Re: [PATCH V4] xfs: make sure sb_fdblocks is non-negative

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

On Thu, Jun 06, 2024 at 11:11:57AM -0700, Wengang Wang wrote:
> A user with a completely full filesystem experienced an unexpected
> shutdown when the filesystem tried to write the superblock during
> runtime.
> kernel shows the following dmesg:
> 
> [    8.176281] XFS (dm-4): Metadata corruption detected at xfs_sb_write_verify+0x60/0x120 [xfs], xfs_sb block 0x0
> [    8.177417] XFS (dm-4): Unmount and run xfs_repair
> [    8.178016] XFS (dm-4): First 128 bytes of corrupted metadata buffer:
> [    8.178703] 00000000: 58 46 53 42 00 00 10 00 00 00 00 00 01 90 00 00  XFSB............
> [    8.179487] 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> [    8.180312] 00000020: cf 12 dc 89 ca 26 45 29 92 e6 e3 8d 3b b8 a2 c3  .....&E)....;...
> [    8.181150] 00000030: 00 00 00 00 01 00 00 06 00 00 00 00 00 00 00 80  ................
> [    8.182003] 00000040: 00 00 00 00 00 00 00 81 00 00 00 00 00 00 00 82  ................
> [    8.182004] 00000050: 00 00 00 01 00 64 00 00 00 00 00 04 00 00 00 00  .....d..........
> [    8.182004] 00000060: 00 00 64 00 b4 a5 02 00 02 00 00 08 00 00 00 00  ..d.............
> [    8.182005] 00000070: 00 00 00 00 00 00 00 00 0c 09 09 03 17 00 00 19  ................
> [    8.182008] XFS (dm-4): Corruption of in-memory data detected.  Shutting down filesystem
> [    8.182010] XFS (dm-4): Please unmount the filesystem and rectify the problem(s)
> 
> When xfs_log_sb writes super block to disk, b_fdblocks is fetched from
> m_fdblocks without any lock. As m_fdblocks can experience a positive ->
> 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 nenative. This patch also takes care of other percpu
> counters in xfs_log_sb.
> 
> Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>

Seems fine to me,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> V3 -> V4: takes care of other percpu counters
> V2 -> V3: break the line to ensure it isn't overly long
> V1 -> V2: add problem symptoms in patch description.
> ---
>  fs/xfs/libxfs/xfs_sb.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index 09e4bf949bf8..6b56f0f6d4c1 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -1038,11 +1038,12 @@ xfs_log_sb(
>  	 * and hence we don't need have to update it here.
>  	 */
>  	if (xfs_has_lazysbcount(mp)) {
> -		mp->m_sb.sb_icount = percpu_counter_sum(&mp->m_icount);
> +		mp->m_sb.sb_icount = percpu_counter_sum_positive(&mp->m_icount);
>  		mp->m_sb.sb_ifree = min_t(uint64_t,
> -				percpu_counter_sum(&mp->m_ifree),
> +				percpu_counter_sum_positive(&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)
> 
>