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

[Jens Axboe <axboe@kernel.dk>]

On 7/9/20 12:36 PM, Kanchan Joshi wrote:
> 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?

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.

-- 
Jens Axboe