR(dentry);
+				goto out_dput;
+			}
+			path_put_conditional(&next, nd);
+			next.mnt = nd->path.mnt;
+			next.dentry = dentry;
+			nd->um_flags &= ~LAST_LOWLEVEL;
+		}
+
 		err = -ENOENT;
 		inode = next.dentry->d_inode;
 		if (!inode)
@@ -962,6 +1399,25 @@ last_component:
 		err = do_lookup(nd, &this, &next);
 		if (err)
 			break;
+
+		if ((nd->um_flags & LAST_LOWLEVEL) &&
+		    ((next.dentry->d_inode &&
+		      S_ISDIR(next.dentry->d_inode->i_mode) &&
+		      (nd->path.mnt != next.mnt)) ||
+		     (nd->flags & LOOKUP_TOPMOST))) {
+			struct dentry *dentry;
+
+			dentry = union_create_topmost(nd, &this, &next);
+			if (IS_ERR(dentry)) {
+				err = PTR_ERR(dentry);
+				goto out_dput;
+			}
+			path_put_conditional(&next, nd);
+			next.mnt = nd->path.mnt;
+			next.dentry = dentry;
+			nd->um_flags &= ~LAST_LOWLEVEL;
+		}
+
 		inode = next.dentry->d_inode;
 		if ((lookup_flags & LOOKUP_FOLLOW)
 		    && inode && inode->i_op->follow_link) {
@@ -1029,6 +1485,7 @@ static int path_init(int dfd, const char
 
 	nd->last_type = LAST_ROOT; /* if there are only slashes... */
 	nd->flags = flags;
+	nd->um_flags = 0;
 	nd->depth = 0;
 	nd->root.mnt = NULL;
 
@@ -1172,61 +1629,437 @@ static int path_lookup_open(int dfd, con
 }
 
 static struct dentry *__lookup_hash(struct qstr *name,
-		struct dentry *base, struct nameidata *nd)
+				    struct dentry *base, struct nameidata *nd)
+{
+	struct dentry *dentry;
+	struct inode *inode;
+	int err;
+
+	inode = base->d_inode;
+
+	/*
+	 * See if the low-level filesystem might want
+	 * to use its own hash..
+	 */
+	if (base->d_op && base->d_op->d_hash) {
+		err = base->d_op->d_hash(base, name);
+		dentry = ERR_PTR(err);
+		if (err < 0)
+			goto out;
+	}
+
+	dentry = cache_lookup(base, name, nd);
+	if (!dentry) {
+		struct dentry *new;
+
+		/* Don't create child dentry for a dead directory. */
+		dentry = ERR_PTR(-ENOENT);
+		if (IS_DEADDIR(inode))
+			goto out;
+
+		new = d_alloc(base, name);
+		dentry = ERR_PTR(-ENOMEM);
+		if (!new)
+			goto out;
+		dentry = inode->i_op->lookup(inode, new, nd);
+		if (!dentry)
+			dentry = new;
+		else
+			dput(new);
+	}
+out:
+	return dentry;
+}
+
+/*
+ * Restricted form of lookup. Doesn't follow links, single-component only,
+ * needs parent already locked. Doesn't follow mounts.
+ * SMP-safe.
+ */
+static int lookup_hash(struct nameidata *nd, struct qstr *name,
+		       struct path *path)
+{
+	int err;
+
+	err = inode_permission(nd->path.dentry->d_inode, MAY_EXEC);
+	if (err)
+		return err;
+	path->mnt = nd->path.mnt;
+	path->dentry =  __lookup_hash(name, nd->path.dentry, nd);
+	if (IS_ERR(path->dentry)) {
+		err = PTR_ERR(path->dentry);
+		path->dentry = NULL;
+		path->mnt = NULL;
+	}
+	return err;
+}
+
+static int __hash_lookup_topmost(struct nameidata *nd, struct qstr *name,
+				 struct path *path)
+{
+	struct path next;
+	int err;
+
+	err = lookup_hash(nd, name, path);
+	if (err)
+		return err;
+
+	if (path->dentry->d_inode || d_is_whiteout(path->dentry))
+		return 0;
+
+	if (IS_OPAQUE(nd->path.dentry->d_inode) && !d_is_fallthru(path->dentry))
+		return 0;
+
+	while (follow_union_down(&nd->path)) {
+		name->hash = full_name_hash(name->name, name->len);
+		if (nd->path.dentry->d_op && nd->path.dentry->d_op->d_hash) {
+			err = nd->path.dentry->d_op->d_hash(nd->path.dentry,
+							    name);
+			if (err < 0)
+				goto out;
+		}
+
+		mutex_lock(&nd->path.dentry->d_inode->i_mutex);
+		err = lookup_hash(nd, name, &next);
+		mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
+		if (err)
+			goto out;
+
+		if (next.dentry->d_inode || d_is_whiteout(next.dentry)) {
+			dput(path->dentry);
+			mntget(next.mnt);
+			*path = next;
+			goto out;
+		}
+
+		if (IS_OPAQUE(nd->path.dentry->d_inode) && !d_is_fallthru(next.dentry))
+			goto out;
+
+		dput(next.dentry);
+	}
+out:
+	if (err)
+		dput(path->dentry);
+	return err;
+}
+
+static int __hash_lookup_build_union(struct nameidata *nd, struct qstr *name,
+				     struct path *path)
+{
+	struct path last = *path;
+	struct path next;
+	int err = 0;
+
+	while (follow_union_down(&nd->path)) {
+		/* We need to recompute the hash for lower layer lookups */
+		name->hash = full_name_hash(name->name, name->len);
+		if (nd->path.dentry->d_op && nd->path.dentry->d_op->d_hash) {
+			err = nd->path.dentry->d_op->d_hash(nd->path.dentry,
+							    name);
+			if (err < 0)
+				goto out;
+		}
+
+		mutex_lock(&nd->path.dentry->d_inode->i_mutex);
+		err = lookup_hash(nd, name, &next);
+		mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
+		if (err)
+			goto out;
+
+		if (d_is_whiteout(next.dentry)) {
+			dput(next.dentry);
+			break;
+		}
+
+		if (!next.dentry->d_inode) {
+			dput(next.dentry);
+			continue;
+		}
+
+		/* only directories can be part of a union stack */
+		if (!S_ISDIR(next.dentry->d_inode->i_mode)) {
+			dput(next.dentry);
+			break;
+		}
+
+		/* now we know we found something "real" */
+		append_to_union(last.mnt, last.dentry, next.mnt, next.dentry);
+
+		if (last.dentry != path->dentry)
+			path_put(&last);
+		last.dentry = next.dentry;
+		last.mnt = mntget(next.mnt);
+	}
+
+	if (last.dentry != path->dentry)
+		path_put(&last);
+out:
+	return err;
+}
+
+int hash_lookup_union(struct nameidata *nd, struct qstr *name,
+			     struct path *path)
+{
+	struct path safe = { .dentry = nd->path.dentry, .mnt = nd->path.mnt };
+	int res ;
+
+	path_get(&safe);
+	res = __hash_lookup_topmost(nd, name, path);
+	if (res)
+		goto out;
+
+	/* only directories can be part of a union stack */
+	if (!path->dentry->d_inode ||
+	    !S_ISDIR(path->dentry->d_inode->i_mode))
+		goto out;
+
+	/* Build the union stack for this part */
+	res = __hash_lookup_build_union(nd, name, path);
+	if (res) {
+		dput(path->dentry);
+		if (path->mnt != safe.mnt)
+			mntput(path->mnt);
+		goto out;
+	}
+
+out:
+	path_put(&nd->path);
+	nd->path.dentry = safe.dentry;
+	nd->path.mnt = safe.mnt;
+	return res;
+}
+
+/**
+ * do_union_hash_lookup() - walk down the union stack and lookup_hash()
+ * @nd: nameidata of parent to lookup from
+ * @name: pathname component to lookup
+ * @path: path to store result of lookup in
+ *
+ * Walk down the union stack and search for single pathname component name. It
+ * is assumed that the caller already did a lookup_hash() in the topmost parent
+ * that gave negative lookup result. Therefore this does call lookup_hash() in
+ * every lower layer (!) of the union stack. If a directory is found the union
+ * stack for that is assembled as well.
+ *
+ * Note:
+ * The caller needs to take care of holding a valid reference to the topmost
+ * parent.
+ * On error we leave @path untouched as well as when we don't find anything.
+ */
+static int do_union_hash_lookup(struct nameidata *nd, struct qstr *name,
+				struct path *path)
+{
+	struct path next;
+	int err = 0;
+
+	while (follow_union_down(&nd->path)) {
+		/* rehash because of d_op->d_hash() by the previous layer */
+		name->hash = full_name_hash(name->name, name->len);
+
+		mutex_lock(&nd->path.dentry->d_inode->i_mutex);
+		err = lookup_hash(nd, name, &next);
+		mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
+
+		if (err)
+			break;
+
+		if (next.dentry->d_inode) {
+			mntget(next.mnt);
+			if (!S_ISDIR(next.dentry->d_inode->i_mode)) {
+				*path = next;
+				break;
+			}
+			err = __hash_lookup_build_union(nd, name, &next);
+			if (err)
+				path_put(&next);
+			else
+				*path = next;
+			break;
+		}
+
+		path_put_conditional(&next, nd);
+
+		if ((IS_OPAQUE(nd->path.dentry->d_inode) &&
+		     !d_is_fallthru(next.dentry)) ||
+		    d_is_whiteout(next.dentry))
+			break;
+	}
+
+	return err;
+}
+
+/**
+ * _hash_lookup_union() - lookup single pathname component
+ * @nd: nameidata of parent to lookup from
+ * @name: pathname component to lookup
+ * @path: path to store result of lookup in
+ *
+ * Returns the topmost parent locked and the target dentry found in the union
+ * or the topmost negative target dentry otherwise.
+ *
+ * Note:
+ * Returns topmost parent locked even on error.
+ */
+static int _hash_lookup_union(struct nameidata *nd, struct qstr *name,
+			      struct path *path)
+{
+	struct path parent = nd->path;
+	struct path topmost;
+	int err;
+
+	mutex_lock(&nd->path.dentry->d_inode->i_mutex);
+	err = lookup_hash(nd, name, path);
+	if (err)
+		return err;
+
+	/* return if we found something and it isn't a directory we are done */
+	if (path->dentry->d_inode && !S_ISDIR(path->dentry->d_inode->i_mode))
+		return 0;
+
+	/* stop lookup if the parent directory is marked opaque */
+	if ((IS_OPAQUE(nd->path.dentry->d_inode) &&
+	     !d_is_fallthru(path->dentry)) ||
+	    d_is_whiteout(path->dentry))
+		return 0;
+
+	if (!strcmp(path->mnt->mnt_sb->s_type->name, "proc") ||
+	    !strcmp(path->mnt->mnt_sb->s_type->name, "sysfs"))
+		return 0;
+
+	mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
+
+	/*
+	 * safe a reference to the topmost parent for walking the union stack
+	 */
+	path_get(&parent);
+	topmost = *path;
+
+	if (path->dentry->d_inode && S_ISDIR(path->dentry->d_inode->i_mode)) {
+		err = __hash_lookup_build_union(nd, name, path);
+		if (err)
+			goto err_lock_parent;
+		goto out_lock_and_revalidate_parent;
+	}
+
+	err = do_union_hash_lookup(nd, name, path);
+	if (err)
+		goto err_lock_parent;
+
+out_lock_and_revalidate_parent:
+	/* seems that we haven't found anything, so return the topmost */
+	path_to_nameidata(&parent, nd);
+	mutex_lock(&nd->path.dentry->d_inode->i_mutex);
+
+	if (topmost.dentry == path->dentry) {
+		spin_lock(&path->dentry->d_lock);
+		if (nd->path.dentry != path->dentry->d_parent) {
+			spin_unlock(&path->dentry->d_lock);
+			dput(path->dentry);
+			name->hash = full_name_hash(name->name, name->len);
+			err = lookup_hash(nd, name, path);
+			if (err)
+				return err;
+			/* FIXME: What if we find a directory here ... */
+			return err;
+		}
+		spin_unlock(&path->dentry->d_lock);
+	} else
+		dput(topmost.dentry);
+
+	return 0;
+
+err_lock_parent:
+	path_to_nameidata(&parent, nd);
+	path_put_conditional(path, nd);
+	mutex_lock(&nd->path.dentry->d_inode->i_mutex);
+	return err;
+}
+
+/**
+ * lookup_rename_source() - lookup the source used by rename
+ *
+ * This is a special version of _hash_lookup_union() which becomes necessary
+ * for finding the source of a rename on union mounts.
+ *
+ * See comment for _hash_lookup_union() above.
+ */
+static int lookup_rename_source(struct nameidata *oldnd,
+				struct nameidata *newnd,
+				struct dentry **trap, struct qstr *name,
+				struct path *old)
 {
-	struct dentry *dentry;
-	struct inode *inode;
+	struct path parent = oldnd->path;
+	struct path topmost;
 	int err;
 
-	inode = base->d_inode;
+	err = lookup_hash(oldnd, name, old);
+	if (err)
+		return err;
+
+	/* return if we found something and it isn't a directory we are done */
+	if (old->dentry->d_inode && !S_ISDIR(old->dentry->d_inode->i_mode))
+		return 0;
+
+	/* stop lookup if the parent directory is marked opaque */
+	if ((IS_OPAQUE(oldnd->path.dentry->d_inode) &&
+	     !d_is_fallthru(old->dentry)) ||
+	    d_is_whiteout(old->dentry))
+		return 0;
+
+	if (!strcmp(old->mnt->mnt_sb->s_type->name, "proc") ||
+	    !strcmp(old->mnt->mnt_sb->s_type->name, "sysfs"))
+		return 0;
+
+	unlock_rename(oldnd->path.dentry, newnd->path.dentry);
 
 	/*
-	 * See if the low-level filesystem might want
-	 * to use its own hash..
+	 * safe a reference to the topmost parent for walking the union stack
 	 */
-	if (base->d_op && base->d_op->d_hash) {
-		err = base->d_op->d_hash(base, name);
-		dentry = ERR_PTR(err);
-		if (err < 0)
-			goto out;
+	path_get(&parent);
+	topmost = *old;
+
+	if (old->dentry->d_inode && S_ISDIR(old->dentry->d_inode->i_mode)) {
+		err = __hash_lookup_build_union(oldnd, name, old);
+		if (err)
+			goto err_lock;
+		goto out_lock_and_revalidate_parent;
 	}
 
-	dentry = cached_lookup(base, name, nd);
-	if (!dentry) {
-		struct dentry *new;
+	err = do_union_hash_lookup(oldnd, name, old);
+	if (err)
+		goto err_lock;
 
