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From: Pavel Begunkov <[email protected]>
To: Matthew Wilcox <[email protected]>
Cc: Christoph Hellwig <[email protected]>,
	Jens Axboe <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected], Kanchan Joshi <[email protected]>,
	Javier Gonzalez <[email protected]>
Subject: Re: [PATCH 0/2] Remove kiocb ki_complete
Date: Thu, 9 Jul 2020 16:49:51 +0300	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

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

  reply	other threads:[~2020-07-09 13:51 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-08 22:26 [PATCH 0/2] Remove kiocb ki_complete Matthew Wilcox (Oracle)
2020-07-08 22:26 ` [PATCH 1/2] fs: Abstract calling the kiocb completion function Matthew Wilcox (Oracle)
2020-07-08 22:37   ` Jens Axboe
2020-07-08 22:40     ` Matthew Wilcox
2020-07-08 22:50       ` Jens Axboe
2020-07-08 22:26 ` [PATCH 2/2] fs: Remove kiocb->ki_complete Matthew Wilcox (Oracle)
2020-07-08 22:38   ` Jens Axboe
2020-07-09  3:25   ` Jens Axboe
2020-07-09  5:23   ` kernel test robot
2020-07-08 22:33 ` [PATCH 0/2] Remove kiocb ki_complete Jens Axboe
2020-07-09 10:17 ` Christoph Hellwig
2020-07-09 11:10   ` Matthew Wilcox
2020-07-09 13:26     ` Christoph Hellwig
2020-07-09 13:32       ` Matthew Wilcox
2020-07-09 13:53         ` Jens Axboe
2020-07-09 13:37       ` Pavel Begunkov
2020-07-09 13:43         ` Matthew Wilcox
2020-07-09 13:49           ` Pavel Begunkov [this message]
2020-07-09 13:53             ` Matthew Wilcox
2020-07-09 13:59               ` Pavel Begunkov
2020-07-09 13:55       ` Jens Axboe

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