ext4: clear mmp sequence number when remounting read-only

Previously, when an MMP-protected file system is remounted read-only,
the kmmpd thread would exit the next time it woke up (a few seconds
later), without resetting the MMP sequence number back to
EXT4_MMP_SEQ_CLEAN.

Fix this by explicitly killing the MMP thread when the file system is
remounted read-only.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Andreas Dilger <adilger@dilger.ca>

fs/ext4/mmp.c

@@ -186,11 +186,8 @@ static int kmmpd(void *data)
 			goto exit_thread;
 		}
 
-		if (sb_rdonly(sb)) {
-			ext4_warning(sb, "kmmpd being stopped since filesystem "
-				     "has been remounted as readonly.");
-			goto exit_thread;
-		}
+		if (sb_rdonly(sb))
+			break;
 
 		diff = jiffies - last_update_time;
 		if (diff < mmp_update_interval * HZ)

fs/ext4/super.c

@@ -5212,6 +5212,8 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 
 			if (sbi->s_journal)
 				ext4_mark_recovery_complete(sb, es);
+			if (sbi->s_mmp_tsk)
+				kthread_stop(sbi->s_mmp_tsk);
 		} else {
 			/* Make sure we can mount this feature set readwrite */
 			if (ext4_has_feature_readonly(sb) ||