Re: [PATCH 0/3] blk-mq/dm-rq: support BLK_MQ_F_BLOCKING for dm-rq

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Dec 22 2021 at 11:16P -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> On Tue, Dec 21, 2021 at 08:21:39AM -0800, Christoph Hellwig wrote:
> > On Tue, Dec 21, 2021 at 10:14:56PM +0800, Ming Lei wrote:
> > > Hello,
> > > 
> > > dm-rq may be built on blk-mq device which marks BLK_MQ_F_BLOCKING, so
> > > dm_mq_queue_rq() may become to sleep current context.
> > > 
> > > Fixes the issue by allowing dm-rq to set BLK_MQ_F_BLOCKING in case that
> > > any underlying queue is marked as BLK_MQ_F_BLOCKING.
> > > 
> > > DM request queue is allocated before allocating tagset, this way is a
> > > bit special, so we need to pre-allocate srcu payload, then use the queue
> > > flag of QUEUE_FLAG_BLOCKING for locking dispatch.
> > 
> > What is the benefit over just forcing bio-based dm-mpath for these
> > devices?
> 
> At least IO scheduler can't be used for bio based dm-mpath, also there should
> be other drawbacks for bio based mpath and request mpath is often the default
> option, maybe Mike has more input about bio vs request dm-mpath.

Yeah, people use request-based for IO scheduling and more capable path
selectors. Imposing bio-based would be a pretty jarring workaround for 
BLK_MQ_F_BLOCKING. request-based DM should properly support it.

I'm on vacation for the next week but will have a look at this
patchset once I'm back.

Thanks,
Mike