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

[Christoph Hellwig]

On Tue, Dec 28, 2021 at 04:30:08PM -0500, Mike Snitzer wrote:
> 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.

Given that nvme-tcp is the only blocking driver that has multipath
driver that driver explicitly does not intend to support dm-multipath
I'm absolutely against adding block layer cruft for this particular
use case.

SCSI even has this:

	        /*
		 * SCSI never enables blk-mq's BLK_MQ_F_BLOCKING flag so
		 * calling synchronize_rcu() once is enough.
		 */
		WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);