[B/C] [PATCH 2/2] block: avoid setting none scheduler if it's already none

[Kai-Heng Feng]

From: Aleksei Zakharov <zakharov.a.g at yandex.ru>

BugLink: https://bugs.launchpad.net/bugs/1815733

There's no reason to freeze queue and remove scheduler
if there's no scheduler already.

Signed-off-by: Aleksei Zakharov <zakharov.a.g at yandex.ru>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(backported from commit fbd72127c975dc8e532ecc73d52f3b1b00935bec)
Signed-off-by: Kai-Heng Feng <kai.heng.feng at canonical.com>
---
 block/elevator.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/elevator.c b/block/elevator.c
index 89a48a3a8c12..14259a524250 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -1090,8 +1090,11 @@ static int __elevator_change(struct request_queue *q, const char *name)
 	/*
 	 * Special case for mq, turn off scheduling
 	 */
-	if (q->mq_ops && !strncmp(name, "none", 4))
+	if (q->mq_ops && !strncmp(name, "none", 4)) {
+		if (!q->elevator)
+			return 0;
 		return elevator_switch(q, NULL);
+	}
 
 	strlcpy(elevator_name, name, sizeof(elevator_name));
 	e = elevator_get(q, strstrip(elevator_name), true);
-- 
2.17.1