-		/* Don't create child dentry for a dead directory. */
-		dentry = ERR_PTR(-ENOENT);
-		if (IS_DEADDIR(inode))
-			goto out;
+out_lock_and_revalidate_parent:
+	path_to_nameidata(&parent, oldnd);
+	*trap = lock_rename(oldnd->path.dentry, newnd->path.dentry);
 
-		new = d_alloc(base, name);
-		dentry = ERR_PTR(-ENOMEM);
-		if (!new)
-			goto out;
-		dentry = inode->i_op->lookup(inode, new, nd);
-		if (!dentry)
-			dentry = new;
-		else
-			dput(new);
-	}
-out:
-	return dentry;
-}
+	/*
+	 * If we return the topmost dentry we have to make sure that it has not
+	 * been moved away while we gave up the topmost parents i_mutex lock.
+	 */
+	if (topmost.dentry == old->dentry) {
+		spin_lock(&old->dentry->d_lock);
+		if (oldnd->path.dentry != old->dentry->d_parent) {
+			spin_unlock(&old->dentry->d_lock);
+			dput(old->dentry);
+			name->hash = full_name_hash(name->name, name->len);
+			err = lookup_hash(oldnd, name, old);
+			if (err)
+				return err;
+			/* FIXME: What if we find a directory here ... */
+			return err;
+		}
+		spin_unlock(&old->dentry->d_lock);
+	} else
+		dput(topmost.dentry);
 
-/*
- * Restricted form of lookup. Doesn't follow links, single-component only,
- * needs parent already locked. Doesn't follow mounts.
- * SMP-safe.
- */
-static struct dentry *lookup_hash(struct nameidata *nd)
-{
-	int err;
+	return 0;
 
-	err = inode_permission(nd->path.dentry->d_inode, MAY_EXEC);
-	if (err)
-		return ERR_PTR(err);
-	return __lookup_hash(&nd->last, nd->path.dentry, nd);
+err_lock:
+	path_to_nameidata(&parent, oldnd);
+	path_put_conditional(old, oldnd);
+	*trap = lock_rename(oldnd->path.dentry, newnd->path.dentry);
+	return err;
 }
 
 static int __lookup_one_len(const char *name, struct qstr *this,
@@ -1502,8 +2335,9 @@ int vfs_create(struct inode *dir, struct
 	return error;
 }
 
-int may_open(struct path *path, int acc_mode, int flag)
+int may_open(struct nameidata *nd, int acc_mode, int flag)
 {
+	struct path *path = &nd->path;
 	struct dentry *dentry = path->dentry;
 	struct inode *inode = dentry->d_inode;
 	int error;
@@ -1529,7 +2363,7 @@ int may_open(struct path *path, int acc_
 		break;
 	}
 
-	error = inode_permission(inode, acc_mode);
+	error = union_permission(path, acc_mode);
 	if (error)
 		return error;
 
@@ -1577,6 +2411,9 @@ int may_open(struct path *path, int acc_
 		if (!error)
 			error = security_path_truncate(path, 0,
 					       ATTR_MTIME|ATTR_CTIME|ATTR_OPEN);
+		/* XXX don't copy up file data */
+		if (is_unionized(path->dentry, path->mnt))
+			error = union_copyup(nd, flag /* XXX not used */);
 		if (!error) {
 			vfs_dq_init(inode);
 
@@ -1623,7 +2460,7 @@ out_unlock:
 	if (error)
 		return error;
 	/* Don't check for write permission, don't truncate */
-	return may_open(&nd->path, 0, flag & ~O_TRUNC);
+	return may_open(nd, 0, flag & ~O_TRUNC);
 }
 
 /*
@@ -1738,12 +2575,10 @@ struct file *do_filp_open(int dfd, const
 	if (flag & O_EXCL)
 		nd.flags |= LOOKUP_EXCL;
 	mutex_lock(&dir->d_inode->i_mutex);
-	path.dentry = lookup_hash(&nd);
-	path.mnt = nd.path.mnt;
+	error = hash_lookup_union(&nd, &nd.last, &path);
 
 do_last:
-	error = PTR_ERR(path.dentry);
-	if (IS_ERR(path.dentry)) {
+	if (error) {
 		mutex_unlock(&dir->d_inode->i_mutex);
 		goto exit;
 	}
@@ -1803,10 +2638,23 @@ do_last:
 	if (path.dentry->d_inode->i_op->follow_link)
 		goto do_link;
 
-	path_to_nameidata(&path, &nd);
 	error = -EISDIR;
 	if (path.dentry->d_inode && S_ISDIR(path.dentry->d_inode->i_mode))
-		goto exit;
+		goto exit_dput;
+
+	/*
+	 * If this file is on a lower layer of the union stack, copy it to the
+	 * topmost layer before opening it
+	 */
+	if (path.dentry->d_inode &&
+	    (path.dentry->d_parent != dir) &&
+	    S_ISREG(path.dentry->d_inode->i_mode)) {
+		error = __union_copyup(&path, &nd, &path);
+		if (error)
+			goto exit_dput;
+	}
+
+	path_to_nameidata(&path, &nd);
 ok:
 	/*
 	 * Consider:
@@ -1824,12 +2672,18 @@ ok:
 		if (error)
 			goto exit;
 	}
-	error = may_open(&nd.path, acc_mode, flag);
+	error = may_open(&nd, acc_mode, flag);
 	if (error) {
 		if (will_write)
 			mnt_drop_write(nd.path.mnt);
 		goto exit;
 	}
+	/* Okay, all permissions go, now copy up */
+	if (!(flag & O_CREAT) && (flag & FMODE_WRITE)) {
+		error = union_copyup(&nd, flag /* XXX not used */);
+		if (error)
+			goto exit;
+	}
 	filp = nameidata_to_filp(&nd, open_flag);
 	if (IS_ERR(filp))
 		ima_counts_put(&nd.path,
@@ -1904,8 +2758,7 @@ do_link:
 	}
 	dir = nd.path.dentry;
 	mutex_lock(&dir->d_inode->i_mutex);
-	path.dentry = lookup_hash(&nd);
-	path.mnt = nd.path.mnt;
+	error = hash_lookup_union(&nd, &nd.last, &path);
 	__putname(nd.last.name);
 	goto do_last;
 }
@@ -1939,7 +2792,8 @@ EXPORT_SYMBOL(filp_open);
  */
 struct dentry *lookup_create(struct nameidata *nd, int is_dir)
 {
-	struct dentry *dentry = ERR_PTR(-EEXIST);
+	struct path path = { .dentry = ERR_PTR(-EEXIST) } ;
+	int err;
 
 	mutex_lock_nested(&nd->path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
 	/*
@@ -1955,11 +2809,13 @@ struct dentry *lookup_create(struct name
 	/*
 	 * Do the final lookup.
 	 */
-	dentry = lookup_hash(nd);
-	if (IS_ERR(dentry))
+	err = hash_lookup_union(nd, &nd->last, &path);
+	if (err) {
+		path.dentry = ERR_PTR(err);
 		goto fail;
+	}
 
-	if (dentry->d_inode)
+	if (path.dentry->d_inode)
 		goto eexist;
 	/*
 	 * Special case - lookup gave negative, but... we had foo/bar/
@@ -1968,15 +2824,17 @@ struct dentry *lookup_create(struct name
 	 * been asking for (non-existent) directory. -ENOENT for you.
 	 */
 	if (unlikely(!is_dir && nd->last.name[nd->last.len])) {
-		dput(dentry);
-		dentry = ERR_PTR(-ENOENT);
+		path_put_conditional(&path, nd);
+		path.dentry = ERR_PTR(-ENOENT);
 	}
-	return dentry;
+	if (nd->path.mnt != path.mnt)
+		mntput(path.mnt);
+	return path.dentry;
 eexist:
-	dput(dentry);
-	dentry = ERR_PTR(-EEXIST);
+	path_put_conditional(&path, nd);
+	path.dentry = ERR_PTR(-EEXIST);
 fail:
-	return dentry;
+	return path.dentry;
 }
 EXPORT_SYMBOL_GPL(lookup_create);
 
@@ -2088,6 +2946,7 @@ SYSCALL_DEFINE3(mknod, const char __user
 int vfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
 {
 	int error = may_create(dir, dentry);
+	int opaque = 0;
 
 	if (error)
 		return error;
@@ -2101,9 +2960,18 @@ int vfs_mkdir(struct inode *dir, struct 
 		return error;
 
 	vfs_dq_init(dir);
+
+	if (d_is_whiteout(dentry))
+		opaque = 1;
+
 	error = dir->i_op->mkdir(dir, dentry, mode);
-	if (!error)
+	if (!error) {
 		fsnotify_mkdir(dir, dentry);
+		if (opaque) {
+			dentry->d_inode->i_flags |= S_OPAQUE;
+			mark_inode_dirty(dentry->d_inode);
+		}
+	}
 	return error;
 }
 
@@ -2149,6 +3017,212 @@ SYSCALL_DEFINE2(mkdir, const char __user
 	return sys_mkdirat(AT_FDCWD, pathname, mode);
 }
 
+
+/* Checks on the victim for whiteout */
+static inline int may_whiteout(struct inode *dir, struct dentry *victim,
+			       int isdir)
+{
+	int err;
+
+	/* from may_create() */
+	if (IS_DEADDIR(dir))
+		return -ENOENT;
+	err = inode_permission(dir, MAY_WRITE | MAY_EXEC);
+	if (err)
+		return err;
+
+	/* from may_delete() */
+	if (IS_APPEND(dir))
+		return -EPERM;
+	if (!victim->d_inode)
+		return 0;
+	if (check_sticky(dir, victim->d_inode) ||
+	    IS_APPEND(victim->d_inode) ||
+	    IS_IMMUTABLE(victim->d_inode))
+		return -EPERM;
+	if (isdir) {
+		if (!S_ISDIR(victim->d_inode->i_mode))
+			return -ENOTDIR;
+		if (IS_ROOT(victim))
+			return -EBUSY;
+	} else if (S_ISDIR(victim->d_inode->i_mode))
+		return -EISDIR;
+	if (victim->d_flags & DCACHE_NFSFS_RENAMED)
+		return -EBUSY;
+	return 0;
+}
+
+/**
+ * vfs_whiteout: creates a white-out for the given directory entry
+ * @dir: parent inode
+ * @dentry: directory entry to white-out
+ *
+ * Simply white-out a given directory entry. This functionality is usually used
+ * in the sense of unlink. Therefore the given dentry can still be in-use and
+ * contains an in-use inode. The filesystem has to do what unlink or rmdir
+ * would in that case. Since the dentry still might be in-use we have to
+ * provide a fresh unhashed dentry that whiteout can fill the new inode into.
+ * In that case the given dentry is dropped and the fresh dentry containing the
+ * whiteout is rehashed instead. If the given dentry is unused, the whiteout
+ * inode is instantiated into it instead.
+ *
+ * After this returns with success, don't make any assumptions about the inode.
+ * Just dput() it dentry.
+ */
+static int vfs_whiteout(struct inode *dir, struct dentry *dentry, int isdir)
+{
+	int err;
+	struct inode *old_inode = dentry->d_inode;
+	struct dentry *parent, *whiteout;
+
+	err = may_whiteout(dir, dentry, isdir);
+	if (err)
+		return err;
+
+	BUG_ON(dentry->d_parent->d_inode != dir);
+
+	if (!dir->i_op || !dir->i_op->whiteout)
+		return -EOPNOTSUPP;
+
+	if (old_inode) {
+		vfs_dq_init(dir);
+
+		mutex_lock(&old_inode->i_mutex);
+		if (isdir)
+			dentry_unhash(dentry);
+		if (d_mountpoint(dentry))
+			err = -EBUSY;
+		else {
+			if (isdir)
+				err = security_inode_rmdir(dir, dentry);
+			else
+				err = security_inode_unlink(dir, dentry);
+		}
+	}
+
+	parent = dget_parent(dentry);
+	whiteout = d_alloc_name(parent, dentry->d_name.name);
+
+	if (!err)
+		err = dir->i_op->whiteout(dir, dentry, whiteout);
+
+	if (old_inode) {
+		mutex_unlock(&old_inode->i_mutex);
+		if (!err) {
+			fsnotify_link_count(old_inode);
+			d_delete(dentry);
+		}
+		if (isdir)
+			dput(dentry);
+	}
+
+	dput(whiteout);
+	dput(parent);
+	return err;
+}
+
+int path_whiteout(struct path *dir_path, struct dentry *dentry, int isdir)
+{
+	int error = mnt_want_write(dir_path->mnt);
+
+	if (!error) {
+		error = vfs_whiteout(dir_path->dentry->d_inode, dentry, isdir);
+		mnt_drop_write(dir_path->mnt);
+	}
+
+	return error;
+}
+EXPORT_SYMBOL(path_whiteout);
+
+/*
+ * This is abusing readdir to check if a union directory is logically empty.
+ * Al Viro barfed when he saw this, but Val said: "Well, at this point I'm
+ * aiming for working, pretty can come later"
+ */
+static int filldir_is_empty(void *__buf, const char *name, int namlen,
+			    loff_t offset, u64 ino, unsigned int d_type)
+{
+	int *is_empty = (int *)__buf;
+
+	switch (namlen) {
+	case 2:
+		if (name[1] != '.')
+			break;
+	case 1:
+		if (name[0] != '.')
+			break;
+		return 0;
+	}
+
+	if (d_type == DT_WHT)
+		return 0;
+
+	(*is_empty) = 0;
+	return 0;
+}
+
+static int directory_is_empty(struct dentry *dentry, struct vfsmount *mnt)
+{
+	struct file *file;
+	int err;
+	int is_empty = 1;
+
+	BUG_ON(!S_ISDIR(dentry->d_inode->i_mode));
+
+	/* references for the file pointer */
+	dget(dentry);
+	mntget(mnt);
+
+	file = dentry_open(dentry, mnt, O_RDONLY, current_cred());
+	if (IS_ERR(file))
+		return 0;
+
+	err = vfs_readdir(file, filldir_is_empty, &is_empty);
+
+	fput(file);
+	return is_empty;
+}
+
+static int do_whiteout(struct nameidata *nd, struct path *path, int isdir)
+{
+	struct path safe = { .dentry = dget(nd->path.dentry),
+			     .mnt = mntget(nd->path.mnt) };
+	struct dentry *dentry = path->dentry;
+	int err;
+
+	err = may_whiteout(nd->path.dentry->d_inode, dentry, isdir);
+	if (err)
+		goto out;
+
+	err = -ENOENT;
+	if (!dentry->d_inode)
+		goto out;
+
+	err = -ENOTEMPTY;
+	if (isdir && !directory_is_empty(path->dentry, path->mnt))
+		goto out;
+
+	if (nd->path.dentry != dentry->d_parent) {
+		dentry = __lookup_hash(&path->dentry->d_name, nd->path.dentry,
+				       nd);
+		err = PTR_ERR(dentry);
+		if (IS_ERR(dentry))
+			goto out;
+
+		dput(path->dentry);
+		if (path->mnt != safe.mnt)
+			mntput(path->mnt);
+		path->mnt = nd->path.mnt;
+		path->dentry = dentry;
+	}
+
+	err = vfs_whiteout(nd->path.dentry->d_inode, dentry, isdir);
+
+out:
+	path_put(&safe);
+	return err;
+}
+
 /*
  * We try to drop the dentry early: we should have
  * a usage count of 2 if we're the only user of this
@@ -2213,7 +3287,7 @@ static long do_rmdir(int dfd, const char
 {
 	int error = 0;
 	char * name;
-	struct dentry *dentry;
+	struct path path;
 	struct nameidata nd;
 
