Re: [PATCH 12/15] block: switch polling to be bio based

From: Ming Lei <ming.lei@redhat.com>
To: Sagi Grimberg <sagi@grimberg.me>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Jeffle Xu <jefflexu@linux.alibaba.com>,
	Damien Le Moal <Damien.LeMoal@wdc.com>,
	Keith Busch <kbusch@kernel.org>,
	"Wunderlich, Mark" <mark.wunderlich@intel.com>,
	"Vasudevan, Anil" <anil.vasudevan@intel.com>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH 12/15] block: switch polling to be bio based
Date: Thu, 13 May 2021 09:26:30 +0800	[thread overview]
Message-ID: <YJyARt6S2JQ9H/Ew@T590> (raw)
In-Reply-To: <45d66945-165c-ae48-69f4-75dc553b0386@grimberg.me>

On Wed, May 12, 2021 at 03:03:40PM -0700, Sagi Grimberg wrote:
> 
> 
> On 5/12/21 6:15 AM, Christoph Hellwig wrote:
> > Replace the blk_poll interface that requires the caller to keep a queue
> > and cookie from the submissions with polling based on the bio.
> > 
> > Polling for the bio itself leads to a few advantages:
> > 
> >   - the cookie construction can made entirely private in blk-mq.c
> >   - the caller does not need to remember the request_queue and cookie
> >     separately and thus sidesteps their lifetime issues
> >   - keeping the device and the cookie inside the bio allows to trivially
> >     support polling BIOs remapping by stacking drivers
> >   - a lot of code to propagate the cookie back up the submission path can
> >     be removed entirely.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>

...

> > +int bio_poll(struct bio *bio, unsigned int flags)
> > +{
> > +	struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> > +	blk_qc_t cookie = READ_ONCE(bio->bi_cookie);
> > +
> > +	if (cookie == BLK_QC_T_NONE ||
> > +	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> > +		return 0;
> > +
> > +	if (current->plug)
> > +		blk_flush_plug_list(current->plug, false);
> > +
> > +	/* not yet implemented, so this should not happen */
> > +	if (WARN_ON_ONCE(!queue_is_mq(q)))
> 
> What happens if the I/O wasn't (yet) queued to the bottom device
> (i.e. no available path in nvme-mpath)?
> 
> In this case the disk would be the mpath device node (which is
> not mq...)

->bi_cookie is only set in blk_mq_start_request() for blk-mq request,
if the I/O isn't queued to bottom device, it won't be polled because
->bi_cookie is still BLK_QC_T_NONE.


Thanks,
Ming


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