block: Merge bio before checking ->cached_rq

It checks if plug->cached_rq is empty before merging bio. But the merge action
doesn't have relationship with plug->cached_rq, it trys to merge bio with
requests within plug->mq_list. Now it checks if ->cached_rq is empty before
merging bio. If it's empty, it will miss the merge chances. So move the merge
function before checking ->cached_rq.

Signed-off-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230209031930.27354-1-xni@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 89b4dd8..08093d4 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2879,15 +2879,16 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 
        if (!plug)
                return NULL;
-       rq = rq_list_peek(&plug->cached_rq);
-       if (!rq || rq->q != q)
-               return NULL;
 
        if (blk_mq_attempt_bio_merge(q, *bio, nsegs)) {
                *bio = NULL;
                return NULL;
        }
 
+       rq = rq_list_peek(&plug->cached_rq);
+       if (!rq || rq->q != q)
+               return NULL;
+
        type = blk_mq_get_hctx_type((*bio)->bi_opf);
        hctx_type = rq->mq_hctx->type;
        if (type != hctx_type &&