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Subject: Re: [PATCH 0/2] Remove kiocb ki_complete
To:     Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@infradead.org>
Cc:     linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-aio@kvack.org, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org, Kanchan Joshi <joshi.k@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>
References: <20200708222637.23046-1-willy@infradead.org>
 <20200709101705.GA2095@infradead.org>
 <20200709111036.GA12769@casper.infradead.org>
 <20200709132611.GA1382@infradead.org>
 <20200709133249.GC12769@casper.infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <cac3062d-3779-5c1b-b3f5-07823c198ead@kernel.dk>
Date:   Thu, 9 Jul 2020 07:53:07 -0600
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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