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

[Mike Snitzer]

On Tue, Dec 28 2021 at  4:30P -0500,
Mike Snitzer <snitzer at redhat.com> wrote:

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

I replied last week and hoped Jens would pick this patchset up after
my Reviewed-by of patch 3/3.

Christoph wasn't back though, so best to kick this thread again.

Thoughts on this patchset?

Thanks,
Mike