Commit ec000220 authored Aug 05, 2017 by Tahsin Erdogan Committed by Theodore Ts'o Aug 05, 2017

ext4: inplace xattr block update fails to deduplicate blocks



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@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>

parent 77a2e84d

fs/ext4/xattr.c

+1 −3

Original line number	Diff line number	Diff line
		@@ -1816,9 +1816,6 @@ ext4_xattr_block_set(handle_t handle, struct inode inode,
		ea_bdebug(bs->bh, "modifying in-place");
		error = ext4_xattr_set_entry(i, s, handle, inode,
		true /* is_block */);
		if (!error)
		ext4_xattr_block_cache_insert(ea_block_cache,
		bs->bh);
		ext4_xattr_block_csum_set(inode, bs->bh);
		unlock_buffer(bs->bh);
		if (error == -EFSCORRUPTED)
		@@ -1974,6 +1971,7 @@ 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_block_cache_insert(ea_block_cache, bs->bh);
		new_bh = bs->bh;
		get_bh(new_bh);
		} else {

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