[PATCH -next 6/6] ext4: fix possible store wrong check interval value in disk when umount

[Ye Bin]

Test follow steps:
1. mkfs.ext4 /dev/sda -O mmp
2. mount /dev/sda  /mnt
3. wait for about 1 minute
4. umount mnt
5. debugfs /dev/sda
6. dump_mmp
7. fsck.ext4 /dev/sda

I found 'check_interval' is range in [5, 10]. And sometime run fsck
print "MMP interval is 10 seconds and total wait time is 42 seconds.
Please wait...".
kmmpd:
...
	if (diff < mmp_update_interval * HZ)
		schedule_timeout_interruptible(mmp_update_interval * HZ - diff);
	 diff = jiffies - last_update_time;
...
	mmp_check_interval = max(min(EXT4_MMP_CHECK_MULT * diff / HZ,
				EXT4_MMP_MAX_CHECK_INTERVAL),
			        EXT4_MMP_MIN_CHECK_INTERVAL);
	mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);
...
We will call ext4_stop_mmpd to stop kmmpd kthread when umount, and
schedule_timeout_interruptible will be interrupted, so 'diff' maybe
little than mmp_update_interval. Then mmp_check_interval will range
in [EXT4_MMP_MAX_CHECK_INTERVAL, EXT4_MMP_CHECK_MULT * diff / HZ].
To solve this issue, if 'diff' little then mmp_update_interval * HZ
just break loop, don't update check interval.

Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
---
 fs/ext4/mmp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index eed854bb6194..41c0a03019db 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -205,6 +205,14 @@ static int kmmpd(void *data)
 			schedule_timeout_interruptible(mmp_update_interval *
 						       HZ - diff);
 			diff = jiffies - last_update_time;
+			/* If 'diff' little 'than mmp_update_interval * HZ', it
+			 * means someone call ext4_stop_mmpd to stop kmmpd
+			 * kthread. We don't need to update mmp_check_interval
+			 * any more, as 'diff' is not exact value.
+			 */
+			if (unlikely(diff < mmp_update_interval * HZ &&
+			    kthread_should_stop()))
+				break;
 		}
 
 		/*
-- 
2.31.1