Re: [RFC] do_iopoll() and *grab_env()

[Jens Axboe <axboe@kernel.dk>]

On 6/12/20 12:33 PM, Pavel Begunkov wrote:
> On 12/06/2020 21:02, Jens Axboe wrote:
>> On 6/12/20 11:55 AM, Jens Axboe wrote:
>>> On 6/12/20 11:30 AM, Pavel Begunkov wrote:
>>>> On 12/06/2020 20:02, Jens Axboe wrote:
>>>>> On 6/11/20 9:54 AM, Pavel Begunkov wrote:
>>>>>> io_do_iopoll() can async punt a request with io_queue_async_work(),
>>>>>> so doing io_req_work_grab_env(). The problem is that iopoll() can
>>>>>> be called from who knows what context, e.g. from a completely
>>>>>> different process with its own memory space, creds, etc.
>>>>>>
>>>>>> io_do_iopoll() {
>>>>>> 	ret = req->poll();
>>>>>> 	if (ret == -EAGAIN)
>>>>>> 		io_queue_async_work()
>>>>>> 	...
>>>>>> }
>>>>>>
>>>>>>
>>>>>> I can't find it handled in io_uring. Can this even happen?
>>>>>> Wouldn't it be better to complete them with -EAGAIN?
>>>>>
>>>>> I don't think a plain -EAGAIN complete would be very useful, it's kind
>>>>> of a shitty thing to pass back to userspace when it can be avoided. For
>>>>> polled IO, we know we're doing O_DIRECT, or using fixed buffers. For the
>>>>> latter, there's no problem in retrying, regardless of context. For the
>>>>> former, I think we'd get -EFAULT mapping the IO at that point, which is
>>>>> probably reasonable. I'd need to double check, though.
>>>>
>>>> It's shitty, but -EFAULT is the best outcome. I care more about not
>>>> corrupting another process' memory if addresses coincide. AFAIK it can
>>>> happen because io_{read,write} will use iovecs for punted re-submission.
>>>>
>>>>
>>>> Unconditional in advance async_prep() is too heavy to be good. I'd love to
>>>> see something more clever, but with -EAGAIN users at least can handle it.
>>>
>>> So how about we just grab ->task for the initial issue, and retry if we
>>> find it through -EAGAIN and ->task == current. That'll be the most
>>> common case, by far, and it'll prevent passes back -EAGAIN when we
>>> really don't have to. If the task is different, then -EAGAIN makes more
>>> sense, because at that point we're passing back -EAGAIN because we
>>> really cannot feasibly handle it rather than just as a convenience.
> 
> Yeah, I was even thinking to drag it through task_work just to call
> *grab_env() there. Looks reasonable to me.
> 
>> Something like this, totally untested. And wants a comment too.
> 
> Looks like it. Would you leave this to me? There is another issue with
> cancellation requiring ->task, It'd be easier to keep them together.

Guess this ties into the next email, on using task_work? I actually
don't think that's a bad idea. If you have a low(er) queue depth device,
the -EAGAIN path is not necessarily that common. And task_work is a lot
more efficient for re-submittal than async work, plus needs to grab less
resources.

So I think you should still run with it...

-- 
Jens Axboe