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

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Message-ID: <ce7d999e-1629-c70d-8bb9-59d7db41a11e@gmail.com>
Date:   Thu, 9 Jul 2020 16:49:51 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 16:43, Matthew Wilcox wrote:
> On Thu, Jul 09, 2020 at 04:37:59PM +0300, Pavel Begunkov wrote:
>> 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?
> 
> No, that's not what cflags are for.  And besides, there's only 32 bits
> there.

It's there to temporarily store cqe->cflags, if a request can't completed
right away. And req->{result,user_data,cflags} are basically an CQE inside
io_kiocb.

So, it is there exactly for that reason, and whatever way it's going to be
encoded in an CQE, io_kiocb can fit it. That was my point.

-- 
Pavel Begunkov