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

[Keith Busch]

On Wed, May 12, 2021 at 03:12:37PM -0700, Keith Busch wrote:
> On Wed, May 12, 2021 at 03:03:40PM -0700, Sagi Grimberg wrote:
> > On 5/12/21 6:15 AM, Christoph Hellwig wrote:
> > > + *
> > > + * Note: the caller must either be the context that submitted @bio, or
> > > + * be in a RCU critical section to prevent freeing of @bio.
> > > + */
> > > +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...)
> 
> The bi_cookie should remain BLK_QC_T_NONE in that case, so we wouldn't
> get to the warning. But if that does happen, it doesn't appear that
> anyone is going to wake up thread that needs to poll for this bio's
> completion when a path becomes available for dispatch. I think it would
> make sense for nvme-mpath to just clear the REQ_POLLED flag if it
> doesn't immediately have viable path.

It looks like there is a way to hit this warning. If you induce a path
error to create a failover, the nvme-mpath will reset the bio's bdev to
the mpath one, which is not MQ. I can occaisionally hit it with a fault
injection test.