[PATCH 6.1 16/28] block: fix hctx checks for batch allocation - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev, Pavel Begunkov <asml.silence@gmail.com>,
	Jens Axboe <axboe@kernel.dk>, Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.1 16/28] block: fix hctx checks for batch allocation
Date: Fri,  3 Feb 2023 11:13:04 +0100	[thread overview]
Message-ID: <20230203101010.657044513@linuxfoundation.org> (raw)
In-Reply-To: <20230203101009.946745030@linuxfoundation.org>

From: Pavel Begunkov <asml.silence@gmail.com>

[ Upstream commit 7746564793978fe2f43b18a302b22dca0ad3a0e8 ]

When there are no read queues read requests will be assigned a
default queue on allocation. However, blk_mq_get_cached_request() is not
prepared for that and will fail all attempts to grab read requests from
the cache. Worst case it doubles the number of requests allocated,
roughly half of which will be returned by blk_mq_free_plug_rqs().

It only affects batched allocations and so is io_uring specific.
For reference, QD8 t/io_uring benchmark improves by 20-35%.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/80d4511011d7d4751b4cf6375c4e38f237d935e3.1673955390.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 block/blk-mq.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 63abbe342b28..83fbc7c54617 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2858,6 +2858,7 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 		struct blk_plug *plug, struct bio **bio, unsigned int nsegs)
 {
 	struct request *rq;
+	enum hctx_type type, hctx_type;
 
 	if (!plug)
 		return NULL;
@@ -2870,7 +2871,10 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 		return NULL;
 	}
 
-	if (blk_mq_get_hctx_type((*bio)->bi_opf) != rq->mq_hctx->type)
+	type = blk_mq_get_hctx_type((*bio)->bi_opf);
+	hctx_type = rq->mq_hctx->type;
+	if (type != hctx_type &&
+	    !(type == HCTX_TYPE_READ && hctx_type == HCTX_TYPE_DEFAULT))
 		return NULL;
 	if (op_is_flush(rq->cmd_flags) != op_is_flush((*bio)->bi_opf))
 		return NULL;
-- 
2.39.0



