block: add struct elv_change_ctx for unifying elevator change

JIRA: https://issues.redhat.com/browse/RHEL-106845

commit 1e9db5c
Author: Ming Lei <ming.lei@redhat.com>
Date:   Mon May 5 22:17:53 2025 +0800

    block: add `struct elv_change_ctx` for unifying elevator change

    Add `struct elv_change_ctx` and prepare for unifying elevator change by
    elevator_change(). With this way, any input & output parameter can
    be provided & observed in top helper.

    This way helps to move kobject add/delete & debugfs register/unregister
    out of ->elevator_lock & freezing queue.

    Reviewed-by: Hannes Reinecke <hare@suse.de>
    Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Link: https://lore.kernel.org/r/20250505141805.2751237-16-ming.lei@redhat.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/elevator.c

@@ -45,6 +45,12 @@
 #include "blk-wbt.h"
 #include "blk-cgroup.h"
 
+/* Holding context data for changing elevator */
+struct elv_change_ctx {
+	const char *name;
+	bool no_uevent;
+};
+
 static DEFINE_SPINLOCK(elv_list_lock);
 static LIST_HEAD(elv_list);
 
@@ -621,16 +627,16 @@ 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.
  */
-static int elevator_switch(struct request_queue *q, const char *name)
+static int elevator_switch(struct request_queue *q, struct elv_change_ctx *ctx)
 {
 	struct elevator_type *new_e = NULL;
 	int ret = 0;
 
 	WARN_ON_ONCE(q->mq_freeze_depth == 0);
 	lockdep_assert_held(&q->elevator_lock);
 
-	if (strncmp(name, "none", 4)) {
-		new_e = elevator_find_get(name);
+	if (strncmp(ctx->name, "none", 4)) {
+		new_e = elevator_find_get(ctx->name);
 		if (!new_e)
 			return -EINVAL;
 	}
@@ -646,7 +652,7 @@ static int elevator_switch(struct request_queue *q, const char *name)
 		ret = blk_mq_init_sched(q, new_e);
 		if (ret)
 			goto out_unfreeze;
-		ret = elv_register_queue(q, true);
+		ret = elv_register_queue(q, !ctx->no_uevent);
 		if (ret) {
 			elevator_exit(q);
 			goto out_unfreeze;
@@ -656,7 +662,7 @@ static int elevator_switch(struct request_queue *q, const char *name)
 		q->elevator = NULL;
 		q->nr_requests = q->tag_set->queue_depth;
 	}
-	blk_add_trace_msg(q, "elv switch: %s", name);
+	blk_add_trace_msg(q, "elv switch: %s", ctx->name);
 
 out_unfreeze:
 	blk_mq_unquiesce_queue(q);
@@ -674,7 +680,7 @@ static int elevator_switch(struct request_queue *q, const char *name)
 /*
  * Switch this queue to the given IO scheduler.
  */
-static int elevator_change(struct request_queue *q, const char *elevator_name)
+static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
 {
 	unsigned int memflags;
 	int ret = 0;
@@ -683,9 +689,8 @@ static int elevator_change(struct request_queue *q, const char *elevator_name)
 
 	memflags = blk_mq_freeze_queue(q);
 	mutex_lock(&q->elevator_lock);
-	if (!(q->elevator && elevator_match(q->elevator->type,
-				elevator_name)))
-		ret = elevator_switch(q, elevator_name);
+	if (!(q->elevator && elevator_match(q->elevator->type, ctx->name)))
+		ret = elevator_switch(q, ctx);
 	mutex_unlock(&q->elevator_lock);
 	blk_mq_unfreeze_queue(q, memflags);
 	return ret;
@@ -701,15 +706,17 @@ void elv_update_nr_hw_queues(struct request_queue *q)
 
 	mutex_lock(&q->elevator_lock);
 	if (q->elevator && !blk_queue_dying(q) && !blk_queue_registered(q)) {
-		const char *name = q->elevator->type->elevator_name;
+		struct elv_change_ctx ctx = {
+			.name = q->elevator->type->elevator_name,
+		};
 
 		/* force to reattach elevator after nr_hw_queue is updated */
-		elevator_switch(q, name);
+		elevator_switch(q, &ctx);
 	}
 	mutex_unlock(&q->elevator_lock);
 }
 
-static void elv_iosched_load_module(char *elevator_name)
+static void elv_iosched_load_module(const char *elevator_name)
 {
 	struct elevator_type *found;
 
@@ -725,7 +732,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 			  size_t count)
 {
 	char elevator_name[ELV_NAME_MAX];
-	char *name;
+	struct elv_change_ctx ctx = {};
 	int ret;
 	struct request_queue *q = disk->queue;
 	struct blk_mq_tag_set *set = q->tag_set;
@@ -740,12 +747,12 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 	 * queue is the one for the device storing the module file.
 	 */
 	strscpy(elevator_name, buf, sizeof(elevator_name));
-	name = strstrip(elevator_name);
+	ctx.name = strstrip(elevator_name);
 
-	elv_iosched_load_module(name);
+	elv_iosched_load_module(ctx.name);
 
 	down_read(&set->update_nr_hwq_lock);
-	ret = elevator_change(q, name);
+	ret = elevator_change(q, &ctx);
 	if (!ret)
 		ret = count;
 	up_read(&set->update_nr_hwq_lock);