Re: "Cannot allocate memory" on ring creation (not RLIMIT_MEMLOCK)

[Dmitry Kadashev <dkadashev@gmail.com>]

On Sun, Dec 20, 2020 at 7:59 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 20/12/2020 00:25, Jens Axboe wrote:
> > On 12/19/20 4:42 PM, Pavel Begunkov wrote:
> >> On 19/12/2020 23:13, Jens Axboe wrote:
> >>> On 12/19/20 2:54 PM, Jens Axboe wrote:
> >>>> On 12/19/20 1:51 PM, Josef wrote:
> >>>>>> And even more so, it's IOSQE_ASYNC on the IORING_OP_READ on an eventfd
> >>>>>> file descriptor. You probably don't want/mean to do that as it's
> >>>>>> pollable, I guess it's done because you just set it on all reads for the
> >>>>>> test?
> >>>>>
> >>>>> yes exactly, eventfd fd is blocking, so it actually makes no sense to
> >>>>> use IOSQE_ASYNC
> >>>>
> >>>> Right, and it's pollable too.
> >>>>
> >>>>> I just tested eventfd without the IOSQE_ASYNC flag, it seems to work
> >>>>> in my tests, thanks a lot :)
> >>>>>
> >>>>>> In any case, it should of course work. This is the leftover trace when
> >>>>>> we should be exiting, but an io-wq worker is still trying to get data
> >>>>>> from the eventfd:
> >>>>>
> >>>>> interesting, btw what kind of tool do you use for kernel debugging?
> >>>>
> >>>> Just poking at it and thinking about it, no hidden magic I'm afraid...
> >>>
> >>> Josef, can you try with this added? Looks bigger than it is, most of it
> >>> is just moving one function below another.
> >>
> >> Hmm, which kernel revision are you poking? Seems it doesn't match
> >> io_uring-5.10, and for 5.11 io_uring_cancel_files() is never called with
> >> NULL files.
> >>
> >> if (!files)
> >>      __io_uring_cancel_task_requests(ctx, task);
> >> else
> >>      io_uring_cancel_files(ctx, task, files);
> >
> > Yeah, I think I messed up. If files == NULL, then the task is going away.
> > So we should cancel all requests that match 'task', not just ones that
> > match task && files.
> >
> > Not sure I have much more time to look into this before next week, but
> > something like that.
> >
> > The problem case is the async worker being queued, long before the task
> > is killed and the contexts go away. But from exit_files(), we're only
> > concerned with canceling if we have inflight. Doesn't look right to me.
>
> In theory all that should be killed in io_ring_ctx_wait_and_kill(),
> of course that's if the ring itself is closed.
>
> Guys, do you share rings between processes? Explicitly like sending
> io_uring fd over a socket, or implicitly e.g. sharing fd tables
> (threads), or cloning with copying fd tables (and so taking a ref
> to a ring).

We do not share rings between processes. Our rings are accessible from different
threads (under locks), but nothing fancy.

> In other words, if you kill all your io_uring applications, does it
> go back to normal?

I'm pretty sure it does not, the only fix is to reboot the box. But I'll find an
affected box and double check just in case.

-- 
Dmitry Kadashev