Re: [PATCH v3 4/4] io_uring: add support for zone-append

[Kanchan Joshi <joshiiitr@gmail.com>]

On Thu, Jul 9, 2020 at 7:36 PM Jens Axboe <axboe@kernel.dk> 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?
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