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From: Pavel Begunkov <[email protected]>
To: Kanchan Joshi <[email protected]>, Jens Axboe <[email protected]>
Cc: "Christoph Hellwig" <[email protected]>,
	"Kanchan Joshi" <[email protected]>,
	[email protected], [email protected], [email protected],
	[email protected], "Matias Bjørling" <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	"Selvakumar S" <[email protected]>,
	"Nitesh Shetty" <[email protected]>,
	"Javier Gonzalez" <[email protected]>
Subject: Re: [PATCH v3 4/4] io_uring: add support for zone-append
Date: Thu, 9 Jul 2020 21:53:47 +0300	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 09/07/2020 21:50, Pavel Begunkov wrote:
> On 09/07/2020 21:36, Kanchan Joshi wrote:
>> On Thu, Jul 9, 2020 at 7:36 PM Jens Axboe <[email protected]> wrote:
>>>
>>> On 7/9/20 8:00 AM, Christoph Hellwig wrote:
>>>> On Thu, Jul 09, 2020 at 07:58:04AM -0600, Jens Axboe wrote:
>>>>>> We don't actually need any new field at all.  By the time the write
>>>>>> returned ki_pos contains the offset after the write, and the res
>>>>>> argument to ->ki_complete contains the amount of bytes written, which
>>>>>> allow us to trivially derive the starting position.
>>
>> Deriving starting position was not the purpose at all.
>> But yes, append-offset is not needed, for a different reason.
>> It was kept for uring specific handling. Completion-result from lower
>> layer was always coming to uring in ret2 via ki_complete(....,ret2).
>> And ret2 goes to CQE (and user-space) without any conversion in between.
>> For polled-completion, there is a short window when we get ret2 but cannot
>> write into CQE immediately, so thought of storing that in append_offset
>> (but should not have done, solving was possible without it).
> 
> fwiw, there are more cases when it's postponed.
> 
>> FWIW, if we move to indirect-offset approach, append_offset gets
>> eliminated automatically, because there is no need to write to CQE
>> itself.
> 
> Right, for the indirect approach we can write offset right after getting it.

Take it back, as you mentioned with task_work, we may need the right context.

BTW, there is one more way to get space for it, and it would also shed 8 bytes
from io_kiocb, but that would need some work to be done.

> If not, then it's somehow stored in an CQE, so may be placed into
> existing req->{result,cflags}, which mimic CQE's fields.
> 
>>
>>>>> Then let's just do that instead of jumping through hoops either
>>>>> justifying growing io_rw/io_kiocb or turning kiocb into a global
>>>>> completion thing.
>>>>
>>>> Unfortunately that is a totally separate issue - the in-kernel offset
>>>> can be trivially calculated.  But we still need to figure out a way to
>>>> pass it on to userspace.  The current patchset does that by abusing
>>>> the flags, which doesn't really work as the flags are way too small.
>>>> So we somewhere need to have an address to do the put_user to.
>>>
>>> Right, we're just trading the 'append_offset' for a 'copy_offset_here'
>>> pointer, which are stored in the same spot...
>>
>> The address needs to be stored somewhere. And there does not seem
>> other option but to use io_kiocb?
>> The bigger problem with address/indirect-offset is to be able to write to it
>> during completion as process-context is different. Will that require entering
>> into task_work_add() world, and may make it costly affair?
>>
>> Using flags have not been liked here, but given the upheaval involved so
>> far I have begun to feel - it was keeping things simple. Should it be
>> reconsidered?
>>
>>
>> --
>> Joshi
>>
> 

-- 
Pavel Begunkov

  reply	other threads:[~2020-07-09 18:55 UTC|newest]

