Re: [PATCH] blk-mq: setup blk_mq_alloc_data.cmd_flags after submit_bio_checks() is done

[Jens Axboe <axboe@kernel.dk>]

On 11/12/21 9:05 AM, Ming Lei wrote:
> On Fri, Nov 12, 2021 at 08:47:01AM -0700, Jens Axboe wrote:
>> On 11/12/21 5:47 AM, Ming Lei wrote:
>>> On Fri, Nov 12, 2021 at 09:44:41AM +0100, Christoph Hellwig wrote:
>>>> On Fri, Nov 12, 2021 at 04:37:19PM +0800, Ming Lei wrote:
>>>>>> 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 <ming.lei@redhat.com>
>>> 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 <ming.lei@redhat.com>
>>> ---
>>>  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;
>>
>> I think we should just check if hctx is the same, that should be enough.
>> We don't need to match the type, just disallow if hw queue has changed.
> 
> But bio doesn't have hw queue. Figuring out exact hw queue seems
> necessary and needs more cpu cycles than getting hctx type.

Thinking about it, if opf and request_queue matches, that should be
enough.

-- 
Jens Axboe