 	error = user_path_parent(dfd, pathname, &nd, &name);
@@ -2235,21 +3309,24 @@ static long do_rmdir(int dfd, const char
 	nd.flags &= ~LOOKUP_PARENT;
 
 	mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
-	dentry = lookup_hash(&nd);
-	error = PTR_ERR(dentry);
-	if (IS_ERR(dentry))
+	error = hash_lookup_union(&nd, &nd.last, &path);
+	if (error)
 		goto exit2;
+	if (is_unionized(nd.path.dentry, nd.path.mnt)) {
+		error = do_whiteout(&nd, &path, 1);
+		goto exit3;
+	}
 	error = mnt_want_write(nd.path.mnt);
 	if (error)
 		goto exit3;
-	error = security_path_rmdir(&nd.path, dentry);
+	error = security_path_rmdir(&nd.path, path.dentry);
 	if (error)
 		goto exit4;
-	error = vfs_rmdir(nd.path.dentry->d_inode, dentry);
+	error = vfs_rmdir(nd.path.dentry->d_inode, path.dentry);
 exit4:
 	mnt_drop_write(nd.path.mnt);
 exit3:
-	dput(dentry);
+	path_put_conditional(&path, &nd);
 exit2:
 	mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
 exit1:
@@ -2304,7 +3381,7 @@ static long do_unlinkat(int dfd, const c
 {
 	int error;
 	char *name;
-	struct dentry *dentry;
+	struct path path;
 	struct nameidata nd;
 	struct inode *inode = NULL;
 
@@ -2319,26 +3396,29 @@ static long do_unlinkat(int dfd, const c
 	nd.flags &= ~LOOKUP_PARENT;
 
 	mutex_lock_nested(&nd.path.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
-	dentry = lookup_hash(&nd);
-	error = PTR_ERR(dentry);
-	if (!IS_ERR(dentry)) {
+	error = hash_lookup_union(&nd, &nd.last, &path);
+	if (!error) {
 		/* Why not before? Because we want correct error value */
 		if (nd.last.name[nd.last.len])
 			goto slashes;
-		inode = dentry->d_inode;
+		inode = path.dentry->d_inode;
 		if (inode)
 			atomic_inc(&inode->i_count);
+		if (is_unionized(nd.path.dentry, nd.path.mnt)) {
+			error = do_whiteout(&nd, &path, 0);
+			goto exit2;
+		}
 		error = mnt_want_write(nd.path.mnt);
 		if (error)
 			goto exit2;
-		error = security_path_unlink(&nd.path, dentry);
+		error = security_path_unlink(&nd.path, path.dentry);
 		if (error)
 			goto exit3;
-		error = vfs_unlink(nd.path.dentry->d_inode, dentry);
+		error = vfs_unlink(nd.path.dentry->d_inode, path.dentry);
 exit3:
 		mnt_drop_write(nd.path.mnt);
 	exit2:
-		dput(dentry);
+		path_put_conditional(&path, &nd);
 	}
 	mutex_unlock(&nd.path.dentry->d_inode->i_mutex);
 	if (inode)
@@ -2349,8 +3429,8 @@ exit1:
 	return error;
 
 slashes:
-	error = !dentry->d_inode ? -ENOENT :
-		S_ISDIR(dentry->d_inode->i_mode) ? -EISDIR : -ENOTDIR;
+	error = !path.dentry->d_inode ? -ENOENT :
+		S_ISDIR(path.dentry->d_inode->i_mode) ? -EISDIR : -ENOTDIR;
 	goto exit2;
 }
 
@@ -2686,11 +3766,96 @@ int vfs_rename(struct inode *old_dir, st
 	return error;
 }
 
+static int vfs_rename_union(struct nameidata *oldnd, struct path *old,
+			    struct nameidata *newnd, struct path *new)
+{
+	struct inode *old_dir = oldnd->path.dentry->d_inode;
+	struct inode *new_dir = newnd->path.dentry->d_inode;
+	struct qstr old_name;
+	char *name;
+	struct dentry *dentry;
+	int error;
+
+	if (old->dentry->d_inode == new->dentry->d_inode)
+		return 0;
+	error = may_whiteout(old_dir, old->dentry, 0);
+	if (error)
+		return error;
+	if (!old_dir->i_op || !old_dir->i_op->whiteout)
+		return -EPERM;
+
+	if (!new->dentry->d_inode)
+		error = may_create(new_dir, new->dentry);
+	else
+		error = may_delete(new_dir, new->dentry, 0);
+	if (error)
+		return error;
+
+	vfs_dq_init(old_dir);
+	vfs_dq_init(new_dir);
+
+	error = -EBUSY;
+	if (d_mountpoint(old->dentry) || d_mountpoint(new->dentry))
+		return error;
+
+	error = -ENOMEM;
+	name = kmalloc(old->dentry->d_name.len, GFP_KERNEL);
+	if (!name)
+		return error;
+	strncpy(name, old->dentry->d_name.name, old->dentry->d_name.len);
+	name[old->dentry->d_name.len] = 0;
+	old_name.len = old->dentry->d_name.len;
+	old_name.hash = old->dentry->d_name.hash;
+	old_name.name = name;
+
+	/* possibly delete the existing new file */
+	if ((newnd->path.dentry == new->dentry->d_parent) &&
+	    new->dentry->d_inode) {
+		/* FIXME: inode may be truncated while we hold a lock */
+		error = vfs_unlink(new_dir, new->dentry);
+		if (error)
+			goto freename;
+
+		dentry = __lookup_hash(&new->dentry->d_name,
+				       newnd->path.dentry, newnd);
+		if (IS_ERR(dentry))
+			goto freename;
+
+		dput(new->dentry);
+		new->dentry = dentry;
+	}
+
+	/* copyup to the new file */
+	error = __union_copyup(old, newnd, new);
+	if (error)
+		goto freename;
+
+	/* whiteout the old file */
+	dentry = __lookup_hash(&old_name, oldnd->path.dentry, oldnd);
+	error = PTR_ERR(dentry);
+	if (IS_ERR(dentry))
+		goto freename;
+	error = vfs_whiteout(old_dir, dentry, 0);
+	dput(dentry);
+
+	/* FIXME: This is acutally unlink() && create() ... */
+/*
+  if (!error) {
+  const char *new_name = old_dentry->d_name.name;
+  fsnotify_move(old_dir, new_dir, old_name.name, new_name, 0,
+  new_dentry->d_inode, old_dentry->d_inode);
+  }
+*/
+freename:
+	kfree(old_name.name);
+	return error;
+}
+
 SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
 		int, newdfd, const char __user *, newname)
 {
 	struct dentry *old_dir, *new_dir;
-	struct dentry *old_dentry, *new_dentry;
+	struct path old, new;
 	struct dentry *trap;
 	struct nameidata oldnd, newnd;
 	char *from;
@@ -2724,16 +3889,28 @@ SYSCALL_DEFINE4(renameat, int, olddfd, c
 
 	trap = lock_rename(new_dir, old_dir);
 
-	old_dentry = lookup_hash(&oldnd);
-	error = PTR_ERR(old_dentry);
-	if (IS_ERR(old_dentry))
+	/*
+	 * For union mounts we need to call a giant lookup_rename_source()
+	 * instead.
+	 * First lock_rename() and look on the topmost fs like you would do in
+	 * the normal rename, if you find something which is not a directory,
+	 * go ahead and lookup target and do normal rename.
+	 * If you find a negative dentry, unlock_rename() and continue as
+	 * _hash_lookup_union() would do without locking the topmost parent
+	 * at the end. After that do lock_rename() of the source parent and the
+	 * target parent and do a copyup with additional whiteout creation at
+	 * the end.
+	 */
+//	error = hash_lookup_union(&oldnd, &oldnd.last, &old);
+	error = lookup_rename_source(&oldnd, &newnd, &trap, &oldnd.last, &old);
+	if (error)
 		goto exit3;
 	/* source must exist */
 	error = -ENOENT;
-	if (!old_dentry->d_inode)
+	if (!old.dentry->d_inode)
 		goto exit4;
 	/* unless the source is a directory trailing slashes give -ENOTDIR */
-	if (!S_ISDIR(old_dentry->d_inode->i_mode)) {
+	if (!S_ISDIR(old.dentry->d_inode->i_mode)) {
 		error = -ENOTDIR;
 		if (oldnd.last.name[oldnd.last.len])
 			goto exit4;
@@ -2742,32 +3919,44 @@ SYSCALL_DEFINE4(renameat, int, olddfd, c
 	}
 	/* source should not be ancestor of target */
 	error = -EINVAL;
-	if (old_dentry == trap)
+	if (old.dentry == trap)
 		goto exit4;
-	new_dentry = lookup_hash(&newnd);
-	error = PTR_ERR(new_dentry);
-	if (IS_ERR(new_dentry))
+	/* target is always on topmost fs, even with unions */
+	error = lookup_hash(&newnd, &newnd.last, &new);
+	if (error)
 		goto exit4;
 	/* target should not be an ancestor of source */
 	error = -ENOTEMPTY;
-	if (new_dentry == trap)
+	if (new.dentry == trap)
+		goto exit5;
+	/* renaming of directories on unions is done by the user-space */
+	error = -EXDEV;
+	if (is_unionized(oldnd.path.dentry, oldnd.path.mnt) &&
+	    S_ISDIR(old.dentry->d_inode->i_mode))
 		goto exit5;
+//	if (is_unionized(newnd.path.dentry, newnd.path.mnt))
+//		goto exit5;
 
 	error = mnt_want_write(oldnd.path.mnt);
 	if (error)
 		goto exit5;
-	error = security_path_rename(&oldnd.path, old_dentry,
-				     &newnd.path, new_dentry);
+	error = security_path_rename(&oldnd.path, old.dentry,
+				     &newnd.path, new.dentry);
 	if (error)
 		goto exit6;
-	error = vfs_rename(old_dir->d_inode, old_dentry,
-				   new_dir->d_inode, new_dentry);
+	if (is_unionized(oldnd.path.dentry, oldnd.path.mnt) &&
+	    (old.dentry->d_parent != oldnd.path.dentry)) {
+		error = vfs_rename_union(&oldnd, &old, &newnd, &new);
+		goto exit6;
+	}
+	error = vfs_rename(old_dir->d_inode, old.dentry,
+				   new_dir->d_inode, new.dentry);
 exit6:
 	mnt_drop_write(oldnd.path.mnt);
 exit5:
-	dput(new_dentry);
+	path_put_conditional(&new, &newnd);
 exit4:
-	dput(old_dentry);
+	path_put_conditional(&old, &oldnd);
 exit3:
 	unlock_rename(new_dir, old_dir);
 exit2:
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -29,6 +29,7 @@
 #include <linux/log2.h>
 #include <linux/idr.h>
 #include <linux/fs_struct.h>
+#include <linux/union.h>
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
 #include "pnode.h"
@@ -150,6 +151,9 @@ struct vfsmount *alloc_vfsmnt(const char
 		INIT_LIST_HEAD(&mnt->mnt_share);
 		INIT_LIST_HEAD(&mnt->mnt_slave_list);
 		INIT_LIST_HEAD(&mnt->mnt_slave);
+#ifdef CONFIG_UNION_MOUNT
+		INIT_LIST_HEAD(&mnt->mnt_unions);
+#endif
 #ifdef CONFIG_SMP
 		mnt->mnt_writers = alloc_percpu(int);
 		if (!mnt->mnt_writers)
@@ -469,6 +473,7 @@ static void __touch_mnt_namespace(struct
 
 static void detach_mnt(struct vfsmount *mnt, struct path *old_path)
 {
+	detach_mnt_union(mnt);
 	old_path->dentry = mnt->mnt_mountpoint;
 	old_path->mnt = mnt->mnt_parent;
 	mnt->mnt_parent = mnt;
@@ -492,6 +497,7 @@ static void attach_mnt(struct vfsmount *
 	list_add_tail(&mnt->mnt_hash, mount_hashtable +
 			hash(path->mnt, path->dentry));
 	list_add_tail(&mnt->mnt_child, &path->mnt->mnt_mounts);
+	attach_mnt_union(mnt, path->mnt, path->dentry);
 }
 
 /*
@@ -514,6 +520,7 @@ static void commit_tree(struct vfsmount 
 	list_add_tail(&mnt->mnt_hash, mount_hashtable +
 				hash(parent, mnt->mnt_mountpoint));
 	list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+	attach_mnt_union(mnt, mnt->mnt_parent, mnt->mnt_mountpoint);
 	touch_mnt_namespace(n);
 }
 
@@ -770,6 +777,7 @@ static void show_mnt_opts(struct seq_fil
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ MNT_RELATIME, ",relatime" },
 		{ MNT_STRICTATIME, ",strictatime" },
+		{ MNT_UNION, ",union" },
 		{ 0, NULL }
 	};
 	const struct proc_fs_info *fs_infop;
@@ -984,6 +992,7 @@ void release_mounts(struct list_head *he
 			struct dentry *dentry;
 			struct vfsmount *m;
 			spin_lock(&vfsmount_lock);
+			detach_mnt_union(mnt);
 			dentry = mnt->mnt_mountpoint;
 			m = mnt->mnt_parent;
 			mnt->mnt_mountpoint = mnt->mnt_root;
@@ -1102,6 +1111,11 @@ static int do_umount(struct vfsmount *mn
 	spin_unlock(&vfsmount_lock);
 	if (retval)
 		security_sb_umount_busy(mnt);
+	/* If this was a union mount, we are no longer a read-only
+	 * user on the underlying mount */
+	if (mnt->mnt_flags & MNT_UNION)
+		mnt->mnt_parent->mnt_sb->s_readonly_users--;
+
 	up_write(&namespace_sem);
 	release_mounts(&umount_list);
 	return retval;
@@ -1426,6 +1440,10 @@ static int do_change_type(struct path *p
 	if (path->dentry != path->mnt->mnt_root)
 		return -EINVAL;
 