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20200705185204epcas5p3adeb4fc3473c5fc0472a7396783c5267@epcas5p3.samsung.com>
2020-07-05 18:47 ` [PATCH v3 0/4] zone-append support in io-uring and aio Kanchan Joshi
     [not found]   ` <CGME20200705185211epcas5p4059d05d2fcedb91829300a7a7d03fda3@epcas5p4.samsung.com>
2020-07-05 18:47     ` [PATCH v3 1/4] fs: introduce FMODE_ZONE_APPEND and IOCB_ZONE_APPEND Kanchan Joshi
     [not found]   ` <CGME20200705185217epcas5p1cc12d4b892f057a1fe06d73a00869daa@epcas5p1.samsung.com>
2020-07-05 18:47     ` [PATCH v3 2/4] block: add zone append handling for direct I/O path Kanchan Joshi
     [not found]   ` <CGME20200705185221epcas5p28b6d060df829b751109265222285da0e@epcas5p2.samsung.com>
2020-07-05 18:47     ` [PATCH v3 3/4] block: enable zone-append for iov_iter of bvec type Kanchan Joshi
     [not found]   ` <CGME20200705185227epcas5p16fba3cb92561794b960184c89fdf2bb7@epcas5p1.samsung.com>
2020-07-05 18:47     ` [PATCH v3 4/4] io_uring: add support for zone-append Kanchan Joshi
2020-07-05 21:00       ` Jens Axboe
2020-07-05 21:09         ` Matthew Wilcox
2020-07-05 21:12           ` Jens Axboe
2020-07-06 14:10             ` Matthew Wilcox
2020-07-06 14:27               ` Jens Axboe
2020-07-06 14:32                 ` Matthew Wilcox
2020-07-06 14:33                   ` Jens Axboe
2020-07-07 15:11                   ` Kanchan Joshi
2020-07-07 15:52                     ` Matthew Wilcox
2020-07-07 16:00                       ` Christoph Hellwig
2020-07-07 20:23                       ` Kanchan Joshi
2020-07-07 20:40                         ` Jens Axboe
2020-07-07 22:18                           ` Matthew Wilcox
2020-07-07 22:37                             ` Jens Axboe
2020-07-08 12:58                               ` Kanchan Joshi
2020-07-08 14:22                                 ` Matthew Wilcox
2020-07-08 16:41                                   ` Kanchan Joshi
2020-07-08 14:54                                 ` Jens Axboe
2020-07-08 14:58                                   ` Matthew Wilcox
2020-07-08 14:59                                     ` Jens Axboe
2020-07-08 15:02                                       ` Matthew Wilcox
2020-07-08 15:06                                         ` Jens Axboe
2020-07-08 16:08                                           ` Javier González
2020-07-08 16:33                                             ` Matthew Wilcox
2020-07-08 16:38                                               ` Jens Axboe
2020-07-08 17:13                                                 ` Kanchan Joshi
2020-07-08 16:43                                               ` Javier González
2020-07-06 13:58         ` Kanchan Joshi
2020-07-09 10:15         ` Christoph Hellwig
2020-07-09 13:58           ` Jens Axboe
2020-07-09 14:00             ` Christoph Hellwig
2020-07-09 14:05               ` Jens Axboe
2020-07-09 18:36                 ` Kanchan Joshi
2020-07-09 18:50                   ` Pavel Begunkov
2020-07-09 18:53                     ` Pavel Begunkov [this message]
2020-07-09 18:50                   ` Jens Axboe
2020-07-09 19:05                     ` Kanchan Joshi
2020-07-10 13:10                       ` Christoph Hellwig
2020-07-10 13:48                         ` Matthew Wilcox
2020-07-10 13:49                           ` Christoph Hellwig
2020-07-10 13:51                             ` Matthew Wilcox
2020-07-10 14:11                               ` Kanchan Joshi
2020-07-20 16:49                                 ` Kanchan Joshi
2020-07-20 17:14                                   ` Matthew Wilcox
2020-07-20 20:17                                     ` Kanchan Joshi
2020-07-21  0:59                                       ` Damien Le Moal
2020-07-21  1:15                                         ` Matthew Wilcox
2020-07-21  1:29                                           ` Jens Axboe
2020-07-21  2:19                                           ` Damien Le Moal
2020-07-10 14:09                         ` Jens Axboe
2020-07-20 16:46                           ` Kanchan Joshi
2020-07-10 13:09                     ` Christoph Hellwig
2020-07-10 13:29                       ` Kanchan Joshi
2020-07-10 13:43                         ` Christoph Hellwig
2020-07-20 17:02                           ` Kanchan Joshi
2020-07-10 13:57                         ` Kanchan Joshi

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