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.

So is there consensus on the USB side that we can just fill res2 with
zero? The single cases that does just do res == res2 puts the error
in res anyway, which is what you'd expect.

If so, then I do think that'd be cleaner than packing two values into
a u64.

-- 
Jens Axboe