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

Andreas Dilger
2017-07-15T09:49:09Z

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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

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