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From: Kanchan Joshi <[email protected]>
To: Jens Axboe <[email protected]>
Cc: "Christoph Hellwig" <[email protected]>,
	"Kanchan Joshi" <[email protected]>,
	[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: Fri, 10 Jul 2020 00:35:43 +0530	[thread overview]
Message-ID: <CA+1E3rLna6VVuwMSHVVEFmrgsTyJN=U4CcZtxSGWYr_UYV7AmQ@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>

On Fri, Jul 10, 2020 at 12:20 AM Jens Axboe <[email protected]> wrote:
>
> On 7/9/20 12:36 PM, 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, if we move to indirect-offset approach, append_offset gets
> > eliminated automatically, because there is no need to write to CQE
> > itself.
> >
> >>>> 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?
>
> That is where it belongs, not sure this was ever questioned. And inside
> io_rw at that.
>
> > 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?
>
> It might, if you have IRQ context for the completion. task_work isn't
> expensive, however. It's not like a thread offload.
>
> > 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?
>
> It's definitely worth considering, especially since we can use cflags
> like Pavel suggested upfront and not need any extra storage. But it
> brings us back to the 32-bit vs 64-bit discussion, and then using blocks
> instead of bytes. Which isn't exactly super pretty.
>
I agree that what we had was not great.
Append required special treatment (conversion for sector to bytes) for io_uring.
And we were planning a user-space wrapper to abstract that.

But good part (as it seems now) was: append result went along with cflags at
virtually no additional cost. And uring code changes became super clean/minimal
with further revisions.
While indirect-offset requires doing allocation/mgmt in application,
io-uring submission
and in completion path (which seems trickier), and those CQE flags
still get written
user-space and serve no purpose for append-write.

-- 
Joshi

  reply	other threads:[~2020-07-09 19:06 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
2020-07-09 18:50                   ` Jens Axboe
2020-07-09 19:05                     ` Kanchan Joshi [this message]
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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