I/O scheduler

Linux v6.6: block/elevator.c
Linux v6.6: block/mq-deadline.c
Linux v6.6: block/kyber-iosched.c

enum elv_merge elv_merge(struct request_queue *q, struct request **req,
		struct bio *bio)
{
	struct elevator_queue *e = q->elevator;
	struct request *__rq;

	/*
	 * Levels of merges:
	 * 	nomerges:  No merges at all attempted
	 * 	noxmerges: Only simple one-hit cache try
	 * 	merges:	   All merge tries attempted
	 */
	if (blk_queue_nomerges(q) || !bio_mergeable(bio))
		return ELEVATOR_NO_MERGE;

	/*
	 * First try one-hit cache.
	 */
	if (q->last_merge && elv_bio_merge_ok(q->last_merge, bio)) {
		enum elv_merge ret = blk_try_merge(q->last_merge, bio);

		if (ret != ELEVATOR_NO_MERGE) {
			*req = q->last_merge;
			return ret;
		}
	}

	if (blk_queue_noxmerges(q))
		return ELEVATOR_NO_MERGE;

	/*
	 * See if our hash lookup can find a potential backmerge.
	 */
	__rq = elv_rqhash_find(q, bio->bi_iter.bi_sector);
	if (__rq && elv_bio_merge_ok(__rq, bio)) {
		*req = __rq;

		if (blk_discard_mergable(__rq))
			return ELEVATOR_DISCARD_MERGE;
		return ELEVATOR_BACK_MERGE;
	}

	if (e->type->ops.request_merge)
		return e->type->ops.request_merge(q, req, bio);

	return ELEVATOR_NO_MERGE;
}

/*
 * initialize elevator private data (deadline_data).
 */
static int dd_init_sched(struct request_queue *q, struct elevator_type *e)
{
	struct deadline_data *dd;
	struct elevator_queue *eq;
	enum dd_prio prio;
	int ret = -ENOMEM;

	eq = elevator_alloc(q, e);
	if (!eq)
		return ret;

	dd = kzalloc_node(sizeof(*dd), GFP_KERNEL, q->node);
	if (!dd)
		goto put_eq;

	eq->elevator_data = dd;

	for (prio = 0; prio <= DD_PRIO_MAX; prio++) {
		struct dd_per_prio *per_prio = &dd->per_prio[prio];

		INIT_LIST_HEAD(&per_prio->dispatch);
		INIT_LIST_HEAD(&per_prio->fifo_list[DD_READ]);
		INIT_LIST_HEAD(&per_prio->fifo_list[DD_WRITE]);
		per_prio->sort_list[DD_READ] = RB_ROOT;
		per_prio->sort_list[DD_WRITE] = RB_ROOT;
	}
	dd->fifo_expire[DD_READ] = read_expire;
	dd->fifo_expire[DD_WRITE] = write_expire;
	dd->writes_starved = writes_starved;
	dd->front_merges = 1;
	dd->last_dir = DD_WRITE;
	dd->fifo_batch = fifo_batch;
	dd->prio_aging_expire = prio_aging_expire;
	spin_lock_init(&dd->lock);
	spin_lock_init(&dd->zone_lock);

	/* We dispatch from request queue wide instead of hw queue */
	blk_queue_flag_set(QUEUE_FLAG_SQ_SCHED, q);

	q->elevator = eq;
	return 0;

put_eq:
	kobject_put(&eq->kobj);
	return ret;
}