Merge branch 'page-refs' (page ref overflow)

Merge page ref overflow branch.

Jann Horn reported that he can overflow the page ref count with
sufficient memory (and a filesystem that is intentionally extremely
slow).

Admittedly it's not exactly easy.  To have more than four billion
references to a page requires a minimum of 32GB of kernel memory just
for the pointers to the pages, much less any metadata to keep track of
those pointers.  Jann needed a total of 140GB of memory and a specially
crafted filesystem that leaves all reads pending (in order to not ever
free the page references and just keep adding more).

Still, we have a fairly straightforward way to limit the two obvious
user-controllable sources of page references: direct-IO like page
references gotten through get_user_pages(), and the splice pipe page
duplication.  So let's just do that.

* branch page-refs:
  fs: prevent page refcount overflow in pipe_buf_get
  mm: prevent get_user_pages() from overflowing page refcount
  mm: add 'try_get_page()' helper function
  mm: make page ref count overflow check tighter and more explicit
static __poll_t fuse_dev_poll(struct file *file, poll_table *wait)
{
	__poll_t mask = EPOLLOUT | EPOLLWRNORM;
	struct fuse_iqueue *fiq;
	struct fuse_dev *fud = fuse_get_dev(file);

	if (!fud)
		return EPOLLERR;

	fiq = &fud->fc->iq;
	poll_wait(file, &fiq->waitq, wait);

	spin_lock(&fiq->waitq.lock);
	if (!fiq->connected)
		mask = EPOLLERR;
	else if (request_pending(fiq))
		mask |= EPOLLIN | EPOLLRDNORM;
	spin_unlock(&fiq->waitq.lock);

	return mask;
}

static __poll_t fuse_dev_poll(struct file *file, poll_table *wait)
{
	__poll_t mask = EPOLLOUT | EPOLLWRNORM;
	struct fuse_iqueue *fiq;
	struct fuse_dev *fud = fuse_get_dev(file);

	if (!fud)
		return EPOLLERR;

	fiq = &fud->fc->iq;
	poll_wait(file, &fiq->waitq, wait);

	spin_lock(&fiq->waitq.lock);
	if (!fiq->connected)
		mask = EPOLLERR;
	else if (request_pending(fiq))
		mask |= EPOLLIN | EPOLLRDNORM;
	spin_unlock(&fiq->waitq.lock);

	return mask;
}
