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

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Message-ID: <e03b9ae9-edf7-3489-7641-8e62e51ad77b@kernel.dk>
Date:   Thu, 9 Jul 2020 07:55:53 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...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 7:26 AM, 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,

Nobody disagrees on that.

> 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.

Per-op data should not spill into the io_kiocb itself. And I absolutely
hate arguments like "oh there's still 14 bytes in there", because then
there's 6, then there's none, and now we're going into the next
cacheline. io_kiocb is already too fat, it should be getting slimmer,
not bigger. And the append write stuff is not nearly interesting
enough to a) grow io_kiocb, b) warrant a special case for op private
data in the io_kiocb itself.


-- 
Jens Axboe