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Date:   Wed, 29 Nov 2017 22:46:51 +0100
From:   Christoph Hellwig <hch@....de>
To:     Richard Weinberger <richard@....at>
Cc:     user-mode-linux-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, axboe@...com,
        anton.ivanov@...bridgegreys.com, hch@....de,
        linux-block@...r.kernel.org
Subject: Re: [PATCH] [RFC] um: Convert ubd driver to blk-mq

On Sun, Nov 26, 2017 at 02:10:53PM +0100, Richard Weinberger wrote:
> MAX_SG is 64, used for blk_queue_max_segments(). This comes from
> a0044bdf60c2 ("uml: batch I/O requests"). Is this still a good/sane
> value for blk-mq?

blk-mq itself doesn't change the tradeoff.

> The driver does IO batching, for each request it issues many UML struct
> io_thread_req request to the IO thread on the host side.
> One io_thread_req per SG page.
> Before the conversion the driver used blk_end_request() to indicate that
> a part of the request is done.
> blk_mq_end_request() does not take a length parameter, therefore we can
> only mark the whole request as done. See the new is_last property on the
> driver.
> Maybe there is a way to partially end requests too in blk-mq?

You can, take a look at scsi_end_request which handles this for blk-mq
and the legacy layer.  That being said I wonder if batching really
makes that much sene if you execute each segment separately?

> Another obstacle with IO batching is that UML IO thread requests can
> fail. Not only due to OOM, also because the pipe between the UML kernel
> process and the host IO thread can return EAGAIN.
> In this case the driver puts the request into a list and retried later
> again when the pipe turns writable.
> I’m not sure whether this restart logic makes sense with blk-mq, maybe
> there is a way in blk-mq to put back a (partial) request?

blk_mq_requeue_request requeues requests that have been partially
exectuted (or not at all for that matter).

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