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

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Message-ID: <CADyq12xg8vxetG8y=M6i2m4qYrtjsRhmpds-3yKTY2icrHG2GA@mail.gmail.com>
Date:   Wed, 15 Apr 2020 18:15:26 -0700
From:   Brian Geffon <bgeffon@...gle.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Hillf Danton <hdanton@...a.com>,
        Peter Xu <peterx@...hat.com>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sonny Rao <sonnyrao@...gle.com>
Subject: Re: Userfaultfd doesn't seem to break out of poll on fd close

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.

Thanks,
Brian