linux-kernel - Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze

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Message-ID: <CAB0TPYEbHeTqd2ZrOyMSMbV+g7r0HMTt2GSpUrRZxM8XsNPi3Q@mail.gmail.com>
Date:   Thu, 15 Aug 2019 21:08:36 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>, kernel-team@...roid.com,
        Narayan Kamath <narayan@...gle.com>,
        Dario Freni <dariofreni@...gle.com>,
        Nikita Ioffe <ioffe@...gle.com>,
        Jiyong Park <jiyong@...gle.com>,
        Martijn Coenen <maco@...gle.com>
Subject: Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze_queue() when possible.

On Thu, Aug 15, 2019 at 6:34 PM Ming Lei <tom.leiming@...il.com> wrote:
> If nothing will change, why does the userspace bother to send
> SET_STATUS?

We don't change transfer, but we do change the offset and sizelimit.
In our specific case, we know there won't be any I/O from userspace at
this point; so from that point of view the freeze wouldn't be
necessary. But I'm not sure how we can make loop aware of that in a
safe way. Ideally we'd just have a way of completely configuring a
loop device before starting the block request queue, but that seems
like a pretty big change.

Martijn

>
>
> Thanks,
> Ming Lei