Re: [PATCH] blk-mq: enable IO poll if .nr_queues of type poll > 0

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

On Tue, Dec 18, 2018 at 07:47:39AM +0100, Christoph Hellwig wrote:
> On Mon, Dec 17, 2018 at 09:34:55PM -0700, Jens Axboe wrote:
> > On 12/17/18 9:15 PM, Ming Lei wrote:
> > > The queue mapping of type poll only exists when set->map[HCTX_TYPE_POLL].nr_queues
> > > is bigger than zero, so enhance the constraint by checking .nr_queues of type poll
> > > before enabling IO poll.
> > > 
> > > Otherwise IO race & timeout can be observed when running block/007.
> > 
> > Looks good to me, but we might consider having a helper for this, since
> > we have this check in 3 places now. But for now this is good, thanks
> > Ming.
> 
> Jens, if you want this I can drop the nvme patch to set nr_maps to 2
> as it wouldn't be needed.  Not sure which one is more elegant, but
> I guess Mings version is a little more fool proof.

For READ queue type, its .nr_maps can be 3 and .nr_queues is zero, so
if check on the two queue type is kept as consistent, we may get good
code readability, IMO.

As Jens mentioned, we may introduce blk_mq_queue_type_supported() helper
to cover both.

Thanks,
Ming