If we have a list of requests in our plug list, send it to the driver in
one go, if possible. The driver must set mq_ops->queue_rqs() to support
this, if not the usual one-by-one path is used.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
  block/blk-mq.c         | 24 +++++++++++++++++++++---
  include/linux/blk-mq.h |  8 ++++++++
  2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 22ec21aa0c22..9ac9174a2ba4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2513,6 +2513,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
  {
  	struct blk_mq_hw_ctx *this_hctx;
  	struct blk_mq_ctx *this_ctx;
+	struct request *rq;
  	unsigned int depth;
  	LIST_HEAD(list);
  

@@ -2521,7 +2522,26 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
  	plug->rq_count = 0;
  
  	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
-		blk_mq_run_dispatch_ops(plug->mq_list->q,
+		struct request_queue *q;
+
+		rq = plug->mq_list;
+		q = rq->q;
+
+		/*
+		 * Peek first request and see if we have a ->queue_rqs() hook.
+		 * If we do, we can dispatch the whole plug list in one go. We
+		 * already know at this point that all requests belong to the
+		 * same queue, caller must ensure that's the case.
+		 */
+		if (q->mq_ops->queue_rqs &&
+		    !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {