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Date:   Tue, 21 Dec 2021 08:21:39 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Mike Snitzer <snitzer@redhat.com>,
        linux-block@vger.kernel.org, dm-devel@redhat.com
Subject: Re: [PATCH 0/3] blk-mq/dm-rq: support BLK_MQ_F_BLOCKING for dm-rq
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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?