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

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Message-ID: <cac3062d-3779-5c1b-b3f5-07823c198ead@kernel.dk>
Date:   Thu, 9 Jul 2020 07:53:07 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     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 7/9/20 7:32 AM, Matthew Wilcox wrote:
> On Thu, Jul 09, 2020 at 02:26:11PM +0100, 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.
> 
> I agree.  Jens doesn't.

Stop putting words in my mouth, especially when they are totally untrue.

I was opposed to growing struct io_rw in io_uring, which is where the
extra append variable belonds, beyond a cacheline. You mentioned you
could probably shave some bits out of struct kiocb, which is how this
completion handling business came about. If kiocb was shrunk, then io_rw
has room for the needed variable.

At no point have I said that whatever we need to shove in there for
io_uring should be in the kiocb, that would not make any sense. I'm
just opposed to growing the per-op data field in io_kiocb beyond
a cacheline. And that's especially true for something like append
writes, which I don't consider super interesting.


-- 
Jens Axboe