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 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.


-- 
Jens Axboe