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 int fuse_request_send_notify_reply(struct fuse_conn *fc,
					  struct fuse_req *req, u64 unique)
{
	int err = -ENODEV;
	struct fuse_iqueue *fiq = &fc->iq;

	__clear_bit(FR_ISREPLY, &req->flags);
	req->in.h.unique = unique;
	spin_lock(&fiq->waitq.lock);
	if (fiq->connected) {
		queue_request(fiq, req);
		err = 0;
	}
	spin_unlock(&fiq->waitq.lock);

	return err;
}

static int fuse_request_send_notify_reply(struct fuse_conn *fc,
					  struct fuse_req *req, u64 unique)
{
	int err = -ENODEV;
	struct fuse_iqueue *fiq = &fc->iq;

	__clear_bit(FR_ISREPLY, &req->flags);
	req->in.h.unique = unique;
	spin_lock(&fiq->waitq.lock);
	if (fiq->connected) {
		queue_request(fiq, req);
		err = 0;
	}
	spin_unlock(&fiq->waitq.lock);

	return err;
}
