linux-kernel - Re: [PATCH] /dev/mem: nowait zero/null ops

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Message-ID: <b03a27cd-1e5a-9444-c406-e15c52d2a066@gmail.com>
Date:   Wed, 8 Sep 2021 11:54:53 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org
Subject: Re: [PATCH] /dev/mem: nowait zero/null ops

On 9/8/21 11:25 AM, Greg Kroah-Hartman wrote:
> On Wed, Sep 08, 2021 at 11:06:51AM +0100, Pavel Begunkov wrote:
>> Make read_iter_zero() to honor IOCB_NOWAIT, so /dev/zero can be
>> advertised as FMODE_NOWAIT. This helps subsystems like io_uring to use
>> it more effectively. Set FMODE_NOWAIT for /dev/null as well, it never
>> waits and therefore trivially meets the criteria.
> 
> I do not understand, why would io_uring need to use /dev/zero

Not directly, users can issue I/O against it via io_uring.
 
> and how is this going to help anything?

For files not supporting nowait io_uring goes through a quite slow path.

> What workload does this help with?

Personally for me it's dumping output and benchmarking (not benchmarking
/dev/zero, of course). But I'd also expect any tool that may be using
it but rewritten with io_uring being able to normally use it without a
performance hit.

-- 
Pavel Begunkov