Re: Userfaultfd doesn't seem to break out of poll on fd close

[Brian Geffon]

Thanks Peter,
I see your point. I'm totally fine if we just leave this at: just
don't do it. lol. I appreciate you guys taking the time to talk
through this.

Brian



Brian

On Wed, Apr 15, 2020 at 6:37 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Wed, Apr 15, 2020 at 06:15:26PM -0700, Brian Geffon wrote:
> > Hi Andrea,
> > Thanks for taking the time to reply.
> >
> > > static int userfaultfd_flush(struct file *file, fl_owner_t id)
> > > {
> > >         struct userfaultfd_ctx *ctx = file->private_data;
> > >         wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
> > > }
> > >
> >
> > Yes, I think that something like this would work for this situation and eventfd.
> >
> > > If eventfd and pipes all behave identical to uffd (they should as they
> > > don't seem to implement flush) I'm not sure if there's good enough
> > > justification to deviate from the default VFS behavior here.
> >
> > Pipes actually behave a little differently, in the case that you close
> > the write end of the pipe the read end will break out of the poll with
> > EPOLLHUP, but I suppose closing the read end while the read end is
> > being polled would be more analogous to what I'm describing here. And
> > this is why it felt weird to me, in these situations the kernel
> > _knows_ that after the close nothing can happen on the file
> > descriptor, so what's the point of keeping it in a poll? As soon as
> > the poll breaks any read, write, ioctl, etc on the fd whether it's a
> > userfaultfd or an eventfd would fail with -EBADF.
> >
> > And all of that I guess makes sense in the case of a non-blocking fd,
> > but what about the case of a blocking file descriptor? Both
> > userfaultfd and eventfd can seemingly be stuck in a read syscall with
> > no way to break them out when the userfaultfd/eventfd has no further
> > utility. Here is an example:
> > https://gist.github.com/bgaff/607302d86d99ac539efca307ce2dd679
> >
> > For my use case adding an eventfd on poll works well, so thank you for
> > that suggestion. But the behavior just seemed odd to me which is why I
> > started this thread.
>
> Hi, Brian,
>
> I think I can understand you on the weirdness when comparing to the
> pipes.  And IIUC that's majorly what POLLHUP is used for - it tells us
> that the channel has closed.  I believe it's the same to a pair of
> send/recv sockets when one end closes the port so the other side can
> get a POLLHUP.
>
> However IMO userfaultfd is not such a channel like pipes, as you have
> already mentioned.  It's not paired ports.  As you've given the other
> example on "closing the read pipe when reading the read pipe" - I'm
> curious what will happen for that.  I feel like it'll happen the same
> way as being blocked, just like what userfaultfd and eventfd are
> doing.  My understanding is that the Linux kernel should be thread
> safe on all these operations so no matter how we use the syscalls and
> in what order the kernel shouldn't break with this.  However IMHO it
> does not mean that it'll guarantee things like "close() will kick all
> existing fd operations".  I don't know whether there's any restriction
> in POSIX or anything for this, but... I won't be too surprised if
> someone tells me there's some OS that will directly crash the process
> if one fd is close()ed during a read()...
>
> Thanks,
>
> --
> Peter Xu
>