[PATCH] blk-mq: move bio merge attempt to blk_mq_submit_bio()

[PATCH] blk-mq: move bio merge attempt to blk_mq_submit_bio()

[Chengming Zhou]

We will try to get request from cache before alloc request, in
both cases will first attempt bio merge.

This patch move this common part to blk_mq_submit_bio(), which
simplify the code and avoid passing in the pointer of bio.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 block/blk-mq.c | 32 ++++++++++++--------------------
 1 file changed, 12 insertions(+), 20 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5ee62b95f3e5..aa091615e20b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2716,8 +2716,7 @@ static bool blk_mq_attempt_bio_merge(struct request_queue *q,
 
 static struct request *blk_mq_get_new_requests(struct request_queue *q,
 					       struct blk_plug *plug,
-					       struct bio *bio,
-					       unsigned int nsegs)
+					       struct bio *bio)
 {
 	struct blk_mq_alloc_data data = {
 		.q		= q,
@@ -2729,9 +2728,6 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
 	if (unlikely(bio_queue_enter(bio)))
 		return NULL;
 
-	if (blk_mq_attempt_bio_merge(q, bio, nsegs))
-		goto queue_exit;
-
 	rq_qos_throttle(q, bio);
 
 	if (plug) {
@@ -2746,13 +2742,13 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
 	rq_qos_cleanup(q, bio);
 	if (bio->bi_opf & REQ_NOWAIT)
 		bio_wouldblock_error(bio);
-queue_exit:
+
 	blk_queue_exit(q);
 	return NULL;
 }
 
 static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
-		struct blk_plug *plug, struct bio **bio, unsigned int nsegs)
+		struct blk_plug *plug, struct bio *bio)
 {
 	struct request *rq;
 
@@ -2762,14 +2758,9 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 	if (!rq || rq->q != q)
 		return NULL;
 
-	if (blk_mq_attempt_bio_merge(q, *bio, nsegs)) {
-		*bio = NULL;
-		return NULL;
-	}
-
-	if (blk_mq_get_hctx_type((*bio)->bi_opf) != rq->mq_hctx->type)
+	if (blk_mq_get_hctx_type(bio->bi_opf) != rq->mq_hctx->type)
 		return NULL;
-	if (op_is_flush(rq->cmd_flags) != op_is_flush((*bio)->bi_opf))
+	if (op_is_flush(rq->cmd_flags) != op_is_flush(bio->bi_opf))
 		return NULL;
 
 	/*
@@ -2778,9 +2769,9 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 	 * before we throttle.
 	 */
 	plug->cached_rq = rq_list_next(rq);
-	rq_qos_throttle(q, *bio);
+	rq_qos_throttle(q, bio);
 
-	rq->cmd_flags = (*bio)->bi_opf;
+	rq->cmd_flags = bio->bi_opf;
 	INIT_LIST_HEAD(&rq->queuelist);
 	return rq;
 }
@@ -2824,11 +2815,12 @@ void blk_mq_submit_bio(struct bio *bio)
 
 	bio_set_ioprio(bio);
 
-	rq = blk_mq_get_cached_request(q, plug, &bio, nr_segs);
+	if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
+		return;
+
+	rq = blk_mq_get_cached_request(q, plug, bio);
 	if (!rq) {
-		if (!bio)
-			return;
-		rq = blk_mq_get_new_requests(q, plug, bio, nr_segs);
+		rq = blk_mq_get_new_requests(q, plug, bio);
 		if (unlikely(!rq))
 			return;
 	}
-- 
2.36.1

Re: [PATCH] blk-mq: move bio merge attempt to blk_mq_submit_bio()

[Jens Axboe]

On 8/12/22 2:39 AM, Chengming Zhou wrote:
> We will try to get request from cache before alloc request, in
> both cases will first attempt bio merge.
> 
> This patch move this common part to blk_mq_submit_bio(), which
> simplify the code and avoid passing in the pointer of bio.

I don't think this will work. If we get a cached request, then we
know we have a reference to the queue. If you swap these around,
then that's no longer the case and we can have a use-after-free if
the queue goes away.

-- 
Jens Axboe

Re: [PATCH] blk-mq: move bio merge attempt to blk_mq_submit_bio()

[Chengming Zhou]

On 2022/8/12 20:40, Jens Axboe wrote:
> On 8/12/22 2:39 AM, Chengming Zhou wrote:
>> We will try to get request from cache before alloc request, in
>> both cases will first attempt bio merge.
>>
>> This patch move this common part to blk_mq_submit_bio(), which
>> simplify the code and avoid passing in the pointer of bio.
> 
> I don't think this will work. If we get a cached request, then we
> know we have a reference to the queue. If you swap these around,
> then that's no longer the case and we can have a use-after-free if
> the queue goes away.
> 

Sorry, thanks for your reply!