linux-kernel - Re: [PATCH 2/2] Revert "mq-deadline: Fix request accounting"

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Date:   Tue, 7 Sep 2021 16:28:49 +0000
From:   Niklas Cassel <Niklas.Cassel@....com>
To:     Bart Van Assche <bvanassche@....org>
CC:     Jens Axboe <axboe@...nel.dk>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Ming Lei <ming.lei@...hat.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] Revert "mq-deadline: Fix request accounting"

On Tue, Sep 07, 2021 at 08:15:03AM -0700, Bart Van Assche wrote:
> On 9/7/21 7:21 AM, Niklas Cassel wrote:
> > blk-mq will no longer call the I/O scheduler .finish_request() callback
> > for requests that were never inserted to the I/O scheduler.
> 
> I do not agree. Even with patch 1/2 from this series applied, finish_request()
> will still be called for requests inserted by blk_insert_cloned_request()
> although these requests are never inserted to the I/O scheduler.
> 
> Bart.

Hello Bart,

Looking at blk_mq_free_request(),
e->type->ops.finish_request() will only be called if RQF_ELVPRIV
is set.

blk_insert_cloned_request() doesn't seem to allocate a request
itself, but instead takes an already cloned request.

So I guess it depends on how the supplied request was cloned.

I would assume if the original request doesn't have RQF_ELVPRIV set,
then neither will the cloned request?

I tried to look at blk_rq_prep_clone(), which seems to be a common
cloning function, but I don't see req->rq_flags being copied
(except for RQF_SPECIAL_PAYLOAD).

Anyway, I don't see how .finish_request() will be called in relation
to blk_insert_cloned_request(). Could you please help me out and
give me an example of a call chain where this can happen?

Kind regards,
Niklas