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

[Pavel Begunkov <asml.silence@gmail.com>]

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 <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, 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