Re: [PATCH] xfs: make sure sb_fdblocks is non-negative

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, May 10, 2024 at 05:34:26PM -0700, Wengang Wang wrote:

You might want to lead off with the origins of this fixpatch:

"A user with a completely full filesystem experienced an unexpected
shutdown when the filesystem tried to write the superblock during
runtime:"

<dmesg dump here>

"When xfs_log_sb writes a superblock to disk, sb_fdblocks is fetched..."

(or so I'm guessing from the other replies in this thread?)

((What was it doing?  Adding the ATTR/ATTR2 feature to the filesystem?))

> when writting super block to disk (in xfs_log_sb), sb_fdblocks is fetched from
> m_fdblocks without any lock. As m_fdblocks can experience a positive -> negativ

"negative"

>  -> positive changing when the FS reaches fullness (see xfs_mod_fdblocks)
> So there is a chance that sb_fdblocks is negative, and because sb_fdblocks is
> type of unsigned long long, it reads super big. And sb_fdblocks being bigger
> than sb_dblocks is a problem during log recovery, xfs_validate_sb_write()
> complains.
> 
> Fix:
> As sb_fdblocks will be re-calculated during mount when lazysbcount is enabled,
> We just need to make xfs_validate_sb_write() happy -- make sure sb_fdblocks is
> not genative.

"negative".

This otherwise looks good to me.

--D

> Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_sb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index 73a4b895de67..199756970383 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -1037,7 +1037,7 @@ xfs_log_sb(
>  		mp->m_sb.sb_ifree = min_t(uint64_t,
>  				percpu_counter_sum(&mp->m_ifree),
>  				mp->m_sb.sb_icount);
> -		mp->m_sb.sb_fdblocks = percpu_counter_sum(&mp->m_fdblocks);
> +		mp->m_sb.sb_fdblocks = percpu_counter_sum_positive(&mp->m_fdblocks);
>  	}
>  
>  	xfs_sb_to_disk(bp->b_addr, &mp->m_sb);
> -- 
> 2.39.3 (Apple Git-146)
> 
>