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>
---
 fs/ext4/xattr.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

On Jul 14, 2017, at 5:25 PM, Tahsin Erdogan <tahsin@google.com> wrote:
> 
> 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>

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
> fs/ext4/xattr.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index cff4f41ced61..ad4ea1cf685f 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -1815,9 +1815,6 @@ 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)
> @@ -1973,6 +1970,7 @@ 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_block_cache_insert(ea_block_cache, bs->bh);
> 			new_bh = bs->bh;
> 			get_bh(new_bh);
> 		} else {
> --
> 2.13.2.932.g7449e964c-goog
> 


Cheers, Andreas

On Sat, Jul 15, 2017 at 02:49:05AM -0700, Andreas Dilger wrote:
> On Jul 14, 2017, at 5:25 PM, Tahsin Erdogan <tahsin@google.com> wrote:
> > 
> > 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>
> 
> Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Thanks, applied.

					- Ted