block: look up the elevator type in elevator_switch

That makes the function nicely self-contained and can be used
to avoid code duplication.

Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-11-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

block/blk-mq.c

@@ -5058,7 +5058,7 @@ static void blk_mq_elv_switch_back(struct list_head *head,
 	kfree(qe);
 
 	mutex_lock(&q->elevator_lock);
-	elevator_switch(q, t);
+	elevator_switch(q, t->elevator_name);
 	/* drop the reference acquired in blk_mq_elv_switch_none */
 	elevator_put(t);
 	mutex_unlock(&q->elevator_lock);

block/blk.h

@@ -322,7 +322,7 @@ bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
 
 bool blk_insert_flush(struct request *rq);
 
-int elevator_switch(struct request_queue *q, struct elevator_type *new_e);
+int elevator_switch(struct request_queue *q, const char *name);
 void elevator_disable(struct request_queue *q);
 void elevator_exit(struct request_queue *q);
 int elv_register_queue(struct request_queue *q, bool uevent);

block/elevator.c

@@ -621,13 +621,18 @@ void elevator_init_mq(struct request_queue *q)
  * If switching fails, we are most likely running out of memory and not able
  * to restore the old io scheduler, so leaving the io scheduler being none.
  */
-int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
+int elevator_switch(struct request_queue *q, const char *name)
 {
+	struct elevator_type *new_e;
 	int ret;
 
 	WARN_ON_ONCE(q->mq_freeze_depth == 0);
 	lockdep_assert_held(&q->elevator_lock);
 
+	new_e = elevator_find_get(name);
+	if (!new_e)
+		return -EINVAL;
+
 	blk_mq_quiesce_queue(q);
 
 	if (q->elevator) {
@@ -654,6 +659,7 @@ int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 			new_e->elevator_name);
 	}
 
+	elevator_put(new_e);
 	return ret;
 }
 
@@ -679,9 +685,6 @@ void elevator_disable(struct request_queue *q)
  */
 static int elevator_change(struct request_queue *q, const char *elevator_name)
 {
-	struct elevator_type *e;
-	int ret;
-
 	/* Make sure queue is not in the middle of being removed */
 	if (!blk_queue_registered(q))
 		return -ENOENT;
@@ -695,12 +698,7 @@ static int elevator_change(struct request_queue *q, const char *elevator_name)
 	if (q->elevator && elevator_match(q->elevator->type, elevator_name))
 		return 0;
 
-	e = elevator_find_get(elevator_name);
-	if (!e)
-		return -EINVAL;
-	ret = elevator_switch(q, e);
-	elevator_put(e);
-	return ret;
+	return elevator_switch(q, elevator_name);
 }
 
 static void elv_iosched_load_module(char *elevator_name)