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

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Date:   Thu, 9 Apr 2020 08:08:59 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Ming Lei <ming.lei@...hat.com>, Tejun Heo <tj@...nel.org>
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

On 09/04/2020 05:38, 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.
> 
>>
>>> 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.

struct request already has such field (see @stats_sectors) because of the same
root-cause. I'd prefer killing it as well by following Ming's way, but otherwise
it could be easily adopted.

-- 
Pavel Begunkov