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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe