Re: [PATCH] blk-mq: check passthrough state before reusing cached requests

[Keith Busch <kbusch@kernel.org>]

On Wed, Sep 16, 2026 at 11:16:55PM +0800, huhai wrote:
> @@ -648,6 +648,8 @@ static struct request *blk_mq_alloc_cached_request(struct request_queue *q,
>  			return NULL;
>  		if (op_is_flush(rq->cmd_flags) != op_is_flush(opf))
>  			return NULL;
> +		if (blk_rq_is_passthrough(rq) != blk_op_is_passthrough(opf))
> +			return NULL;

I see how this addresses your observation, but I don't think this is the
right way to fix it. The batch of requests allocated to prime the cache
shouldn't be locked into a specific type of command that can use it,
IMO. This is a bit more involved of a change, but it should allow more
flexible use of the cached requests. Here's a quick idea to attempt
that:

---
diff --git a/block/blk-mq.c b/block/blk-mq.c
index a26a11c73ee3e..c1fd20807da30 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -447,17 +447,28 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	WRITE_ONCE(rq->deadline, 0);
 	req_ref_set(rq, 1);
 
-	if (rq->rq_flags & RQF_USE_SCHED) {
-		struct elevator_queue *e = data->q->elevator;
+	return rq;
+}
+
+static bool blk_op_bypass_sched(blk_opf_t opf)
+{
+	return (opf & REQ_OP_MASK) == REQ_OP_FLUSH ||
+	       blk_op_is_passthrough(opf);
+}
 
-		INIT_HLIST_NODE(&rq->hash);
-		RB_CLEAR_NODE(&rq->rb_node);
+static void blk_mq_set_rq_sched(struct request *rq, blk_opf_t opf)
+{
+	struct elevator_queue *e = rq->q->elevator;
 
-		if (e->type->ops.prepare_request)
-			e->type->ops.prepare_request(rq);
-	}
+	if (!e || blk_op_bypass_sched(opf))
+		return;
 
-	return rq;
+	rq->rq_flags |= RQF_USE_SCHED;
+	INIT_HLIST_NODE(&rq->hash);
+	RB_CLEAR_NODE(&rq->rb_node);
+
+	if (e->type->ops.prepare_request)
+		e->type->ops.prepare_request(rq);
 }
 
 static inline struct request *
@@ -518,12 +529,10 @@ static void blk_mq_limit_depth(struct blk_mq_alloc_data *data)
 	 * Flush/passthrough requests are special and go directly to the
 	 * dispatch list, they are not subject to the async_depth limit.
 	 */
-	if ((data->cmd_flags & REQ_OP_MASK) == REQ_OP_FLUSH ||
-	    blk_op_is_passthrough(data->cmd_flags))
+	if (blk_op_bypass_sched(data->cmd_flags))
 		return;
 
 	WARN_ON_ONCE(data->flags & BLK_MQ_REQ_RESERVED);
-	data->rq_flags |= RQF_USE_SCHED;
 
 	/*
 	 * By default, sync requests have no limit, and async requests are
@@ -563,6 +572,7 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 		rq = __blk_mq_alloc_requests_batch(data);
 		if (rq) {
 			blk_mq_rq_time_init(rq, alloc_time_ns);
+			blk_mq_set_rq_sched(rq, data->cmd_flags);
 			return rq;
 		}
 		data->nr_tags = 1;
@@ -591,6 +601,7 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
 		blk_mq_inc_active_requests(data->hctx);
 	rq = blk_mq_rq_ctx_init(data, blk_mq_tags_from_data(data), tag);
 	blk_mq_rq_time_init(rq, alloc_time_ns);
+	blk_mq_set_rq_sched(rq, data->cmd_flags);
 	return rq;
 }
 
@@ -637,8 +648,6 @@ static struct request *blk_mq_alloc_cached_request(struct request_queue *q,
 		if (plug->nr_ios == 1)
 			return NULL;
 		rq = blk_mq_rq_cache_fill(q, plug, opf, flags);
-		if (!rq)
-			return NULL;
 	} else {
 		rq = rq_list_peek(&plug->cached_rqs);
 		if (!rq || rq->q != q)
@@ -646,15 +655,14 @@ static struct request *blk_mq_alloc_cached_request(struct request_queue *q,
 
 		if (blk_mq_get_hctx_type(opf) != rq->mq_hctx->type)
 			return NULL;
-		if (op_is_flush(rq->cmd_flags) != op_is_flush(opf))
-			return NULL;
 
 		rq_list_pop(&plug->cached_rqs);
 		blk_mq_rq_time_init(rq, blk_time_get_ns());
+		rq->cmd_flags = opf;
+		INIT_LIST_HEAD(&rq->queuelist);
+		blk_mq_set_rq_sched(rq, opf);
 	}
 
-	rq->cmd_flags = opf;
-	INIT_LIST_HEAD(&rq->queuelist);
 	return rq;
 }
 
@@ -3060,8 +3068,6 @@ static struct request *blk_mq_get_cached_request(struct blk_plug *plug,
 	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;
 	rq_list_pop(&plug->cached_rqs);
 	return rq;
 }
@@ -3166,6 +3172,7 @@ void blk_mq_submit_bio(struct bio *bio)
 		blk_mq_rq_time_init(rq, blk_time_get_ns());
 		rq->cmd_flags = bio->bi_opf;
 		INIT_LIST_HEAD(&rq->queuelist);
+		blk_mq_set_rq_sched(rq, bio->bi_opf);
 	} else {
 		rq = blk_mq_get_new_requests(q, plug, bio);
 		if (unlikely(!rq)) {
--