f2fs: don't start checkpoint thread in readonly mountpoint

In readonly mountpoint, there should be no write IOs include checkpoint
IO, so that it's not needed to create kernel checkpoint thread.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -2069,8 +2069,10 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 		}
 	}
 
-	if (!test_opt(sbi, DISABLE_CHECKPOINT) &&
-			test_opt(sbi, MERGE_CHECKPOINT)) {
+	if ((*flags & SB_RDONLY) || test_opt(sbi, DISABLE_CHECKPOINT) ||
+			!test_opt(sbi, MERGE_CHECKPOINT)) {
+		f2fs_stop_ckpt_thread(sbi);
+	} else {
 		err = f2fs_start_ckpt_thread(sbi);
 		if (err) {
 			f2fs_err(sbi,
@@ -2078,8 +2080,6 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 			    err);
 			goto restore_gc;
 		}
-	} else {
-		f2fs_stop_ckpt_thread(sbi);
 	}
 
 	/*
@@ -3835,7 +3835,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 
 	/* setup checkpoint request control and start checkpoint issue thread */
 	f2fs_init_ckpt_req_control(sbi);
-	if (!test_opt(sbi, DISABLE_CHECKPOINT) &&
+	if (!f2fs_readonly(sb) && !test_opt(sbi, DISABLE_CHECKPOINT) &&
 			test_opt(sbi, MERGE_CHECKPOINT)) {
 		err = f2fs_start_ckpt_thread(sbi);
 		if (err) {