Re: [PATCH v3] xfs: fix sb write verify for lazysbcount

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

On Thu, Nov 03, 2022 at 11:47:36AM +0800, Long Li wrote:
> When lazysbcount is enabled, fsstress and loop mount/unmount test report
> the following problems:
> 
> XFS (loop0): SB summary counter sanity check failed
> XFS (loop0): Metadata corruption detected at xfs_sb_write_verify+0x13b/0x460,
> 	xfs_sb block 0x0
> XFS (loop0): Unmount and run xfs_repair
> XFS (loop0): First 128 bytes of corrupted metadata buffer:
> 00000000: 58 46 53 42 00 00 10 00 00 00 00 00 00 28 00 00  XFSB.........(..
> 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00000020: 69 fb 7c cd 5f dc 44 af 85 74 e0 cc d4 e3 34 5a  i.|._.D..t....4Z
> 00000030: 00 00 00 00 00 20 00 06 00 00 00 00 00 00 00 80  ..... ..........
> 00000040: 00 00 00 00 00 00 00 81 00 00 00 00 00 00 00 82  ................
> 00000050: 00 00 00 01 00 0a 00 00 00 00 00 04 00 00 00 00  ................
> 00000060: 00 00 0a 00 b4 b5 02 00 02 00 00 08 00 00 00 00  ................
> 00000070: 00 00 00 00 00 00 00 00 0c 09 09 03 14 00 00 19  ................
> XFS (loop0): Corruption of in-memory data (0x8) detected at _xfs_buf_ioapply
> 	+0xe1e/0x10e0 (fs/xfs/xfs_buf.c:1580).  Shutting down filesystem.
> XFS (loop0): Please unmount the filesystem and rectify the problem(s)
> XFS (loop0): log mount/recovery failed: error -117
> XFS (loop0): log mount failed
> 
> This corruption will shutdown the file system and the file system will
> no longer be mountable. The following script can reproduce the problem,
> but it may take a long time.
> 
>  #!/bin/bash
> 
>  device=/dev/sda
>  testdir=/mnt/test
>  round=0
> 
>  function fail()
>  {
> 	 echo "$*"
> 	 exit 1
>  }
> 
>  mkdir -p $testdir
>  while [ $round -lt 10000 ]
>  do
> 	 echo "******* round $round ********"
> 	 mkfs.xfs -f $device
> 	 mount $device $testdir || fail "mount failed!"
> 	 fsstress -d $testdir -l 0 -n 10000 -p 4 >/dev/null &
> 	 sleep 4
> 	 killall -w fsstress
> 	 umount $testdir
> 	 xfs_repair -e $device > /dev/null
> 	 if [ $? -eq 2 ];then
> 		 echo "ERR CODE 2: Dirty log exception during repair."
> 		 exit 1
> 	 fi
> 	 round=$(($round+1))
>  done
> 
> With lazysbcount is enabled, There is no additional lock protection for
> reading m_ifree and m_icount in xfs_log_sb(), if other cpu modifies the
> m_ifree, this will make the m_ifree greater than m_icount. For example,
> consider the following sequence and ifreedelta is postive:
> 
>  CPU0				 CPU1
>  xfs_log_sb			 xfs_trans_unreserve_and_mod_sb
>  ----------			 ------------------------------
>  percpu_counter_sum(&mp->m_icount)
> 				 percpu_counter_add_batch(&mp->m_icount,
> 						idelta, XFS_ICOUNT_BATCH)
> 				 percpu_counter_add(&mp->m_ifree, ifreedelta);
>  percpu_counter_sum(&mp->m_ifree)
> 
> After this, incorrect inode count (sb_ifree > sb_icount) will be writen to
> the log. In the subsequent writing of sb, incorrect inode count (sb_ifree >
> sb_icount) will fail to pass the boundary check in xfs_validate_sb_write()
> that cause the file system shutdown.
> 
> When lazysbcount is enabled, we don't need to guarantee that Lazy sb
> counters are completely correct, but we do need to guarantee that sb_ifree
> <= sb_icount. On the other hand, the constraint that m_ifree <= m_icount
> must be satisfied any time that there /cannot/ be other threads allocating
> or freeing inode chunks. If the constraint is violated under these
> circumstances, sb_i{count,free} (the ondisk superblock inode counters)
> maybe incorrect and need to be marked sick at unmount, the count will
> be rebuilt on the next mount.
> 
> Fixes: 8756a5af1819 ("libxfs: add more bounds checking to sb sanity checks")
> Signed-off-by: Long Li <leo.lilong@huawei.com>
> ---
> v3:
> - Corrected the description of the cause of the problem 
> - Add a check for m_icount and m_ifree at unmout
> v2:
> - Add scripts that could reproduce the problem
> - Guaranteed that ifree will never be logged as being greater than icount
> 
>  fs/xfs/libxfs/xfs_sb.c |  4 +++-
>  fs/xfs/xfs_mount.c     | 15 +++++++++++++++
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index a20cade590e9..1eeecf2eb2a7 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -972,7 +972,9 @@ xfs_log_sb(
>  	 */
>  	if (xfs_has_lazysbcount(mp)) {
>  		mp->m_sb.sb_icount = percpu_counter_sum(&mp->m_icount);
> -		mp->m_sb.sb_ifree = percpu_counter_sum(&mp->m_ifree);
> +		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);
>  	}
>  
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index e8bb3c2e847e..fb87ffb48f7f 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -538,6 +538,20 @@ xfs_check_summary_counts(
>  	return 0;
>  }
>  
> +static void
> +xfs_unmount_check(

I'd have called this xfs_ifree_unmount or something, but as this is a
fix for a race condition and I'd like to get this all into 6.1 before
-rc4 so I can start working on 6.2, so I'll change the name and commit
it.  Thank you for digging into this.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> +	struct xfs_mount	*mp)
> +{
> +	if (xfs_is_shutdown(mp))
> +		return;
> +
> +	if (percpu_counter_sum(&mp->m_ifree) >
> +			percpu_counter_sum(&mp->m_icount)) {
> +		xfs_alert(mp, "ifree/icount mismatch at unmount");
> +		xfs_fs_mark_sick(mp, XFS_SICK_FS_COUNTERS);
> +	}
> +}
> +
>  /*
>   * Flush and reclaim dirty inodes in preparation for unmount. Inodes and
>   * internal inode structures can be sitting in the CIL and AIL at this point,
> @@ -1077,6 +1091,7 @@ xfs_unmountfs(
>  	if (error)
>  		xfs_warn(mp, "Unable to free reserved block pool. "
>  				"Freespace may not be correct on next mount.");
> +	xfs_unmount_check(mp);
>  
>  	xfs_log_unmount(mp);
>  	xfs_da_unmount(mp);
> -- 
> 2.31.1
>