Re: [PATCH 2/2] zonefs: use zone-append for AIO as well

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 22/07/2020 16:52, Christoph Hellwig wrote:
> On Wed, Jul 22, 2020 at 12:43:21PM +0000, Johannes Thumshirn wrote:
>> On 21/07/2020 07:54, Christoph Hellwig wrote:
>>> On Mon, Jul 20, 2020 at 04:48:50PM +0000, Johannes Thumshirn wrote:
>>>> On 20/07/2020 15:45, Christoph Hellwig wrote:
>>>>> On Mon, Jul 20, 2020 at 10:21:18PM +0900, Johannes Thumshirn wrote:
>>>>>> On a successful completion, the position the data is written to is
>>>>>> returned via AIO's res2 field to the calling application.
>>>>>
>>>>> That is a major, and except for this changelog, undocumented ABI
>>>>> change.  We had the whole discussion about reporting append results
>>>>> in a few threads and the issues with that in io_uring.  So let's
>>>>> have that discussion there and don't mix it up with how zonefs
>>>>> writes data.  Without that a lot of the boilerplate code should
>>>>> also go away.
>>>>>
>>>>
>>>> OK maybe I didn't remember correctly, but wasn't this all around 
>>>> io_uring and how we'd report the location back for raw block device
>>>> access?
>>>
>>> Report the write offset.  The author seems to be hell bent on making
>>> it block device specific, but that is a horrible idea as it is just
>>> as useful for normal file systems (or zonefs).
>>
>> After having looked into io_uring I don't this there is anything that
>> prevents io_uring from picking up the write offset from ki_complete's
>> res2 argument. As of now io_uring ignores the filed but that can be 
>> changed.
> 
> Sure.  Except for the fact that the io_uring CQE doesn't have space
> for it.  See the currently ongoing discussion on that..

That one I was aware of, but I thought once that discussion has settled
the write offset can be copied from res2 into what ever people have agreed
on by then.

> 
>> So the only thing that needs to be done from a zonefs perspective is 
>> documenting the use of res2 and CC linux-aio and linux-abi (including
>> an update of the io_getevents man page).
>>
>> Or am I completely off track now?
> 
> Yes.  We should not have a different ABI just for zonefs.  We need to
> support this feature in a generic way and not as a weird one off for
> one filesystem and only with the legacy AIO interface.

OK, will have a look.

> Either way please make sure you properly separate the interface (
> using Write vs Zone Append in zonefs) from the interface (returning
> the actually written offset from appending writes), as they are quite
> separate issues.

So doing async RWF_APPEND writes using Zone Append isn't the problem here,
it's "only" the reporting of the write offset back to user-space? So once
we have sorted this out we can start issuing zone appends for zonefs async
writes?

Thanks,
	Johannes