xfs: fix sb write verify for lazysbcount

Source kernel commit: 7cecd500d90164419add650e26cc1de03a7a66cb

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>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/libxfs/xfs_sb.c b/libxfs/xfs_sb.c
index fc33dc4a2a..d05f0e6e4b 100644
--- a/libxfs/xfs_sb.c
+++ b/libxfs/xfs_sb.c
@@ -970,7 +970,9 @@ 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);
 	}