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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

-- 
Pavel Begunkov