linux-kernel - [RFC PATCH 2/2] elevator: restore the old io scheduler if failed to switch to the new one

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Message-Id: <29281ffcdd756bdbdfcee8769cd8b2eb867b74e2.1668772991.git.nickyc975@zju.edu.cn>
Date:   Fri, 18 Nov 2022 20:09:54 +0800
From:   Jinlong Chen <nickyc975@....edu.cn>
To:     axboe@...nel.dk
Cc:     hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, nickyc975@....edu.cn
Subject: [RFC PATCH 2/2] elevator: restore the old io scheduler if failed to switch to the new one

If we failed to switch to the new io scheduler, we should try to restore
the old one instead of just switching to none.

This also makes elevator_switch match its document.

Signed-off-by: Jinlong Chen <nickyc975@....edu.cn>
---
 block/elevator.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 517857a9a68f..b7bd0b8468bd 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -672,6 +672,7 @@ static int __elevator_apply(struct request_queue *q, struct elevator_type *e)
  */
 int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 {
+	struct elevator_type *old_e = NULL;
 	int ret;
 
 	lockdep_assert_held(&q->sysfs_lock);
@@ -680,17 +681,37 @@ int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 	blk_mq_quiesce_queue(q);
 
 	if (q->elevator) {
+		old_e = q->elevator->type;
+		/*
+		 * Keep a reference so we can fallback on failure.
+		 */
+		__elevator_get(old_e);
 		elv_unregister_queue(q);
 		elevator_exit(q);
 	}
 
 	ret = __elevator_apply(q, new_e);
-	if (ret)
-		goto out_unfreeze;
+	if (likely(!ret)) {
+		blk_add_trace_msg(q, "elv switch: %s", new_e->elevator_name);
+	} else if (old_e) {
+		int err;
+
+		err = __elevator_apply(q, old_e);
+		if (unlikely(err)) {
+			blk_add_trace_msg(q,
+				"elv switch failed: %s (%d), fallback failed: %s (%d)",
+				new_e->elevator_name, ret, old_e->elevator_name, err
+			);
+		}
+	}
 
-	blk_add_trace_msg(q, "elv switch: %s", new_e->elevator_name);
+	if (old_e) {
+		/*
+		 * Done, release the reference we kept.
+		 */
+		elevator_put(old_e);
+	}
 
-out_unfreeze:
 	blk_mq_unquiesce_queue(q);
 	blk_mq_unfreeze_queue(q);
 	return ret;
-- 
2.31.1