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

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Message-ID: <fdcf70aaf53b4d3040bed95535846edc0120bfea.camel@fb.com>
Date:   Tue, 22 Feb 2022 10:52:17 +0000
From:   Dylan Yudaken <dylany@...com>
To:     "haoxu@...ux.alibaba.com" <haoxu@...ux.alibaba.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "asml.silence@...il.com" <asml.silence@...il.com>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>
CC:     Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] io_uring: pre-increment f_pos on rw

On Tue, 2022-02-22 at 15:34 +0800, Hao Xu wrote:
> 
> 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..
> 

It seems to be ok - as in nothing crashes when f_pos is past the end of
the file - but I really am not an expert on these things so am happy to
receive feedback on this.