+	/* Don't change the type of union mounts */
+	if (IS_MNT_UNION(path->mnt))
+		return -EINVAL;
+
 	down_write(&namespace_sem);
 	if (type == MS_SHARED) {
 		err = invent_group_ids(mnt, recurse);
@@ -1444,10 +1462,65 @@ static int do_change_type(struct path *p
 }
 
 /*
+ * Mount-time check of upper and lower layer file systems to see if we
+ * can union mount one on the other.
+ *
+ * Union mounts must follow these rules:
+ *
+ * - The lower layer must be read-only.  This avoids lots of nasty
+ *   unsolvable races where file system structures disappear suddenly.
+ *   XXX - Checking the vfsmnt for read-only is a temporary hack; the
+ *   file system could be mounted read-write elsewhere.  We need to
+ *   enforce read-only at the superblock level (patches coming).
+ *
+ * - The upper layer must be writable.  This isn't an absolute
+ *   requirement; right now we need it to make readdir() work since we
+ *   copy up directory entries to the top level.  A possible
+ *   workaround is to mount a tmpfs file system transparently over the
+ *   top.
+ *
+ * - The upper layer must support whiteouts and fallthrus (if it is
+ *   writeable).
+ *
+ * - The lower layer must not also be a union mount.  This is just to
+ *   make life simpler for now, there is no inherent limitation on the
+ *   number of layers.
+ *
+ * XXX - Check other mount flags for incompatibilities - I'm sure
+ * there are some.
+ */
+
+static int
+check_union_mnt(struct path *mntpnt, struct vfsmount *top_mnt, int mnt_flags)
+{
+	struct vfsmount *lower_mnt = mntpnt->mnt;
+
+	/* Is this even a union mount? */
+	if (!(mnt_flags & MNT_UNION))
+		return 0;
+
+	/* Lower layer must be read-only and not a union mount */
+	if (!(lower_mnt->mnt_sb->s_flags & MS_RDONLY) ||
+	    (lower_mnt->mnt_flags & MNT_UNION))
+		return -EBUSY;
+
+	/* Upper layer must be writable */
+	if (mnt_flags & MNT_READONLY)
+		return -EROFS;
+
+	/* Upper layer must support whiteouts and fallthrus */
+	if (!(top_mnt->mnt_sb->s_flags & MS_WHITEOUT))
+		return -EINVAL;
+
+	/* All good! */
+	return 0;
+}
+
+/*
  * do loopback mount.
  */
-static int do_loopback(struct path *path, char *old_name,
-				int recurse)
+static int do_loopback(struct path *path, char *old_name, int recurse,
+		       int mnt_flags)
 {
 	struct path old_path;
 	struct vfsmount *mnt = NULL;
@@ -1477,6 +1550,13 @@ static int do_loopback(struct path *path
 	if (!mnt)
 		goto out;
 
+	err = check_union_mnt(&old_path, mnt, mnt_flags);
+	if (err)
+		goto out;
+
+	if (mnt_flags & MNT_UNION)
+		mnt->mnt_flags |= MNT_UNION;
+
 	err = graft_tree(mnt, path);
 	if (err) {
 		LIST_HEAD(umount_list);
@@ -1486,6 +1566,10 @@ static int do_loopback(struct path *path
 		release_mounts(&umount_list);
 	}
 
+	/* If this is a union mount, add ourselves to the readonly users */
+	if (mnt_flags & MNT_UNION)
+		mnt->mnt_parent->mnt_sb->s_readonly_users++;
+
 out:
 	up_write(&namespace_sem);
 	path_put(&old_path);
@@ -1570,6 +1654,13 @@ static int do_move_mount(struct path *pa
 	if (err)
 		return err;
 
+	/* moving to or from a union mount is not supported */
+	err = -EINVAL;
+	if (IS_MNT_UNION(path->mnt))
+		goto exit;
+	if (IS_MNT_UNION(old_path.mnt))
+		goto exit;
+
 	down_write(&namespace_sem);
 	while (d_mountpoint(path->dentry) &&
 	       follow_down(path))
@@ -1627,6 +1718,7 @@ out:
 	up_write(&namespace_sem);
 	if (!err)
 		path_put(&parent_path);
+exit:
 	path_put(&old_path);
 	return err;
 }
@@ -1684,10 +1776,18 @@ int do_add_mount(struct vfsmount *newmnt
 	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
 		goto unlock;
 
+	err = check_union_mnt(path, newmnt, mnt_flags);
+	if (err)
+		goto unlock;
+
 	newmnt->mnt_flags = mnt_flags;
 	if ((err = graft_tree(newmnt, path)))
 		goto unlock;
 
+	/* If this is a union mount, add ourselves to the readonly users */
+	if (mnt_flags & MNT_UNION)
+		newmnt->mnt_parent->mnt_sb->s_readonly_users++;
+
 	if (fslist) /* add to the specified expiration list */
 		list_add_tail(&newmnt->mnt_expire, fslist);
 
@@ -1940,10 +2040,12 @@ long do_mount(char *dev_name, char *dir_
 		mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
 	if (flags & MS_RDONLY)
 		mnt_flags |= MNT_READONLY;
+	if (flags & MS_UNION)
+		mnt_flags |= MNT_UNION;
 
 	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
 		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
-		   MS_STRICTATIME);
+		   MS_STRICTATIME | MS_UNION);
 
 	/* ... and get the mountpoint */
 	retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);
@@ -1959,7 +2061,8 @@ long do_mount(char *dev_name, char *dir_
 		retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
 				    data_page);
 	else if (flags & MS_BIND)
-		retval = do_loopback(&path, dev_name, flags & MS_REC);
+		retval = do_loopback(&path, dev_name, flags & MS_REC,
+				     mnt_flags);
 	else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
 		retval = do_change_type(&path, flags);
 	else if (flags & MS_MOVE)
@@ -2196,6 +2299,8 @@ SYSCALL_DEFINE2(pivot_root, const char _
 	if (d_unlinked(old.dentry))
 		goto out2;
 	error = -EBUSY;
+	follow_union_down(&new);
+	follow_union_down(&root);
 	if (new.mnt == root.mnt ||
 	    old.mnt == root.mnt)
 		goto out2; /* loop, on the same file system  */
--- a/fs/nfsctl.c
+++ b/fs/nfsctl.c
@@ -38,10 +38,10 @@ static struct file *do_open(char *name, 
 		return ERR_PTR(error);
 
 	if (flags == O_RDWR)
-		error = may_open(&nd.path, MAY_READ|MAY_WRITE,
-					   FMODE_READ|FMODE_WRITE);
+		error = may_open(&nd, MAY_READ|MAY_WRITE,
+				 FMODE_READ|FMODE_WRITE);
 	else
-		error = may_open(&nd.path, MAY_WRITE, FMODE_WRITE);
+		error = may_open(&nd, MAY_WRITE, FMODE_WRITE);
 
 	if (!error)
 		return dentry_open(nd.path.dentry, nd.path.mnt, flags,
--- a/fs/nfsd/nfs3xdr.c
+++ b/fs/nfsd/nfs3xdr.c
@@ -898,6 +898,11 @@ encode_entry(struct readdir_cd *ccd, con
 	int		elen;		/* estimated entry length in words */
 	int		num_entry_words = 0;	/* actual number of words */
 
+	if (d_type == DT_WHT) {
+		cd->common.err = nfs_ok;
+		return 0;
+	}
+
 	if (cd->offset) {
 		u64 offset64 = offset;
 
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -2261,7 +2261,7 @@ nfsd4_encode_dirent(void *ccdv, const ch
 	__be32 nfserr = nfserr_toosmall;
 
 	/* In nfsv4, "." and ".." never make it onto the wire.. */
-	if (name && isdotent(name, namlen)) {
+	if (d_type == DT_WHT || (name && isdotent(name, namlen))) {
 		cd->common.err = nfs_ok;
 		return 0;
 	}
--- a/fs/nfsd/nfsxdr.c
+++ b/fs/nfsd/nfsxdr.c
@@ -513,6 +513,10 @@ nfssvc_encode_entry(void *ccdv, const ch
 			namlen, name, offset, ino);
 	 */
 
+	if (d_type == DT_WHT) {
+		cd->common.err = nfs_ok;
+		return 0;
+	}
 	if (offset > ~((u32) 0)) {
 		cd->common.err = nfserr_fbig;
 		return -EINVAL;
--- a/fs/open.c
+++ b/fs/open.c
@@ -30,6 +30,7 @@
 #include <linux/audit.h>
 #include <linux/falloc.h>
 #include <linux/fs_struct.h>
+#include <linux/union.h>
 
 int vfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
@@ -224,69 +225,69 @@ int do_truncate(struct dentry *dentry, l
 	return ret;
 }
 
-static long do_sys_truncate(const char __user *pathname, loff_t length)
+static int __do_ftruncate(struct file *file, unsigned long length, int small)
 {
-	struct path path;
-	struct inode *inode;
+	struct inode * inode;
+	struct dentry *dentry;
 	int error;
 
 	error = -EINVAL;
-	if (length < 0)	/* sorry, but loff_t says... */
+	if (length < 0)
 		goto out;
+	/* explicitly opened as large or we are on 64-bit box */
+	if (file->f_flags & O_LARGEFILE)
+		small = 0;
 
-	error = user_path(pathname, &path);
-	if (error)
+	dentry = file->f_path.dentry;
+	inode = dentry->d_inode;
+	error = -EINVAL;
+	if (!S_ISREG(inode->i_mode) || !(file->f_mode & FMODE_WRITE))
 		goto out;
-	inode = path.dentry->d_inode;
-
-	/* For directories it's -EISDIR, for other non-regulars - -EINVAL */
-	error = -EISDIR;
-	if (S_ISDIR(inode->i_mode))
-		goto dput_and_out;
 
 	error = -EINVAL;
-	if (!S_ISREG(inode->i_mode))
-		goto dput_and_out;
-
-	error = mnt_want_write(path.mnt);
-	if (error)
-		goto dput_and_out;
+	/* Cannot ftruncate over 2^31 bytes without large file support */
+	if (small && length > MAX_NON_LFS)
 
-	error = inode_permission(inode, MAY_WRITE);
-	if (error)
-		goto mnt_drop_write_and_out;
+		goto out;
 
 	error = -EPERM;
 	if (IS_APPEND(inode))
-		goto mnt_drop_write_and_out;
+		goto out;
 
-	error = get_write_access(inode);
-	if (error)
-		goto mnt_drop_write_and_out;
+	error = locks_verify_truncate(inode, file, length);
+	if (!error)
+		error = security_path_truncate(&file->f_path, length,
+					       ATTR_MTIME|ATTR_CTIME);
+	if (!error)
+		/* Already copied up for union, opened with write */
+		error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, file);
+out:
+	return error;
+}
 
-	/*
-	 * Make sure that there are no leases.  get_write_access() protects
-	 * against the truncate racing with a lease-granting setlease().
-	 */
-	error = break_lease(inode, FMODE_WRITE);
-	if (error)
-		goto put_write_and_out;
+static long do_sys_truncate(const char __user *pathname, loff_t length)
+{
+	struct file *file;
+	char *tmp;
+	int error;
 
-	error = locks_verify_truncate(inode, NULL, length);
-	if (!error)
-		error = security_path_truncate(&path, length, 0);
-	if (!error) {
-		vfs_dq_init(inode);
-		error = do_truncate(path.dentry, length, 0, NULL);
-	}
+	error = -EINVAL;
+	if (length < 0)	/* sorry, but loff_t says... */
+		return error;
 
-put_write_and_out:
-	put_write_access(inode);
-mnt_drop_write_and_out:
-	mnt_drop_write(path.mnt);
-dput_and_out:
-	path_put(&path);
-out:
+	tmp = getname(pathname);
+	if (IS_ERR(tmp))
+		return PTR_ERR(tmp);
+
+	file = filp_open(tmp, O_RDWR | O_LARGEFILE, 0);
+	putname(tmp);
+
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	error = __do_ftruncate(file, length, 0);
+
+	fput(file);
 	return error;
 }
 
@@ -297,45 +298,16 @@ SYSCALL_DEFINE2(truncate, const char __u
 
 static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
 {
-	struct inode * inode;
-	struct dentry *dentry;
 	struct file * file;
 	int error;
 
-	error = -EINVAL;
-	if (length < 0)
-		goto out;
 	error = -EBADF;
 	file = fget(fd);
 	if (!file)
 		goto out;
 
-	/* explicitly opened as large or we are on 64-bit box */
-	if (file->f_flags & O_LARGEFILE)
-		small = 0;
-
-	dentry = file->f_path.dentry;
-	inode = dentry->d_inode;
-	error = -EINVAL;
-	if (!S_ISREG(inode->i_mode) || !(file->f_mode & FMODE_WRITE))
-		goto out_putf;
-
-	error = -EINVAL;
-	/* Cannot ftruncate over 2^31 bytes without large file support */
-	if (small && length > MAX_NON_LFS)
-		goto out_putf;
+	error = __do_ftruncate(file, length, small);
 
-	error = -EPERM;
-	if (IS_APPEND(inode))
-		goto out_putf;
-
-	error = locks_verify_truncate(inode, file, length);
-	if (!error)
-		error = security_path_truncate(&file->f_path, length,
-					       ATTR_MTIME|ATTR_CTIME);
-	if (!error)
-		error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, file);
-out_putf:
 	fput(file);
 out:
 	return error;
@@ -494,7 +466,8 @@ SYSCALL_DEFINE3(faccessat, int, dfd, con
 			goto out_path_release;
 	}
 
-	res = inode_permission(inode, mode | MAY_ACCESS);
+	res = union_permission(&path, mode | MAY_ACCESS);
+
 	/* SuS v2 requires we report a read only fs too */
 	if (res || !(mode & S_IWOTH) || special_file(inode->i_mode))
 		goto out_path_release;
@@ -508,7 +481,8 @@ SYSCALL_DEFINE3(faccessat, int, dfd, con
 	 * inherently racy and know that the fs may change
 	 * state before we even see this result.
 	 */
-	if (__mnt_is_readonly(path.mnt))
+	if ((!is_unionized(path.dentry, path.mnt) &&
+	     (__mnt_is_readonly(path.mnt))))
 		res = -EROFS;
 
 out_path_release:
@@ -554,20 +528,19 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd
 	error = -EBADF;
 	file = fget(fd);
 	if (!file)
-		goto out;
+		return error;
 
 	inode = file->f_path.dentry->d_inode;
 
 	error = -ENOTDIR;
 	if (!S_ISDIR(inode->i_mode))
-		goto out_putf;
+		goto out;
 
 	error = inode_permission(inode, MAY_EXEC | MAY_ACCESS);
 	if (!error)
 		set_fs_pwd(current->fs, &file->f_path);
-out_putf:
-	fput(file);
 out:
+	fput(file);
 	return error;
 }
 
