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

[Ming Lei <ming.lei@redhat.com>]

On Fri, Nov 12, 2021 at 09:08:39AM -0700, Jens Axboe wrote:
> 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.

I think that is same with hctx->type check: POLLED & OP needs to be
same between the request and bio, and op_is_flush(), or could you
explain how to run the exact check on opf?


Thanks,
Ming