blk-mq: special case cached requests less

Share the main merge / split / integrity preparation code between the
cached request vs newly allocated request cases, and add comments
explaining the cached request handling.

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/20240124092658.2258309-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ad57a25..c53a196 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2967,29 +2967,24 @@ void blk_mq_submit_bio(struct bio *bio)
        struct blk_plug *plug = blk_mq_plug(bio);
        const int is_sync = op_is_sync(bio->bi_opf);
        struct blk_mq_hw_ctx *hctx;
-       struct request *rq = NULL;
        unsigned int nr_segs = 1;
+       struct request *rq;
        blk_status_t ret;
 
        bio = blk_queue_bounce(bio, q);
 
+       /*
+        * If the plug has a cached request for this queue, try use it.
+        *
+        * The cached request already holds a q_usage_counter reference and we
+        * don't have to acquire a new one if we use it.
+        */
        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);
-                       if (!bio)
-                               return;
-               }
-               if (!bio_integrity_prep(bio))
-                       return;
-               if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
+       if (!rq) {
+               if (unlikely(bio_queue_enter(bio)))
                        return;
-               blk_mq_use_cached_rq(rq, plug, bio);
-               goto done;
        }
 
-       if (unlikely(bio_queue_enter(bio)))
-               return;
        if (unlikely(bio_may_exceed_limits(bio, &q->limits))) {
                bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
                if (!bio)
@@ -3001,11 +2996,14 @@ void blk_mq_submit_bio(struct bio *bio)
        if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
                goto queue_exit;
 
-       rq = blk_mq_get_new_requests(q, plug, bio, nr_segs);
-       if (unlikely(!rq))
-               goto queue_exit;
+       if (!rq) {
+               rq = blk_mq_get_new_requests(q, plug, bio, nr_segs);
+               if (unlikely(!rq))
+                       goto queue_exit;
+       } else {
+               blk_mq_use_cached_rq(rq, plug, bio);
+       }
 
-done:
        trace_block_getrq(bio);
 
        rq_qos_track(q, rq, bio);
@@ -3039,7 +3037,12 @@ done:
        return;
 
 queue_exit:
-       blk_queue_exit(q);
+       /*
+        * Don't drop the queue reference if we were trying to use a cached
+        * request and thus didn't acquire one.
+        */
+       if (!rq)
+               blk_queue_exit(q);
 }
 
 #ifdef CONFIG_BLK_MQ_STACKING