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -16,6 +16,7 @@
 #include <linux/security.h>
 #include <linux/syscalls.h>
 #include <linux/unistd.h>
+#include <linux/union.h>
 
 #include <asm/uaccess.h>
 
@@ -36,9 +37,24 @@ int vfs_readdir(struct file *file, filld
 
 	res = -ENOENT;
 	if (!IS_DEADDIR(inode)) {
+		/*
+		 * XXX Think harder about locking for
+		 * union_copyup_dir.  Currently we lock the topmost
+		 * directory and hold that lock while sequentially
+		 * acquiring and dropping locks for the directories
+		 * below this one in the union stack.
+		 */
+		if (is_unionized(file->f_path.dentry, file->f_path.mnt) &&
+		    !IS_OPAQUE(inode)) {
+			res = union_copyup_dir(&file->f_path);
+			if (res)
+				goto out_unlock;
+		}
+
 		res = file->f_op->readdir(file, buf, filler);
 		file_accessed(file);
 	}
+out_unlock:
 	mutex_unlock(&inode->i_mutex);
 out:
 	return res;
@@ -77,6 +93,9 @@ static int fillonedir(void * __buf, cons
 	struct old_linux_dirent __user * dirent;
 	unsigned long d_ino;
 
+	if (d_type == DT_WHT)
+		return 0;
+
 	if (buf->result)
 		return -EINVAL;
 	d_ino = ino;
@@ -154,6 +173,9 @@ static int filldir(void * __buf, const c
 	unsigned long d_ino;
 	int reclen = ALIGN(NAME_OFFSET(dirent) + namlen + 2, sizeof(long));
 
+	if (d_type == DT_WHT)
+		return 0;
+
 	buf->error = -EINVAL;	/* only used if we fail.. */
 	if (reclen > buf->count)
 		return -EINVAL;
@@ -239,6 +261,9 @@ static int filldir64(void * __buf, const
 	struct getdents_callback64 * buf = (struct getdents_callback64 *) __buf;
 	int reclen = ALIGN(NAME_OFFSET(dirent) + namlen + 1, sizeof(u64));
 
+	if (d_type == DT_WHT)
+		return 0;
+
 	buf->error = -EINVAL;	/* only used if we fail.. */
 	if (reclen > buf->count)
 		return -EINVAL;
--- a/fs/super.c
+++ b/fs/super.c
@@ -596,6 +596,15 @@ int do_remount_sb(struct super_block *sb
 	}
 	remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
 
+	/* If we are remounting read/write, make sure that none of the
+	   users require read-only for correct operation (such as
+	   union mounts). */
+	if (remount_rw && sb->s_readonly_users) {
+		printk(KERN_INFO "%s: In use by %d read-only user(s)\n",
+		       sb->s_id, sb->s_readonly_users);
+		return -EROFS;
+	}
+
 	if (sb->s_op->remount_fs) {
 		retval = sb->s_op->remount_fs(sb, &flags, data);
 		if (retval)
@@ -953,6 +962,11 @@ vfs_kern_mount(struct file_system_type *
 	WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
 		"negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);
 
