linux-kernel - [PATCH 4.9 64/86] ext4: inplace xattr block update fails to deduplicate blocks

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Date:   Fri, 16 Mar 2018 16:23:27 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Tahsin Erdogan <tahsin@...gle.com>,
        Theodore Tso <tytso@....edu>,
        Andreas Dilger <adilger@...ger.ca>,
        Tommi Rantala <tommi.t.rantala@...ia.com>
Subject: [PATCH 4.9 64/86] ext4: inplace xattr block update fails to deduplicate blocks

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tahsin Erdogan <tahsin@...gle.com>

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@...gle.com>
Signed-off-by: Theodore Ts'o <tytso@....edu>
Reviewed-by: Andreas Dilger <adilger@...ger.ca>
Signed-off-by: Tommi Rantala <tommi.t.rantala@...ia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -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 {