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

[Jens Axboe <axboe@kernel.dk>]

On 10/23/21 3:09 AM, Greg Kroah-Hartman wrote:
> On Fri, Oct 22, 2021 at 09:44:32AM -0600, Jens Axboe wrote:
>> 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.
> 
> I think yes, we should try that, and if something breaks, be ready to
> provide a fix for it.

I've split the change in two, one that gets rid of the special arguments
on the USB side, and one that then drops it for the whole kernel since
everybody passes 0 at that point. Should make it easy to pinpoint, if
need be.

-- 
Jens Axboe