On Fri, Nov 12, 2021 at 09:44:41AM +0100, Christoph Hellwig wrote:

quoted

On Fri, Nov 12, 2021 at 04:37:19PM +0800, Ming Lei wrote:

quoted

quoted

can only be used for reads, and no fua can be set if the preallocating
I/O didn't use fua, etc.

What are the pitfalls of just chanigng cmd_flags?

Then we need to check cmd_flags carefully, such as hctx->type has to
be same, flush & passthrough flags has to be same, that said all
->cmd_flags used for allocating rqs have to be same with the following
bio->bi_opf.

In usual cases, I guess all IOs submitted from same plug batch should be
same type. If not, we can switch to change cmd_flags.

Jens: is this a limit fitting into your use cases?

I guess as a quick fix this rejecting different flags is probably the
best we can do for now, but I suspect we'll want to eventually relax
them.

rw mixed workload will be affected, so I think we need to switch to
change cmd_flags, how about the following patch?

From 9ab77b7adee768272944c20b7cffc8abdb85a35b Mon Sep 17 00:00:00 2001
From: Ming Lei <redacted>
Date: Fri, 12 Nov 2021 08:14:38 +0800
Subject: [PATCH] blk-mq: fix filesystem I/O request allocation

submit_bio_checks() may update bio->bi_opf, so we have to initialize
blk_mq_alloc_data.cmd_flags with bio->bi_opf after submit_bio_checks()
returns when allocating new request.

In case of using cached request, fallback to allocate new request if
cached rq isn't compatible with the incoming bio, otherwise change
rq->cmd_flags with incoming bio->bi_opf.

Fixes: 900e080752025f00 ("block: move queue enter logic into blk_mq_submit_bio()")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <redacted>
---
 block/blk-mq.c | 39 ++++++++++++++++++++++++++++++---------
 block/blk-mq.h | 26 +++++++++++++++-----------
 2 files changed, 45 insertions(+), 20 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f511db395c7f..3ab34c4f20da 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2521,12 +2521,8 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
 	};
 	struct request *rq;
 
-	if (unlikely(bio_queue_enter(bio)))
-		return NULL;
-	if (unlikely(!submit_bio_checks(bio)))
-		goto put_exit;
 	if (blk_mq_attempt_bio_merge(q, bio, nsegs, same_queue_rq))
-		goto put_exit;
+		return NULL;
 
 	rq_qos_throttle(q, bio);
 

@@ -2543,19 +2539,32 @@ 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);
-put_exit:
-	blk_queue_exit(q);
+
 	return NULL;
 }
 
+static inline bool blk_mq_can_use_cached_rq(struct request *rq,
+		struct bio *bio)
+{
+	if (blk_mq_get_hctx_type(bio->bi_opf) != rq->mq_hctx->type)
+		return false;
+
+	if (op_is_flush(rq->cmd_flags) != op_is_flush(bio->bi_opf))
+		return false;
+
+	return true;