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

[Ming Lei]

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.



Thanks,
Ming