linux-kernel - Re: [PATCH 0/2] Remove kiocb ki

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Message-ID: <ffbd272c-32f3-8c8c-6395-5eab47725929@gmail.com>
Date:   Thu, 9 Jul 2020 16:37:59 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kanchan Joshi <joshi.k@...sung.com>,
        Javier Gonzalez <javier.gonz@...sung.com>
Subject: Re: [PATCH 0/2] Remove kiocb ki_complete

On 09/07/2020 16:26, Christoph Hellwig wrote:
> On Thu, Jul 09, 2020 at 12:10:36PM +0100, Matthew Wilcox wrote:
>> On Thu, Jul 09, 2020 at 11:17:05AM +0100, Christoph Hellwig wrote:
>>> I really don't like this series at all.  If saves a single pointer
>>> but introduces a complicated machinery that just doesn't follow any
>>> natural flow.  And there doesn't seem to be any good reason for it to
>>> start with.
>>
>> Jens doesn't want the kiocb to grow beyond a single cacheline, and we
>> want the ability to set the loff_t in userspace for an appending write,
>> so the plan was to replace the ki_complete member in kiocb with an
>> loff_t __user *ki_posp.
>>
>> I don't think it's worth worrying about growing kiocb, personally,
>> but this seemed like the easiest way to make room for a new pointer.
> 
> The user offset pointer has absolutely no business in the the kiocb
> itself - it is a io_uring concept which needs to go into the io_kiocb,
> which has 14 bytes left in the last cache line in my build.  It would
> fit in very well there right next to the result and user pointer.

After getting a valid offset, io_uring shouldn't do anything but
complete the request. And as io_kiocb implicitly contains a CQE entry,
not sure we need @append_offset in the first place.

Kanchan, could you take a look if you can hide it in req->cflags?

-- 
Pavel Begunkov