blk-mq: introduce a blk_mq_peek_cached_request helper

Add a new helper to check if there is suitable cached request in
blk_mq_submit_bio.  This removes open coded logic in blk_mq_submit_bio
and moves some checks that so far are in blk_mq_use_cached_rq to
be performed earlier.  This avoids the case where we first do check
with the cached request but then later end up allocating a new one
anyway and need to grab a queue reference.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240124092658.2258309-3-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -2909,22 +2909,31 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
 }
 
 /*
- * Check if we can use the passed on request for submitting the passed in bio,
- * and remove it from the request list if it can be used.
+ * Check if there is a suitable cached request and return it.
  */
-static bool blk_mq_use_cached_rq(struct request *rq, struct blk_plug *plug,
-		struct bio *bio)
+static struct request *blk_mq_peek_cached_request(struct blk_plug *plug,
+		struct request_queue *q, blk_opf_t opf)
 {
-	enum hctx_type type = blk_mq_get_hctx_type(bio->bi_opf);
-	enum hctx_type hctx_type = rq->mq_hctx->type;
+	enum hctx_type type = blk_mq_get_hctx_type(opf);
+	struct request *rq;
 
-	WARN_ON_ONCE(rq_list_peek(&plug->cached_rq) != rq);
+	if (!plug)
+		return NULL;
+	rq = rq_list_peek(&plug->cached_rq);
+	if (!rq || rq->q != q)
+		return NULL;
+	if (type != rq->mq_hctx->type &&
+	    (type != HCTX_TYPE_READ || rq->mq_hctx->type != HCTX_TYPE_DEFAULT))
+		return NULL;
+	if (op_is_flush(rq->cmd_flags) != op_is_flush(opf))
+		return NULL;
+	return rq;
+}
 
-	if (type != hctx_type &&
-	    !(type == HCTX_TYPE_READ && hctx_type == HCTX_TYPE_DEFAULT))
-		return false;
-	if (op_is_flush(rq->cmd_flags) != op_is_flush(bio->bi_opf))
-		return false;
+static void blk_mq_use_cached_rq(struct request *rq, struct blk_plug *plug,
+		struct bio *bio)
+{
+	WARN_ON_ONCE(rq_list_peek(&plug->cached_rq) != rq);
 
 	/*
 	 * If any qos ->throttle() end up blocking, we will have flushed the
@@ -2937,7 +2946,6 @@ static bool blk_mq_use_cached_rq(struct request *rq, struct blk_plug *plug,
 	blk_mq_rq_time_init(rq, 0);
 	rq->cmd_flags = bio->bi_opf;
 	INIT_LIST_HEAD(&rq->queuelist);
-	return true;
 }
 
 /**
@@ -2965,11 +2973,7 @@ void blk_mq_submit_bio(struct bio *bio)
 
 	bio = blk_queue_bounce(bio, q);
 
-	if (plug) {
-		rq = rq_list_peek(&plug->cached_rq);
-		if (rq && rq->q != q)
-			rq = NULL;
-	}
+	rq = blk_mq_peek_cached_request(plug, q, bio->bi_opf);
 	if (rq) {
 		if (unlikely(bio_may_exceed_limits(bio, &q->limits))) {
 			bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
@@ -2980,10 +2984,10 @@ void blk_mq_submit_bio(struct bio *bio)
 			return;
 		if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
 			return;
-		if (blk_mq_use_cached_rq(rq, plug, bio))
+		blk_mq_use_cached_rq(rq, plug, bio);
 		goto done;
-		percpu_ref_get(&q->q_usage_counter);
-	} else {
+	}
+
 	if (unlikely(bio_queue_enter(bio)))
 		return;
 	if (unlikely(bio_may_exceed_limits(bio, &q->limits))) {
@@ -2993,7 +2997,6 @@ void blk_mq_submit_bio(struct bio *bio)
 	}
 	if (!bio_integrity_prep(bio))
 		goto queue_exit;
-	}
 
 	if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
 		goto queue_exit;