Re: [PATCH] fs: kill unused ret2 argument from iocb->ki_complete()

[Jens Axboe <axboe@kernel.dk>]

On 10/21/21 10:40 AM, John Keeping wrote:
> On Wed, 20 Oct 2021 19:49:07 +0200
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 
>> On Wed, Oct 20, 2021 at 11:35:27AM -0600, Jens Axboe wrote:
>>> On 10/20/21 11:30 AM, Christoph Hellwig wrote:  
>>>> On Wed, Oct 20, 2021 at 10:49:07AM -0600, Jens Axboe wrote:  
>>>>> It's not used for anything, and we're wasting time passing in zeroes
>>>>> where we could just ignore it instead. Update all ki_complete users in
>>>>> the kernel to drop that last argument.
>>>>>
>>>>> The exception is the USB gadget code, which passes in non-zero. But
>>>>> since nobody every looks at ret2, it's still pointless.  
>>>>
>>>> Yes, the USB gadget passes non-zero, and aio passes that on to
>>>> userspace.  So this is an ABI change.  Does it actually matter?
>>>> I don't know, but you could CC the relevant maintainers and list
>>>> to try to figure that out.  
>>>
>>> True, guess it does go out to userspace. Greg, is anyone using
>>> it on the userspace side?  
>>
>> I really do not know (adding linux-usb@vger)  My interactions with the
>> gadget code have not been through the aio api, thankfully :)
>>
>> Odds are it's fine, I think that something had to be passed in there so
>> that was chosen?  If the aio code didn't do anything with it, I can't
>> see where the gadget code gets it back at anywhere, but I might be
>> looking in the wrong place.
>>
>> Anyone else here know?
> 
> I really doubt anyone uses io_event::res2 with FunctionFS gadgets.  The
> examples in tools/usb/ffs-aio-example/ either check just "res" or ignore
> the status completely.
> 
> The only other program I can find using aio FunctionFS is adbd which
> also checks res and ignores res2 [1].  Other examples I know of just use
> synchronous I/O.

We might consider doing a separate change to just skip status reporting,
and then once a few releases have passed and if nobody has complained,
then we can drop it entirely. If we do it separately, it'd be an easy
revert of just that part if we do run into trouble.

At least for now we'll just have the one argument bundling the two parts
for aio, so there's not a great need to eliminate the status thing on
the USB gadget side. But if it is unused, then it should get removed
regardless.

-- 
Jens Axboe