[PATCH] ext4: inplace xattr block update fails to deduplicate blocks

[Tahsin Erdogan]

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.

Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
Signed-off-by: Tommi Rantala <tommi.t.rantala@xxxxxxxxx>
---
 fs/ext4/xattr.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 3eeed8f0aa06..3fadfabcac39 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -837,8 +837,6 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
 				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 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
 		} 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 {
-- 
2.14.3


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