linux-kernel - Re: [PATCH v2 4/4] io_uring: pre-increment f

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Message-ID: <9cd3cc84-a2a0-a827-3fb8-bd2928eabd28@linux.alibaba.com>
Date:   Tue, 22 Feb 2022 15:34:40 +0800
From:   Hao Xu <haoxu@...ux.alibaba.com>
To:     Dylan Yudaken <dylany@...com>, Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 4/4] io_uring: pre-increment f_pos on rw


On 2/21/22 22:16, Dylan Yudaken wrote:
> In read/write ops, preincrement f_pos when no offset is specified, and
> then attempt fix up the position after IO completes if it completed less
> than expected. This fixes the problem where multiple queued up IO will all
> obtain the same f_pos, and so perform the same read/write.
>
> This is still not as consistent as sync r/w, as it is able to advance the
> file offset past the end of the file. It seems it would be quite a
> performance hit to work around this limitation - such as by keeping track
> of concurrent operations - and the downside does not seem to be too
> problematic.
>
> The attempt to fix up the f_pos after will at least mean that in situations
> where a single operation is run, then the position will be consistent.
>
It's a little bit weird, when a read req returns x bytes read while f_pos

moves ahead y bytes where x isn't equal to y. Don't know if this causes

problems..