Patch "ext4: inplace xattr block update fails to deduplicate blocks" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

ext4: inplace xattr block update fails to deduplicate blocks

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ext4-inplace-xattr-block-update-fails-to-deduplicate-blocks.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tahsin Erdogan <[email protected]>
Date: Sat, 5 Aug 2017 22:41:42 -0400
Subject: ext4: inplace xattr block update fails to deduplicate blocks

From: Tahsin Erdogan <[email protected]>

commit ec00022030da5761518476096626338bd67df57a upstream.

When an xattr block has a single reference, block is updated inplace and it is reinserted to the cache. Later, a cache lookup is performed to see whether an existing block has the same contents. This cache lookup will most of the time return the just inserted entry so deduplication is not achieved.

Running the following test script will produce two xattr blocks which can be observed in "File ACL: " line of debugfs output:

mke2fs -b 1024 -I 128 -F -O extent /dev/sdb 1G
mount /dev/sdb /mnt/sdb

touch /mnt/sdb/{x,y}

setfattr -n user.1 -v aaa /mnt/sdb/x
setfattr -n user.2 -v bbb /mnt/sdb/x

setfattr -n user.1 -v aaa /mnt/sdb/y
setfattr -n user.2 -v bbb /mnt/sdb/y

debugfs -R 'stat x' /dev/sdb | cat
debugfs -R 'stat y' /dev/sdb | cat

This patch defers the reinsertion to the cache so that we can locate other blocks with the same contents.

---
 fs/ext4/xattr.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

@@ -837,8 +837,6 @@ ext4_xattr_block_set(handle_t *handle, s
 				if (!IS_LAST_ENTRY(s->first))
 					ext4_xattr_rehash(header(s->base),
 							  s->here);
-				ext4_xattr_cache_insert(ext4_mb_cache,
-					bs->bh);
 			}
 			ext4_xattr_block_csum_set(inode, bs->bh);
 			unlock_buffer(bs->bh);

@@ -959,6 +957,7 @@ inserted:
 		} else if (bs->bh && s->base == bs->bh->b_data) {
 			/* We were modifying this block in-place. */
 			ea_bdebug(bs->bh, "keeping this block");
+			ext4_xattr_cache_insert(ext4_mb_cache, bs->bh);
 			new_bh = bs->bh;
 			get_bh(new_bh);
 		} else {

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ext4-inplace-xattr-block-update-fails-to-deduplicate-blocks.patch