+	error = -EROFS;
+	if (!(flags & MS_RDONLY) &&
+	    (mnt->mnt_sb->s_readonly_users))
+		goto out_sb;
+
 	mnt->mnt_mountpoint = mnt->mnt_root;
 	mnt->mnt_parent = mnt;
 	up_write(&mnt->mnt_sb->s_umount);
--- /dev/null
+++ b/fs/union.c
@@ -0,0 +1,981 @@
+/*
+ * VFS based union mount for Linux
+ *
+ * Copyright (C) 2004-2007 IBM Corporation, IBM Deutschland Entwicklung GmbH.
+ * Copyright (C) 2007-2009 Novell Inc.
+ *
+ *   Author(s): Jan Blunck (j.blunck@tu-harburg.de)
+ *              Valerie Aurora <vaurora@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#include <linux/bootmem.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/hash.h>
+#include <linux/fs.h>
+#include <linux/mount.h>
+#include <linux/fs_struct.h>
+#include <linux/union.h>
+#include <linux/namei.h>
+#include <linux/file.h>
+#include <linux/mm.h>
+#include <linux/quotaops.h>
+#include <linux/dnotify.h>
+#include <linux/security.h>
+#include <linux/pipe_fs_i.h>
+#include <linux/splice.h>
+
+/*
+ * This is borrowed from fs/inode.c. The hashtable for lookups. Somebody
+ * should try to make this good - I've just made it work.
+ */
+static unsigned int union_hash_mask __read_mostly;
+static unsigned int union_hash_shift __read_mostly;
+static struct hlist_head *union_hashtable __read_mostly;
+static unsigned int union_rhash_mask __read_mostly;
+static unsigned int union_rhash_shift __read_mostly;
+static struct hlist_head *union_rhashtable __read_mostly;
+
+/*
+ * Locking Rules:
+ * - dcache_lock (for union_rlookup() only)
+ * - union_lock
+ */
+DEFINE_SPINLOCK(union_lock);
+
+static struct kmem_cache *union_cache __read_mostly;
+
+static unsigned long hash(struct dentry *dentry, struct vfsmount *mnt)
+{
+	unsigned long tmp;
+
+	tmp = ((unsigned long)mnt * (unsigned long)dentry) ^
+		(GOLDEN_RATIO_PRIME + (unsigned long)mnt) / L1_CACHE_BYTES;
+	tmp = tmp ^ ((tmp ^ GOLDEN_RATIO_PRIME) >> union_hash_shift);
+	return tmp & union_hash_mask;
+}
+
+static __initdata unsigned long union_hash_entries;
+
+static int __init set_union_hash_entries(char *str)
+{
+	if (!str)
+		return 0;
+	union_hash_entries = simple_strtoul(str, &str, 0);
+	return 1;
+}
+
+__setup("union_hash_entries=", set_union_hash_entries);
+
+static int __init init_union(void)
+{
+	int loop;
+
+	union_cache = KMEM_CACHE(union_mount, SLAB_PANIC | SLAB_MEM_SPREAD);
+	union_hashtable = alloc_large_system_hash("Union-cache",
+						  sizeof(struct hlist_head),
+						  union_hash_entries,
+						  14,
+						  0,
+						  &union_hash_shift,
+						  &union_hash_mask,
+						  0);
+
+	for (loop = 0; loop < (1 << union_hash_shift); loop++)
+		INIT_HLIST_HEAD(&union_hashtable[loop]);
+
+
+	union_rhashtable = alloc_large_system_hash("rUnion-cache",
+						  sizeof(struct hlist_head),
+						  union_hash_entries,
+						  14,
+						  0,
+						  &union_rhash_shift,
+						  &union_rhash_mask,
+						  0);
+
+	for (loop = 0; loop < (1 << union_rhash_shift); loop++)
+		INIT_HLIST_HEAD(&union_rhashtable[loop]);
+
+	return 0;
+}
+
+fs_initcall(init_union);
+
+struct union_mount *union_alloc(struct dentry *this, struct vfsmount *this_mnt,
+				struct dentry *next, struct vfsmount *next_mnt)
+{
+	struct union_mount *um;
+
+	BUG_ON(!S_ISDIR(this->d_inode->i_mode));
+	BUG_ON(!S_ISDIR(next->d_inode->i_mode));
+
+	um = kmem_cache_alloc(union_cache, GFP_ATOMIC);
+	if (!um)
+		return NULL;
+
+	atomic_set(&um->u_count, 1);
+	INIT_LIST_HEAD(&um->u_unions);
+	INIT_LIST_HEAD(&um->u_list);
+	INIT_HLIST_NODE(&um->u_hash);
+	INIT_HLIST_NODE(&um->u_rhash);
+
+	um->u_this.mnt = this_mnt;
+	um->u_this.dentry = this;
+	um->u_next.mnt = mntget(next_mnt);
+	um->u_next.dentry = dget(next);
+
+	return um;
+}
+
+struct union_mount *union_get(struct union_mount *um)
+{
+	BUG_ON(!atomic_read(&um->u_count));
+	atomic_inc(&um->u_count);
+	return um;
+}
+
+static int __union_put(struct union_mount *um)
+{
+	if (!atomic_dec_and_test(&um->u_count))
+		return 0;
+
+	BUG_ON(!hlist_unhashed(&um->u_hash));
+	BUG_ON(!hlist_unhashed(&um->u_rhash));
+
+	kmem_cache_free(union_cache, um);
+	return 1;
+}
+
+void union_put(struct union_mount *um)
+{
+	struct path tmp = um->u_next;
+
+	if (__union_put(um))
+		path_put(&tmp);
+}
+
+static void __union_hash(struct union_mount *um)
+{
+	hlist_add_head(&um->u_hash, union_hashtable +
+		       hash(um->u_this.dentry, um->u_this.mnt));
+	hlist_add_head(&um->u_rhash, union_rhashtable +
+		       hash(um->u_next.dentry, um->u_next.mnt));
+}
+
+static void __union_unhash(struct union_mount *um)
+{
+	hlist_del_init(&um->u_hash);
+	hlist_del_init(&um->u_rhash);
+}
+
+struct union_mount *union_lookup(struct dentry *dentry, struct vfsmount *mnt)
+{
+	struct hlist_head *head = union_hashtable + hash(dentry, mnt);
+	struct hlist_node *node;
+	struct union_mount *um;
+
+	hlist_for_each_entry(um, node, head, u_hash) {
+		if ((um->u_this.dentry == dentry) &&
+		    (um->u_this.mnt == mnt))
+			return um;
+	}
+
+	return NULL;
+}
+
+struct union_mount *union_rlookup(struct dentry *dentry, struct vfsmount *mnt)
+{
+	struct hlist_head *head = union_rhashtable + hash(dentry, mnt);
+	struct hlist_node *node;
+	struct union_mount *um;
+
+	hlist_for_each_entry(um, node, head, u_rhash) {
+		if ((um->u_next.dentry == dentry) &&
+		    (um->u_next.mnt == mnt))
+			return um;
+	}
+
+	return NULL;
+}
+
+/*
+ * is_unionized - check if a dentry lives on a union mounted file system
+ *
+ * This tests if a dentry is living on an union mounted file system by walking
+ * the file system hierarchy.
+ */
+int is_unionized(struct dentry *dentry, struct vfsmount *mnt)
+{
+	struct path this = { .mnt = mntget(mnt),
+			     .dentry = dget(dentry) };
+	struct vfsmount *tmp;
+
+	do {
+		/* check if there is an union mounted on top of us */
+		spin_lock(&vfsmount_lock);
+		list_for_each_entry(tmp, &this.mnt->mnt_mounts, mnt_child) {
+			if (!(tmp->mnt_flags & MNT_UNION))
+				continue;
+			/* Isn't this a bug? */
+			if (this.dentry->d_sb != tmp->mnt_mountpoint->d_sb)
+				continue;
+			if (is_subdir(this.dentry, tmp->mnt_mountpoint)) {
+				spin_unlock(&vfsmount_lock);
+				path_put(&this);
+				return 1;
+			}
+		}
+		spin_unlock(&vfsmount_lock);
+
+		/* check our mountpoint next */
+		tmp = mntget(this.mnt->mnt_parent);
+		dput(this.dentry);
+		this.dentry = dget(this.mnt->mnt_mountpoint);
+		mntput(this.mnt);
+		this.mnt = tmp;
+	} while (this.mnt != this.mnt->mnt_parent);
+
+	path_put(&this);
+	return 0;
+}
+
+int append_to_union(struct vfsmount *mnt, struct dentry *dentry,
+		    struct vfsmount *dest_mnt, struct dentry *dest_dentry)
+{
+	struct union_mount *this, *um;
+
+	BUG_ON(!IS_MNT_UNION(mnt));
+
+	this = union_alloc(dentry, mnt, dest_dentry, dest_mnt);
+	if (!this)
+		return -ENOMEM;
+
+	spin_lock(&union_lock);
+	um = union_lookup(dentry, mnt);
+	if (um) {
+		BUG_ON((um->u_next.dentry != dest_dentry) ||
+		       (um->u_next.mnt != dest_mnt));
+		spin_unlock(&union_lock);
+		union_put(this);
+		return 0;
+	}
+	list_add(&this->u_list, &mnt->mnt_unions);
+	list_add(&this->u_unions, &dentry->d_unions);
+	dest_dentry->d_unionized++;
+	__union_hash(this);
+	spin_unlock(&union_lock);
+	return 0;
+}
+
+/*
+ * follow_union_down - follow the union stack one layer down
+ *
+ * This is called to traverse the union stack from one layer to the next
+ * overlayed one. follow_union_down() is called by various lookup functions
+ * that are aware of union mounts.
+ *
+ * Returns non-zero if followed to the next layer, zero otherwise.
+ */
+int follow_union_down(struct path *path)
+{
+	struct union_mount *um;
+
+	if (!IS_MNT_UNION(path->mnt))
+		return 0;
+
+	spin_lock(&union_lock);
+	um = union_lookup(path->dentry, path->mnt);
+	spin_unlock(&union_lock);
+	if (um) {
+		path_get(&um->u_next);
+		dput(path->dentry);
+		path->dentry = um->u_next.dentry;
+		mntput(path->mnt);
+		path->mnt = um->u_next.mnt;
+		return 1;
+	}
+	return 0;
+}
+
+/*
+ * follow_union_mount - follow the union stack to the topmost layer
+ *
+ * This is called to traverse the union stack to the topmost layer. This is
+ * necessary for following parent pointers in an union mount.
+ *
+ * Returns none zero if followed to the topmost layer, zero otherwise.
+ */
+int follow_union_mount(struct path *path)
+{
+	struct union_mount *um;
+	int res = 0;
+
+	while (IS_UNION(path->dentry)) {
+		spin_lock(&dcache_lock);
+		spin_lock(&union_lock);
+		um = union_rlookup(path->dentry, path->mnt);
+		if (um)
+			path_get(&um->u_this);
+		spin_unlock(&union_lock);
+		spin_unlock(&dcache_lock);
+
+		/*
+		 * Q: Aaargh, how do I validate the topmost dentry pointer?
+		 * A: Eeeeasy! We took the dcache_lock and union_lock. Since
+		 *    this protects from any dput'ng going on, we know that the
+		 *    dentry is valid since the union is unhashed under
+		 *    dcache_lock too.
+		 */
+		if (!um)
+			break;
+		dput(path->dentry);
+		path->dentry = um->u_this.dentry;
+		mntput(path->mnt);
+		path->mnt = um->u_this.mnt;
+		res = 1;
+	}
+
+	return res;
+}
+
+/*
+ * Union mount copyup support
+ */
+
+extern int hash_lookup_union(struct nameidata *, struct qstr *, struct path *);
+extern void follow_mount(struct path *);
+
+/*
+ * union_relookup_topmost - lookup and create the topmost path to dentry
+ * @nd: pointer to nameidata
+ * @flags: lookup flags
+ */
+static int union_relookup_topmost(struct nameidata *nd, int flags)
+{
+	int err;
+	char *kbuf, *name;
+	struct nameidata this;
+
+	kbuf = (char *)__get_free_page(GFP_KERNEL);
+	if (!kbuf)
+		return -ENOMEM;
+
+	name = d_path(&nd->path, kbuf, PAGE_SIZE);
+	err = PTR_ERR(name);
+	if (IS_ERR(name))
+		goto free_page;
+
+	err = path_lookup(name, flags|LOOKUP_CREATE|LOOKUP_TOPMOST, &this);
+	if (err)
+		goto free_page;
+
+	path_put(&nd->path);
+	nd->path.dentry = this.path.dentry;
+	nd->path.mnt = this.path.mnt;
+
+	/*
+	 * the nd->flags should be unchanged
+	 */
+	BUG_ON(this.um_flags & LAST_LOWLEVEL);
+	nd->um_flags &= ~LAST_LOWLEVEL;
+ free_page:
+	free_page((unsigned long)kbuf);
+	return err;
+}
+
+static void __update_fs_pwd(struct path *path, struct dentry *dentry,
+			    struct vfsmount *mnt)
+{
+	struct path old = { NULL, NULL };
+
+	write_lock(&current->fs->lock);
+	if (current->fs->pwd.dentry == path->dentry) {
+		old = current->fs->pwd;
+		path_get(&current->fs->pwd);
+	}
+	write_unlock(&current->fs->lock);
+
+	if (old.dentry)
+		path_put(&old);
+
+	return;
+}
+
+/**
+ * union_permission  -  check for access rights to a given inode
+ * @inode:	inode to check permission on
+ * @mask:	right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
+ *
+ * In a union mount, the top layer is always read-write and the bottom
+ * is always read-only.  Ignore the read-only flag on the lower fs.
+ *
+ * Only need for certain activities, like checking to see if write
+ * access is ok.
+ */
+
+int union_permission(struct path *path, int mask)
+{
+	struct inode *inode = path->dentry->d_inode;
+
+	if (!is_unionized(path->dentry, path->mnt))
+		return inode_permission(inode, mask);
+
+	/* Tell __inode_permission to ignore MS_RDONLY */
+	return __inode_permission(inode, mask, 0);
+}
+
+/*
+ * union_create_topmost - create the topmost path component
+ * @nd: pointer to nameidata of the base directory
+ * @name: pointer to file name
+ * @path: pointer to path of the overlaid file
+ *
+ * This is called by __link_path_walk() to create the directories on a path
+ * when it is called with LOOKUP_TOPMOST.
+ */
+struct dentry *union_create_topmost(struct nameidata *nd, struct qstr *name,
+				    struct path *path)
+{
+	struct dentry *dentry, *parent = nd->path.dentry;
+	int res, mode = path->dentry->d_inode->i_mode;
+
+	if (parent->d_sb == path->dentry->d_sb)
+		return ERR_PTR(-EEXIST);
+
+	mutex_lock(&parent->d_inode->i_mutex);
+	dentry = lookup_one_len(name->name, nd->path.dentry, name->len);
+	if (IS_ERR(dentry))
+		goto out_unlock;
+
+	switch (mode & S_IFMT) {
+	case S_IFREG:
+		/*
+		 * FIXME: Does this make any sense in this case?
+		 * Special case - lookup gave negative, but... we had foo/bar/
+		 * From the vfs_mknod() POV we just have a negative dentry -
+		 * all is fine. Let's be bastards - you had / on the end,you've
+		 * been asking for (non-existent) directory. -ENOENT for you.
+		 */
+		if (name->name[name->len] && !dentry->d_inode) {
+			dput(dentry);
+			dentry = ERR_PTR(-ENOENT);
+			goto out_unlock;
+		}
+
+		res = vfs_create(parent->d_inode, dentry, mode, nd);
+		if (res) {
+			dput(dentry);
+			dentry = ERR_PTR(res);
+			goto out_unlock;
+		}
+		break;
+	case S_IFDIR:
+		res = vfs_mkdir(parent->d_inode, dentry, mode);
+		if (res) {
+			dput(dentry);
+			dentry = ERR_PTR(res);
+			goto out_unlock;
+		}
+
+		res = append_to_union(nd->path.mnt, dentry, path->mnt,
+				      path->dentry);
+		if (res) {
+			dput(dentry);
+			dentry = ERR_PTR(res);
+			goto out_unlock;
+		}
+		break;
+	default:
+		dput(dentry);
+		dentry = ERR_PTR(-EINVAL);
+		goto out_unlock;
+	}
+
+	/* FIXME: Really necessary ??? */
+/*	__update_fs_pwd(path, dentry, nd->path.mnt); */
+
+ out_unlock:
+	mutex_unlock(&parent->d_inode->i_mutex);
+	return dentry;
+}
+
+static int union_copy_file(struct dentry *old_dentry, struct vfsmount *old_mnt,
+			   struct dentry *new_dentry, struct vfsmount *new_mnt)
+{
+	int ret;
+	size_t size;
+	loff_t offset;
+	struct file *old_file, *new_file;
+	const struct cred *cred = current_cred();
+
+	dget(old_dentry);
+	mntget(old_mnt);
+	old_file = dentry_open(old_dentry, old_mnt, O_RDONLY, cred);
+	if (IS_ERR(old_file))
+		return PTR_ERR(old_file);
+
+	dget(new_dentry);
+	mntget(new_mnt);
+	new_file = dentry_open(new_dentry, new_mnt, O_WRONLY, cred);
+	ret = PTR_ERR(new_file);
+	if (IS_ERR(new_file))
+		goto fput_old;
+
+	/* XXX be smart by using a length param, which indicates max
+	 * data we'll want (e.g., we are about to truncate to 0 or 10
+	 * bytes or something */
+	size = i_size_read(old_file->f_path.dentry->d_inode);
+	if (((size_t)size != size) || ((ssize_t)size != size)) {
+		ret = -EFBIG;
+		goto fput_new;
+	}
+
+	offset = 0;
+	ret = do_splice_direct(old_file, &offset, new_file, size,
+			       SPLICE_F_MOVE);
+	if (ret >= 0)
+		ret = 0;
+ fput_new:
+	fput(new_file);
+ fput_old:
+	fput(old_file);
+	return ret;
+}
+
+/**
+ * __union_copyup - copy a file to the topmost directory
+ * @old: pointer to path of the old file name
+ * @new_nd: pointer to nameidata of the topmost directory
+ * @new: pointer to path of the new file name
+ *
+ * The topmost directory @new_nd must already be locked. Creates the topmost
+ * file if it doesn't exist yet.
+ */
+int __union_copyup(struct path *old, struct nameidata *new_nd,
+		   struct path *new)
+{
+	struct dentry *dentry;
+	int error;
+
+	/* Maybe this should be -EINVAL */
+	if (S_ISDIR(old->dentry->d_inode->i_mode))
+		return -EISDIR;
+
+	if (new_nd->path.dentry != new->dentry->d_parent) {
+		mutex_lock(&new_nd->path.dentry->d_inode->i_mutex);
+		dentry = lookup_one_len(new->dentry->d_name.name,
+					new_nd->path.dentry,
+					new->dentry->d_name.len);
+		mutex_unlock(&new_nd->path.dentry->d_inode->i_mutex);
+		if (IS_ERR(dentry))
+			return PTR_ERR(dentry);
+		error = -EEXIST;
+		if (dentry->d_inode)
+			goto out_dput;
+	} else
+		dentry = dget(new->dentry);
+
+	if (!dentry->d_inode) {
+		error = vfs_create(new_nd->path.dentry->d_inode, dentry,
+				   old->dentry->d_inode->i_mode, new_nd);
+		if (error)
+			goto out_dput;
+	}
+
+	BUG_ON(!S_ISREG(old->dentry->d_inode->i_mode));
+	error = union_copy_file(old->dentry, old->mnt, dentry,
+				new_nd->path.mnt);
+	if (error) {
+		/* FIXME: are there return value we should not
+		 * BUG() on ? */
+		BUG_ON(vfs_unlink(new_nd->path.dentry->d_inode,
+				  dentry));
+		goto out_dput;
+	}
+
+	dput(new->dentry);
+	new->dentry = dentry;
+	if (new->mnt != new_nd->path.mnt)
+		mntput(new->mnt);
+	new->mnt = new_nd->path.mnt;
+	return error;
+
+out_dput:
+	dput(dentry);
+	return error;
+}
+
+/*
+ * union_copyup - copy a file to the topmost layer of the union stack
+ * @nd: nameidata pointer to the file
+ * @flags: flags given to open_namei
+ */
+int union_copyup(struct nameidata *nd, int flags /* XXX not used */)
+{
+	struct qstr this;
+	char *name;
+	struct dentry *dir;
+	struct path path;
+	int err;
+
+	if (!is_unionized(nd->path.dentry, nd->path.mnt))
+		return 0;
+	if (!S_ISREG(nd->path.dentry->d_inode->i_mode))
+		return 0;
+
+	/* safe the name for hash_lookup_union() */
+	this.len = nd->path.dentry->d_name.len;
+	this.hash = nd->path.dentry->d_name.hash;
+	name = kmalloc(this.len + 1, GFP_KERNEL);
+	if (!name)
+		return -ENOMEM;
+	this.name = name;
+	memcpy(name, nd->path.dentry->d_name.name, nd->path.dentry->d_name.len);
+	name[this.len] = 0;
+
+	err = union_relookup_topmost(nd, nd->flags|LOOKUP_PARENT);
+	if (err) {
+		kfree(name);
+		return err;
+	}
+	nd->flags &= ~LOOKUP_PARENT;
+
+	dir = nd->path.dentry;
+	mutex_lock(&dir->d_inode->i_mutex);
+	err = hash_lookup_union(nd, &this, &path);
+	mutex_unlock(&dir->d_inode->i_mutex);
+	kfree(name);
+	if (err)
+		return err;
+
+	err = -ENOENT;
+	if (!path.dentry->d_inode)
+		goto exit_dput;
+
+	/* Necessary?! I guess not ... */
+	follow_mount(&path);
+
+	err = -ENOENT;
+	if (!path.dentry->d_inode)
+		goto exit_dput;
+
+	err = -EISDIR;
+	if (!S_ISREG(path.dentry->d_inode->i_mode))
+		goto exit_dput;
+
+	if (path.dentry->d_parent != nd->path.dentry) {
+		err = __union_copyup(&path, nd, &path);
+		if (err)
+			goto exit_dput;
+	}
+
+	dput(nd->path.dentry);
+	if (nd->path.mnt != path.mnt)
+		mntput(nd->path.mnt);
+	nd->path = path;
+	return 0;
+
+exit_dput:
+	dput(path.dentry);
+	if (path.mnt != nd->path.mnt)
+		mntput(path.mnt);
+	return err;
+}
+
+/*
+ * This must be called when unhashing a dentry. This is called with dcache_lock
+ * and unhashes all unions this dentry is in.
+ */
+void __d_drop_unions(struct dentry *dentry)
+{
+	struct union_mount *this, *next;
+
+	spin_lock(&union_lock);
+	list_for_each_entry_safe(this, next, &dentry->d_unions, u_unions)
+		__union_unhash(this);
+	spin_unlock(&union_lock);
+}
+EXPORT_SYMBOL_GPL(__d_drop_unions);
+
+/*
+ * This must be called after __d_drop_unions() without holding any locks.
+ * Note: The dentry might still be reachable via a lookup but at that time it
+ * already a negative dentry. Otherwise it would be unhashed. The union_mount
+ * structure itself is still reachable through mnt->mnt_unions (which we
+ * protect against with union_lock).
+ */
+void shrink_d_unions(struct dentry *dentry)
+{
+	struct union_mount *this, *next;
+
+repeat:
+	spin_lock(&union_lock);
+	list_for_each_entry_safe(this, next, &dentry->d_unions, u_unions) {
+		BUG_ON(!hlist_unhashed(&this->u_hash));
+		BUG_ON(!hlist_unhashed(&this->u_rhash));
+		list_del(&this->u_list);
+		list_del(&this->u_unions);
+		this->u_next.dentry->d_unionized--;
+		spin_unlock(&union_lock);
+		union_put(this);
+		goto repeat;
+	}
+	spin_unlock(&union_lock);
+}
+
+extern void __dput(struct dentry *, struct list_head *, int);
+
+/*
+ * This is the special variant for use in dput() only.
+ */
+void __shrink_d_unions(struct dentry *dentry, struct list_head *list)
+{
+	struct union_mount *this, *next;
+
+	BUG_ON(!d_unhashed(dentry));
+
+repeat:
+	spin_lock(&union_lock);
+	list_for_each_entry_safe(this, next, &dentry->d_unions, u_unions) {
+		struct dentry *n_dentry = this->u_next.dentry;
+		struct vfsmount *n_mnt = this->u_next.mnt;
+
+		BUG_ON(!hlist_unhashed(&this->u_hash));
+		BUG_ON(!hlist_unhashed(&this->u_rhash));
+		list_del(&this->u_list);
+		list_del(&this->u_unions);
+		this->u_next.dentry->d_unionized--;
+		spin_unlock(&union_lock);
+		if (__union_put(this)) {
+			__dput(n_dentry, list, 0);
+			mntput(n_mnt);
+		}
+		goto repeat;
+	}
+	spin_unlock(&union_lock);
+}
+
+/*
+ * Remove all union_mounts structures belonging to this vfsmount from the
+ * union lookup hashtable and so on ...
+ */
+void shrink_mnt_unions(struct vfsmount *mnt)
+{
+	struct union_mount *this, *next;
+
+repeat:
+	spin_lock(&union_lock);
+	list_for_each_entry_safe(this, next, &mnt->mnt_unions, u_list) {
+		if (this->u_this.dentry == mnt->mnt_root)
+			continue;
+		__union_unhash(this);
+		list_del(&this->u_list);
+		list_del(&this->u_unions);
+		this->u_next.dentry->d_unionized--;
+		spin_unlock(&union_lock);
+		union_put(this);
+		goto repeat;
+	}
+	spin_unlock(&union_lock);
+}
+
+int attach_mnt_union(struct vfsmount *mnt, struct vfsmount *dest_mnt,
+		     struct dentry *dest_dentry)
+{
+	if (!IS_MNT_UNION(mnt))
+		return 0;
+
+	return append_to_union(mnt, mnt->mnt_root, dest_mnt, dest_dentry);
+}
+
+void detach_mnt_union(struct vfsmount *mnt)
+{
+	struct union_mount *um;
+
+	if (!IS_MNT_UNION(mnt))
+		return;
+
+	shrink_mnt_unions(mnt);
+
+	spin_lock(&union_lock);
+	um = union_lookup(mnt->mnt_root, mnt);
+	__union_unhash(um);
+	list_del(&um->u_list);
+	list_del(&um->u_unions);
+	um->u_next.dentry->d_unionized--;
+	spin_unlock(&union_lock);
+	union_put(um);
+	return;
+}
+
+/**
+ * union_copyup_dir_one - copy up a single directory entry
+ *
+ * Individual directory entry copyup function for union_copyup_dir.
+ * We get the entries from higher level layers first.
+ */
+
+static int union_copyup_dir_one(void *buf, const char *name, int namlen,
+				loff_t offset, u64 ino, unsigned int d_type)
+{
+	struct dentry *topmost_dentry = (struct dentry *) buf;
+	struct dentry *dentry;
+	int err = 0;
+
+	switch (namlen) {
+	case 2:
+		if (name[1] != '.')
+			break;
+	case 1:
+		if (name[0] != '.')
+			break;
+		return 0;
+	}
+
+	/* Lookup this entry in the topmost directory */
+	dentry = lookup_one_len(name, topmost_dentry, namlen);
+
+	if (IS_ERR(dentry)) {
+		printk(KERN_INFO "error looking up %s\n", dentry->d_name.name);
+		goto out;
+	}
+
+	/*
+	 * If the entry already exists, one of the following is true:
+	 * it was already copied up (due to an earlier lookup), an
+	 * entry with the same name already exists on the topmost file
+	 * system, it is a whiteout, or it is a fallthru.  In each
+	 * case, the top level entry masks any entries from lower file
+	 * systems, so don't copy up this entry.
+	 */
+	if (dentry->d_inode || d_is_whiteout(dentry) ||
+	    d_is_fallthru(dentry)) {
+		printk(KERN_INFO "skipping copy of %s\n", dentry->d_name.name);
+		goto out_dput;
+	}
+
+	/*
+	 * If the entry doesn't exist, create a fallthru entry in the
+	 * topmost file system.  All possible directory types are
+	 * used, so each file system must implement its own way of
+	 * storing a fallthru entry.
+	 */
+	printk(KERN_INFO "creating fallthru for %s\n", dentry->d_name.name);
+	err = topmost_dentry->d_inode->i_op->fallthru(topmost_dentry->d_inode,
+						      dentry);
+	/* FIXME */
+	BUG_ON(err);
+	/*
+	 * At this point, we have a negative dentry marked as fallthru
+	 * in the cache.  We could potentially lookup the entry lower
+	 * level file system and turn this into a positive dentry
+	 * right now, but it is not clear that would be a performance
+	 * win and adds more opportunities to fail.
+	 */
+out_dput:
+	dput(dentry);
+out:
+	return 0;
+}
+
+/**
+ * union_copyup_dir - copy up low-level directory entries to topmost dir
+ *
+ * readdir() is difficult to support on union file systems for two
+ * reasons: We must eliminate duplicates and apply whiteouts, and we
+ * must return something in f_pos that lets us restart in the same
+ * place when we return.  Our solution is to, on first readdir() of
+ * the directory, copy up all visible entries from the low-level file
+ * systems and mark the entries that refer to low-level file system
+ * objects as "fallthru" entries.
+ */
+
+int union_copyup_dir(struct path *topmost_path)
+{
+	struct dentry *topmost_dentry = topmost_path->dentry;
+	struct path path = *topmost_path;
+	int res = 0;
+
+	/*
+	 * Skip opaque dirs.
+	 */
+	if (IS_OPAQUE(topmost_dentry->d_inode))
+		return 0;
+
+	/*
+	 * Mark this dir opaque to show that we have already copied up
+	 * the lower entries.  Only fallthru entries pass through to
+	 * the underlying file system.
+	 *
+	 * XXX Deal with the lower file system changing.  This could
+	 * be through running a tool over the top level file system to
+	 * make directories transparent again, or we could check the
+	 * mtime of the underlying directory.
+	 */
+
+	topmost_dentry->d_inode->i_flags |= S_OPAQUE;
+	mark_inode_dirty(topmost_dentry->d_inode);
+
+	/*
+	 * Loop through each dir on each level copying up the entries
+	 * to the topmost.
+	 */
+
+	/* Don't drop the caller's reference to the topmost path */
+	path_get(&path);
+	while (follow_union_down(&path)) {
+		struct file * ftmp;
+		struct inode * inode;
+
+		/* XXX Permit fallthrus on lower-level? Would need to
+		 * pass in opaque flag to union_copyup_dir_one() and
+		 * only copy up fallthru entries there.  We allow
+		 * fallthrus in lower level opaque directories on
+		 * lookup, so for consistency we should do one or the
+		 * other in both places. */
+		if (IS_OPAQUE(path.dentry->d_inode))
+			break;
+
+		/* dentry_open() doesn't get a path reference itself */
+		path_get(&path);
+		ftmp = dentry_open(path.dentry, path.mnt,
+				   O_RDONLY | O_DIRECTORY | O_NOATIME,
+				   current_cred());
+		if (IS_ERR(ftmp)) {
+			printk (KERN_ERR "unable to open dir %s for "
+				"directory copyup: %ld\n",
+				path.dentry->d_name.name, PTR_ERR(ftmp));
+			continue;
+		}
+
+		inode = path.dentry->d_inode;
+		mutex_lock(&inode->i_mutex);
+
+		res = -ENOENT;
+		if (IS_DEADDIR(inode))
+			goto out_fput;
+		/*
+		 * Read the whole directory, calling our directory
+		 * entry copyup function on each entry.  Pass in the
+		 * topmost dentry as our private data so we can create
+		 * new entries in the topmost directory.
+		 */
+		res = ftmp->f_op->readdir(ftmp, topmost_dentry,
+					  union_copyup_dir_one);
+out_fput:
+		mutex_unlock(&inode->i_mutex);
+		fput(ftmp);
+
+		if (res)
+			break;
+	}
+	path_put(&path);
+	return res;
+}
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -101,6 +101,15 @@ struct dentry {
 	struct dentry *d_parent;	/* parent directory */
 	struct qstr d_name;
 
