linux-kernel - Re: [PATCH v3] ext4: Fix bug on in ext4_es_cache_extent as ext4_split_extent

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Date:   Fri, 30 Apr 2021 14:58:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ye Bin <yebin10@...wei.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] ext4: Fix bug on in ext4_es_cache_extent as
 ext4_split_extent_at failed

On Wed 28-04-21 16:51:58, Ye Bin wrote:
> We got follow bug_on when run fsstress with injecting IO fault:
> [130747.323114] kernel BUG at fs/ext4/extents_status.c:762!
> [130747.323117] Internal error: Oops - BUG: 0 [#1] SMP
> ......
> [130747.334329] Call trace:
> [130747.334553]  ext4_es_cache_extent+0x150/0x168 [ext4]
> [130747.334975]  ext4_cache_extents+0x64/0xe8 [ext4]
> [130747.335368]  ext4_find_extent+0x300/0x330 [ext4]
> [130747.335759]  ext4_ext_map_blocks+0x74/0x1178 [ext4]
> [130747.336179]  ext4_map_blocks+0x2f4/0x5f0 [ext4]
> [130747.336567]  ext4_mpage_readpages+0x4a8/0x7a8 [ext4]
> [130747.336995]  ext4_readpage+0x54/0x100 [ext4]
> [130747.337359]  generic_file_buffered_read+0x410/0xae8
> [130747.337767]  generic_file_read_iter+0x114/0x190
> [130747.338152]  ext4_file_read_iter+0x5c/0x140 [ext4]
> [130747.338556]  __vfs_read+0x11c/0x188
> [130747.338851]  vfs_read+0x94/0x150
> [130747.339110]  ksys_read+0x74/0xf0
> 
> If call ext4_ext_insert_extent failed but new extent already inserted, we just
> update "ex->ee_len = orig_ex.ee_len", this will lead to extent overlap, then
> cause bug on when cache extent.

Thanks for the patch but I'm still not quite sure, how overlapping extents
in the extent tree can lead to triggering BUG_ON(lblk + len - 1 < lblk) in
ext4_es_cache_extent().  Can you ellaborate a bit more how this happens?

> If call ext4_ext_insert_extent failed don't update ex->ee_len with old value.
> Maybe there will lead to block leak, but it can be fixed by fsck later.
> 
> After we fixed above issue with v2 patch, but we got the same issue.
> ext4_split_extent_at:
> {
>         ......
>         err = ext4_ext_insert_extent(handle, inode, ppath, &newex, flags);
>         if (err == -ENOSPC && (EXT4_EXT_MAY_ZEROOUT & split_flag)) {
>             ......
>             ext4_ext_try_to_merge(handle, inode, path, ex); ->step(1)
>             err = ext4_ext_dirty(handle, inode, path + path->p_depth); ->step(2)
>             if (err)
>                 goto fix_extent_len;
>         ......
>         }
>         ......
> fix_extent_len:
>         ex->ee_len = orig_ex.ee_len; ->step(3)
>         ......
> }
> If step(1) have been merged, but step(2) dirty extent failed, then go to
> fix_extent_len label to fix ex->ee_len with orig_ex.ee_len. But "ex" may not be
> old one, will cause overwritten. Then will trigger the same issue as previous.
> If step(2) failed, just return error, don't fix ex->ee_len with old value.
> 
> Signed-off-by: Ye Bin <yebin10@...wei.com>
> ---
>  fs/ext4/extents.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 77c84d6f1af6..d4aa24a09d8b 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3238,15 +3238,12 @@ static int ext4_split_extent_at(handle_t *handle,
>  		ex->ee_len = cpu_to_le16(ee_len);
>  		ext4_ext_try_to_merge(handle, inode, path, ex);
>  		err = ext4_ext_dirty(handle, inode, path + path->p_depth);
> -		if (err)
> -			goto fix_extent_len;
> -
> -		/* update extent status tree */
> -		err = ext4_zeroout_es(inode, &zero_ex);
> -
> -		goto out;
> -	} else if (err)
> +		if (!err)
> +		        /* update extent status tree */
> +		        err = ext4_zeroout_es(inode, &zero_ex);
> +	} else if (err && err != -EROFS) {

I fail to see why EROFS is special here. Can you explain a bit please?

>  		goto fix_extent_len;
> +	}
>  
>  out:
>  	ext4_ext_show_leaf(inode, path);

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR