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

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Date:   Sat, 15 Jul 2017 02:49:05 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Tahsin Erdogan <tahsin@...gle.com>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: inplace xattr block update fails to deduplicate
 blocks

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

Reviewed-by: Andreas Dilger <adilger@...ger.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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