+#ifdef CONFIG_UNION_MOUNT
+	/*
+	 * The following fields are used by the VFS based union mount
+	 * implementation. Both are protected by union_lock!
+	 */
+	struct list_head d_unions;	/* list of union_mount's */
+	unsigned int d_unionized;	/* unions referencing this dentry */
+#endif
+
 	struct list_head d_lru;		/* LRU list */
 	/*
 	 * d_child and d_rcu can share memory
@@ -186,6 +195,9 @@ d_iput:		no		no		no       yes
 
 #define DCACHE_FSNOTIFY_PARENT_WATCHED	0x0080 /* Parent inode is watched by some fsnotify listener */
 
+#define DCACHE_WHITEOUT		0x0100	/* This negative dentry is a whiteout */
+#define DCACHE_FALLTHRU		0x0200	/* Keep looking in the file system below */
+
 extern spinlock_t dcache_lock;
 extern seqlock_t rename_lock;
 
@@ -205,12 +217,20 @@ extern seqlock_t rename_lock;
  * __d_drop requires dentry->d_lock.
  */
 
+#ifdef CONFIG_UNION_MOUNT
+extern void __d_drop_unions(struct dentry *);
+#endif
+
 static inline void __d_drop(struct dentry *dentry)
 {
 	if (!(dentry->d_flags & DCACHE_UNHASHED)) {
 		dentry->d_flags |= DCACHE_UNHASHED;
 		hlist_del_rcu(&dentry->d_hash);
 	}
+#ifdef CONFIG_UNION_MOUNT
+	/* remove dentry from the union hashtable */
+	__d_drop_unions(dentry);
+#endif
 }
 
 static inline void d_drop(struct dentry *dentry)
@@ -358,6 +378,16 @@ static inline int d_unlinked(struct dent
 	return d_unhashed(dentry) && !IS_ROOT(dentry);
 }
 
+static inline int d_is_whiteout(struct dentry *dentry)
+{
+	return (dentry->d_flags & DCACHE_WHITEOUT);
+}
+
+static inline int d_is_fallthru(struct dentry *dentry)
+{
+	return (dentry->d_flags & DCACHE_FALLTHRU);
+}
+
 static inline struct dentry *dget_parent(struct dentry *dentry)
 {
 	struct dentry *ret;
--- a/include/linux/ext2_fs.h
+++ b/include/linux/ext2_fs.h
@@ -189,6 +189,7 @@ struct ext2_group_desc
 #define EXT2_NOTAIL_FL			FS_NOTAIL_FL	/* file tail should not be merged */
 #define EXT2_DIRSYNC_FL			FS_DIRSYNC_FL	/* dirsync behaviour (directories only) */
 #define EXT2_TOPDIR_FL			FS_TOPDIR_FL	/* Top of directory hierarchies*/
+#define EXT2_OPAQUE_FL			0x00040000
 #define EXT2_RESERVED_FL		FS_RESERVED_FL	/* reserved for ext2 lib */
 
 #define EXT2_FL_USER_VISIBLE		FS_FL_USER_VISIBLE	/* User visible flags */
@@ -503,10 +504,12 @@ struct ext2_super_block {
 #define EXT3_FEATURE_INCOMPAT_RECOVER		0x0004
 #define EXT3_FEATURE_INCOMPAT_JOURNAL_DEV	0x0008
 #define EXT2_FEATURE_INCOMPAT_META_BG		0x0010
+#define EXT2_FEATURE_INCOMPAT_WHITEOUT		0x0020
 #define EXT2_FEATURE_INCOMPAT_ANY		0xffffffff
 
 #define EXT2_FEATURE_COMPAT_SUPP	EXT2_FEATURE_COMPAT_EXT_ATTR
 #define EXT2_FEATURE_INCOMPAT_SUPP	(EXT2_FEATURE_INCOMPAT_FILETYPE| \
+					 EXT2_FEATURE_INCOMPAT_WHITEOUT| \
 					 EXT2_FEATURE_INCOMPAT_META_BG)
 #define EXT2_FEATURE_RO_COMPAT_SUPP	(EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \
 					 EXT2_FEATURE_RO_COMPAT_LARGE_FILE| \
@@ -573,6 +576,8 @@ enum {
 	EXT2_FT_FIFO,
 	EXT2_FT_SOCK,
 	EXT2_FT_SYMLINK,
+	EXT2_FT_WHT,
+	EXT2_FT_FALLTHRU,
 	EXT2_FT_MAX
 };
 
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -188,6 +188,7 @@ struct inodes_stat_t {
 #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
 #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
 #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
+#define MS_UNION	256
 #define MS_NOATIME	1024	/* Do not update access times. */
 #define MS_NODIRATIME	2048	/* Do not update directory access times */
 #define MS_BIND		4096
@@ -205,6 +206,7 @@ struct inodes_stat_t {
 #define MS_KERNMOUNT	(1<<22) /* this is a kern_mount call */
 #define MS_I_VERSION	(1<<23) /* Update inode I_version field */
 #define MS_STRICTATIME	(1<<24) /* Always perform atime updates */
+#define MS_WHITEOUT	(1<<26) /* fs does support white-out filetype */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 
@@ -231,6 +233,7 @@ struct inodes_stat_t {
 #define S_NOCMTIME	128	/* Do not update file c/mtime */
 #define S_SWAPFILE	256	/* Do not truncate: swapon got its bmaps */
 #define S_PRIVATE	512	/* Inode is fs-internal */
+#define S_OPAQUE	1024	/* Directory is opaque */
 
 /*
  * Note that nosuid etc flags are inode-specific: setting some file-system
@@ -266,6 +269,8 @@ struct inodes_stat_t {
 #define IS_SWAPFILE(inode)	((inode)->i_flags & S_SWAPFILE)
 #define IS_PRIVATE(inode)	((inode)->i_flags & S_PRIVATE)
 
+#define IS_OPAQUE(inode)	((inode)->i_flags & S_OPAQUE)
+
 /* the read-only stuff doesn't really belong here, but any other place is
    probably as bad and I don't want to create yet another include file. */
 
@@ -1380,6 +1385,11 @@ struct super_block {
 	 * generic_show_options()
 	 */
 	char *s_options;
+
+	/*
+	 * Users who require read-only access - e.g., union mounts
+	 */
+	int s_readonly_users;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);
@@ -1517,6 +1527,8 @@ struct inode_operations {
 	int (*mkdir) (struct inode *,struct dentry *,int);
 	int (*rmdir) (struct inode *,struct dentry *);
 	int (*mknod) (struct inode *,struct dentry *,int,dev_t);
+	int (*whiteout) (struct inode *, struct dentry *, struct dentry *);
+	int (*fallthru) (struct inode *, struct dentry *);
 	int (*rename) (struct inode *, struct dentry *,
 			struct inode *, struct dentry *);
 	int (*readlink) (struct dentry *, char __user *,int);
@@ -2108,6 +2120,7 @@ extern void emergency_remount(void);
 extern sector_t bmap(struct inode *, sector_t);
 #endif
 extern int notify_change(struct dentry *, struct iattr *);
+extern int __inode_permission(struct inode *inode, int mask, int rofs);
 extern int inode_permission(struct inode *, int);
 extern int generic_permission(struct inode *, int,
 		int (*check_acl)(struct inode *, int));
@@ -2135,7 +2148,7 @@ extern void free_write_pipe(struct file 
 
 extern struct file *do_filp_open(int dfd, const char *pathname,
 		int open_flag, int mode, int acc_mode);
-extern int may_open(struct path *, int, int);
+extern int may_open(struct nameidata *, int, int);
 
 extern int kernel_read(struct file *, loff_t, char *, unsigned long);
 extern struct file * open_exec(const char *);
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -35,6 +35,7 @@ struct mnt_namespace;
 #define MNT_SHARED	0x1000	/* if the vfsmount is a shared mount */
 #define MNT_UNBINDABLE	0x2000	/* if the vfsmount is a unbindable mount */
 #define MNT_PNODE_MASK	0x3000	/* propagation flag mask */
+#define MNT_UNION	0x4000	/* if the vfsmount is a union mount */
 
