linux-kernel - Re: [PATCH 2/5] block: add request->io_data

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Message-ID: <20200413135621.GB60335@mtj.duckdns.org>
Date:   Mon, 13 Apr 2020 09:56:21 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...r.kernel.org,
        cgroups@...r.kernel.org, newella@...com, josef@...icpanda.com
Subject: Re: [PATCH 2/5] block: add request->io_data_len

Hello,

On Thu, Apr 09, 2020 at 10:38:57AM +0800, Ming Lei wrote:
> On Wed, Apr 08, 2020 at 10:11:19PM -0400, Tejun Heo wrote:
> > On Thu, Apr 09, 2020 at 09:44:06AM +0800, Ming Lei wrote:
> > > Almost all __blk_mq_end_request() follow blk_update_request(), so the
> > > completed bytes can be passed to __blk_mq_end_request(), then we can
> > > avoid to introduce this field.
> > 
> > But on some drivers blk_update_request() may be called multiple times before
> > __blk_mq_end_request() is called and what's needed here is the total number of
> > bytes in the whole request, not just in the final completion.
> 
> OK.
> 
> Another choice might be to record request bytes in rq's payload
> when calling .queue_rq() only for these drivers.

There sure are multiple ways to skin a cat.

> > > Also there is just 20 callers of __blk_mq_end_request(), looks this kind
> > > of change shouldn't be too big.
> > 
> > This would work iff we get rid of partial completions and if we get rid of
> > partial completions, we might as well stop exposing blk_update_request() and
> > __blk_mq_end_request().
> 
> Indeed, we can store the completed bytes in request payload, so looks killing
> partial completion shouldn't be too hard.

There's a reason why we've had partial completions. On slower IO devices, like
floppy, partial completions actually are advantageous. I'm not arguing this
still holds up as a valid justification but getting rid of partial completions
isn't just a decision about plumbing details either.

Thanks.

-- 
tejun