linux-kernel - Re: [PATCH RFC 1/3] block: add flag 'nowait

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Date:   Tue, 2 Jun 2020 21:58:22 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        dm-devel@...hat.com, linux-raid@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH RFC 1/3] block: add flag 'nowait_requests' into queue
 limits

On Mon, Jun 01, 2020 at 03:37:09PM +0300, Konstantin Khlebnikov wrote:
> Add flag for marking bio-based queues which support REQ_NOWAIT.
> Set for all request based (mq) devices.
> 
> Stacking device should set it after blk_set_stacking_limits() if method
> make_request() itself doesn't delay requests or handles REQ_NOWAIT.

I don't think this belongs into the queue limits.  For example a
stacking driver that always defers requests to a workqueue can support
REQ_NOWAIT entirely independent of the underlying devices.  I think
this just needs to be a simple queue flag.