 struct vfsmount {
 	struct list_head mnt_hash;
@@ -53,6 +54,9 @@ struct vfsmount {
 	struct list_head mnt_slave_list;/* list of slave mounts */
 	struct list_head mnt_slave;	/* slave list entry */
 	struct vfsmount *mnt_master;	/* slave is on master->mnt_slave_list */
+#ifdef CONFIG_UNION_MOUNT
+	struct list_head mnt_unions;	/* list of union_mount structures */
+#endif
 	struct mnt_namespace *mnt_ns;	/* containing namespace */
 	int mnt_id;			/* mount identifier */
 	int mnt_group_id;		/* peer group identifier */
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -20,6 +20,7 @@ struct nameidata {
 	struct qstr	last;
 	struct path	root;
 	unsigned int	flags;
+	unsigned int	um_flags;
 	int		last_type;
 	unsigned	depth;
 	char *saved_names[MAX_NESTED_LINKS + 1];
@@ -35,6 +36,9 @@ struct nameidata {
  */
 enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 
+#define LAST_UNION             0x01
+#define LAST_LOWLEVEL          0x02
+
 /*
  * The bitmask for a lookup event:
  *  - follow links at the end
@@ -49,6 +53,8 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LA
 #define LOOKUP_CONTINUE		 4
 #define LOOKUP_PARENT		16
 #define LOOKUP_REVAL		64
+#define LOOKUP_TOPMOST	       128
+
 /*
  * Intent data
  */
--- /dev/null
+++ b/include/linux/union.h
@@ -0,0 +1,84 @@
+/*
+ * VFS based union mount for Linux
+ *
+ * Copyright (C) 2004-2007 IBM Corporation, IBM Deutschland Entwicklung GmbH.
+ * Copyright (C) 2007 Novell Inc.
+ *   Author(s): Jan Blunck (j.blunck@tu-harburg.de)
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ */
+#ifndef __LINUX_UNION_H
+#define __LINUX_UNION_H
+#ifdef __KERNEL__
+
+#include <linux/list.h>
+#include <asm/atomic.h>
+
+struct dentry;
+struct vfsmount;
+
+#ifdef CONFIG_UNION_MOUNT
+
+/*
+ * The new union mount structure.
+ */
+struct union_mount {
+	atomic_t u_count;		/* reference count */
+	struct mutex u_mutex;
+	struct list_head u_unions;	/* list head for d_unions */
+	struct list_head u_list;	/* list head for mnt_unions */
+	struct hlist_node u_hash;	/* list head for seaching */
+	struct hlist_node u_rhash;	/* list head for reverse seaching */
+
+	struct path u_this;		/* this is me */
+	struct path u_next;		/* this is what I overlay */
+};
+
+#define IS_UNION(dentry)	(!list_empty(&(dentry)->d_unions) || \
+				 (dentry)->d_unionized)
+#define IS_MNT_UNION(mnt)	((mnt)->mnt_flags & MNT_UNION)
+
+extern int is_unionized(struct dentry *, struct vfsmount *);
+extern int append_to_union(struct vfsmount *, struct dentry *,
+			   struct vfsmount *, struct dentry *);
+extern int follow_union_down(struct path *);
+extern int follow_union_mount(struct path *);
+extern void __d_drop_unions(struct dentry *);
+extern void shrink_d_unions(struct dentry *);
+extern void __shrink_d_unions(struct dentry *, struct list_head *);
+extern int attach_mnt_union(struct vfsmount *, struct vfsmount *,
+			    struct dentry *);
+extern void detach_mnt_union(struct vfsmount *);
+extern struct dentry *union_create_topmost(struct nameidata *, struct qstr *,
+					   struct path *);
+extern int __union_copyup(struct path *, struct nameidata *, struct path *);
+extern int union_copyup(struct nameidata *, int);
+extern int union_copyup_dir(struct path *path);
+extern int union_permission(struct path *, int);
+
+#else /* CONFIG_UNION_MOUNT */
+
+#define IS_UNION(x)			(0)
+#define IS_MNT_UNION(x)			(0)
+#define is_unionized(x, y)		(0)
+#define append_to_union(x1, y1, x2, y2)	({ BUG(); (0); })
+#define follow_union_down(x)		({ (0); })
+#define follow_union_mount(x)	({ (0); })
+#define __d_drop_unions(x)		do { } while (0)
+#define shrink_d_unions(x)		do { } while (0)
+#define __shrink_d_unions(x,y)		do { } while (0)
+#define attach_mnt_union(x, y, z)	do { } while (0)
+#define detach_mnt_union(x)		do { } while (0)
+#define union_create_topmost(x, y, z)	({ BUG(); (NULL); })
+#define __union_copyup(x, y, z)		({ BUG(); (0); })
+#define union_copyup(x, y)		({ (0); })
+#define union_copyup_dir(x)		({ BUG(); (0); })
+#define union_permission(x, y)		inode_permission(x->dentry->d_inode, y)
+
+#endif	/* CONFIG_UNION_MOUNT */
+#endif	/* __KERNEL__ */
+#endif	/* __LINUX_UNION_H */
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1798,6 +1798,118 @@ static int shmem_statfs(struct dentry *d
 	return 0;
 }
 
+static int shmem_rmdir(struct inode *dir, struct dentry *dentry);
+static int shmem_unlink(struct inode *dir, struct dentry *dentry);
+
+/*
+ * Create a dentry to signify a whiteout.
+ */
+static int shmem_whiteout(struct inode *dir, struct dentry *old_dentry,
+			  struct dentry *new_dentry)
+{
+	struct shmem_sb_info *sbinfo = SHMEM_SB(dir->i_sb);
+	struct dentry *dentry;
+
+	if (!(dir->i_sb->s_flags & MS_WHITEOUT))
+		return -EPERM;
+
+	/* This gives us a proper initialized negative dentry */
+	dentry = simple_lookup(dir, new_dentry, NULL);
+	if (dentry && IS_ERR(dentry))
+		return PTR_ERR(dentry);
+
+	/*
+	 * No ordinary (disk based) filesystem counts whiteouts as inodes;
+	 * but each new link needs a new dentry, pinning lowmem, and
+	 * tmpfs dentries cannot be pruned until they are unlinked.
+	 */
+	if (sbinfo->max_inodes) {
+		spin_lock(&sbinfo->stat_lock);
+		if (!sbinfo->free_inodes) {
+			spin_unlock(&sbinfo->stat_lock);
+			return -ENOSPC;
+		}
+		sbinfo->free_inodes--;
+		spin_unlock(&sbinfo->stat_lock);
+	}
+
+	if (old_dentry->d_inode || d_is_fallthru(old_dentry)) {
+		if (old_dentry->d_inode && S_ISDIR(old_dentry->d_inode->i_mode))
+			shmem_rmdir(dir, old_dentry);
+		else
+			shmem_unlink(dir, old_dentry);
+	}
+
+	dir->i_size += BOGO_DIRENT_SIZE;
+	dir->i_ctime = dir->i_mtime = CURRENT_TIME;
+	/* Extra pinning count for the created dentry */
+	dget(new_dentry);
+	spin_lock(&new_dentry->d_lock);
+	new_dentry->d_flags |= DCACHE_WHITEOUT;
+	spin_unlock(&new_dentry->d_lock);
+	return 0;
+}
+
+static void shmem_d_instantiate(struct inode *dir, struct dentry *dentry,
+				struct inode *inode);
+
+/*
+ * Create a dentry to signify a fallthru.  A fallthru lets us read the
+ * low-level dentries into the dcache once on the first readdir() and
+ * then
+ */
+static int shmem_fallthru(struct inode *dir, struct dentry *dentry)
+{
+	struct shmem_sb_info *sbinfo = SHMEM_SB(dir->i_sb);
+
+	/* FIXME: this is stupid */
+	if (!(dir->i_sb->s_flags & MS_WHITEOUT))
+		return -EPERM;
+
+	if (dentry->d_inode || d_is_fallthru(dentry) || d_is_whiteout(dentry))
+		return -EEXIST;
+
+	/*
+	 * Each new link needs a new dentry, pinning lowmem, and tmpfs
+	 * dentries cannot be pruned until they are unlinked.
+	 */
+	if (sbinfo->max_inodes) {
+		spin_lock(&sbinfo->stat_lock);
+		if (!sbinfo->free_inodes) {
+			spin_unlock(&sbinfo->stat_lock);
+			return -ENOSPC;
+		}
+		sbinfo->free_inodes--;
+		spin_unlock(&sbinfo->stat_lock);
+	}
+
+	shmem_d_instantiate(dir, dentry, NULL);
+	dir->i_ctime = dir->i_mtime = CURRENT_TIME;
+
+	spin_lock(&dentry->d_lock);
+	dentry->d_flags |= DCACHE_FALLTHRU;
+	spin_unlock(&dentry->d_lock);
+	return 0;
+}
+
+static void shmem_d_instantiate(struct inode *dir, struct dentry *dentry,
+				struct inode *inode)
+{
+	if (d_is_whiteout(dentry)) {
+		/* Re-using an existing whiteout */
+		shmem_free_inode(dir->i_sb);
+		if (S_ISDIR(inode->i_mode))
+			inode->i_mode |= S_OPAQUE;
+	} else if (d_is_fallthru(dentry)) {
+		shmem_free_inode(dir->i_sb);
+	} else {
+		/* New dentry */
+		dir->i_size += BOGO_DIRENT_SIZE;
+		dget(dentry); /* Extra count - pin the dentry in core */
+	}
+	/* Will clear DCACHE_WHITEOUT and DCACHE_FALLTHRU flags */
+	d_instantiate(dentry, inode);
+}
 /*
  * File creation. Allocate an inode, and we're done..
  */
@@ -1822,15 +1934,16 @@ shmem_mknod(struct inode *dir, struct de
 			iput(inode);
 			return error;
 		}
+
 		if (dir->i_mode & S_ISGID) {
 			inode->i_gid = dir->i_gid;
 			if (S_ISDIR(mode))
 				inode->i_mode |= S_ISGID;
 		}
-		dir->i_size += BOGO_DIRENT_SIZE;
+
+		shmem_d_instantiate(dir, dentry, inode);
+
 		dir->i_ctime = dir->i_mtime = CURRENT_TIME;
-		d_instantiate(dentry, inode);
-		dget(dentry); /* Extra count - pin the dentry in core */
 	}
 	return error;
 }
@@ -1868,12 +1981,11 @@ static int shmem_link(struct dentry *old
 	if (ret)
 		goto out;
 
-	dir->i_size += BOGO_DIRENT_SIZE;
+	shmem_d_instantiate(dir, dentry, inode);
+
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
 	inc_nlink(inode);
 	atomic_inc(&inode->i_count);	/* New dentry reference */
-	dget(dentry);		/* Extra pinning count for the created dentry */
-	d_instantiate(dentry, inode);
 out:
 	return ret;
 }
@@ -1882,21 +1994,63 @@ static int shmem_unlink(struct inode *di
 {
 	struct inode *inode = dentry->d_inode;
 
-	if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
-		shmem_free_inode(inode->i_sb);
+	if (d_is_whiteout(dentry) || d_is_fallthru(dentry) ||
+	    (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode)))
+		shmem_free_inode(dir->i_sb);
 
+	if (inode) {
+		inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
+		drop_nlink(inode);
+	}
 	dir->i_size -= BOGO_DIRENT_SIZE;
-	inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
-	drop_nlink(inode);
 	dput(dentry);	/* Undo the count from "create" - this does all the work */
 	return 0;
 }
 
+static void shmem_dir_unlink_whiteouts(struct inode *dir, struct dentry *dentry)
+{
+	if (!dentry->d_inode)
+		return;
+
+	/* Remove whiteouts from logical empty directory */
+	if (S_ISDIR(dentry->d_inode->i_mode) &&
+	    dentry->d_inode->i_sb->s_flags & MS_WHITEOUT) {
+		struct dentry *child, *next;
+		LIST_HEAD(list);
+
+		spin_lock(&dcache_lock);
+		list_for_each_entry(child, &dentry->d_subdirs, d_u.d_child) {
+			spin_lock(&child->d_lock);
+			/* Unlink fallthrus too */
+			if (d_is_whiteout(child) || d_is_fallthru(child)) {
+				__d_drop(child);
+				if (!list_empty(&child->d_lru)) {
+					list_del(&child->d_lru);
+					dentry_stat.nr_unused--;
+				}
+				list_add(&child->d_lru, &list);
+			}
+			spin_unlock(&child->d_lock);
+		}
+		spin_unlock(&dcache_lock);
+
+		list_for_each_entry_safe(child, next, &list, d_lru) {
+			spin_lock(&child->d_lock);
+			list_del_init(&child->d_lru);
+			spin_unlock(&child->d_lock);
+
+			shmem_unlink(dentry->d_inode, child);
+		}
+	}
+}
+
 static int shmem_rmdir(struct inode *dir, struct dentry *dentry)
 {
 	if (!simple_empty(dentry))
 		return -ENOTEMPTY;
 
+	/* Remove whiteouts from logical empty directory */
+	shmem_dir_unlink_whiteouts(dir, dentry);
 	drop_nlink(dentry->d_inode);
 	drop_nlink(dir);
 	return shmem_unlink(dir, dentry);
@@ -1905,7 +2059,7 @@ static int shmem_rmdir(struct inode *dir
 /*
  * The VFS layer already does all the dentry stuff for rename,
  * we just have to decrement the usage count for the target if
- * it exists so that the VFS layer correctly free's it when it
+ * it exists so that the VFS layer correctly frees it when it
  * gets overwritten.
  */
 static int shmem_rename(struct inode *old_dir, struct dentry *old_dentry, struct inode *new_dir, struct dentry *new_dentry)
@@ -1916,7 +2070,12 @@ static int shmem_rename(struct inode *ol
 	if (!simple_empty(new_dentry))
 		return -ENOTEMPTY;
 
+	if (d_is_whiteout(new_dentry))
+		shmem_unlink(new_dir, new_dentry);
+
 	if (new_dentry->d_inode) {
+		/* Remove whiteouts from logical empty directory */
+		shmem_dir_unlink_whiteouts(new_dir, new_dentry);
 		(void) shmem_unlink(new_dir, new_dentry);
 		if (they_are_dirs)
 			drop_nlink(old_dir);
@@ -1981,12 +2140,12 @@ static int shmem_symlink(struct inode *d
 		unlock_page(page);
 		page_cache_release(page);
 	}
+
+	shmem_d_instantiate(dir, dentry, inode);
+
 	if (dir->i_mode & S_ISGID)
 		inode->i_gid = dir->i_gid;
-	dir->i_size += BOGO_DIRENT_SIZE;
 	dir->i_ctime = dir->i_mtime = CURRENT_TIME;
-	d_instantiate(dentry, inode);
-	dget(dentry);
 	return 0;
 }
 
@@ -2363,6 +2522,12 @@ int shmem_fill_super(struct super_block 
 	if (!root)
 		goto failed_iput;
 	sb->s_root = root;
+
+#ifdef CONFIG_TMPFS
+	if (!(sb->s_flags & MS_NOUSER))
+		sb->s_flags |= MS_WHITEOUT;
+#endif
+
 	return 0;
 
 failed_iput:
@@ -2463,6 +2628,8 @@ static const struct inode_operations shm
 	.rmdir		= shmem_rmdir,
 	.mknod		= shmem_mknod,
 	.rename		= shmem_rename,
+	.whiteout       = shmem_whiteout,
+	.fallthru       = shmem_fallthru,
 #endif
 #ifdef CONFIG_TMPFS_POSIX_ACL
 	.setattr	= shmem_notify_change,