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Date:   Tue, 11 Jan 2022 00:34:30 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Mike Snitzer <snitzer@redhat.com>
Cc:     Ming Lei <ming.lei@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, dm-devel@redhat.com,
        linux-block@vger.kernel.org
Subject: Re: [dm-devel] [PATCH 0/3] blk-mq/dm-rq: support BLK_MQ_F_BLOCKING
 for dm-rq
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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);