userfaultfd: non-cooperative: avoid MADV_DONTNEED race condition

MADV_DONTNEED must be notified to userland before the pages are zapped.

This allows userland to immediately stop adding pages to the userfaultfd
ranges before the pages are actually zapped or there could be
non-zeropage leftovers as result of concurrent UFFDIO_COPY run in
between zap_page_range and madvise_userfault_dontneed (both
MADV_DONTNEED and UFFDIO_COPY runs under the mmap_sem for reading, so
they can run concurrently).

Link: http://lkml.kernel.org/r/20161216144821.5183-15-aarcange@redhat.comSigned-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/madvise.c

@@ -477,8 +477,8 @@ static long madvise_dontneed(struct vm_area_struct *vma,
 	if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
 		return -EINVAL;
 
-	zap_page_range(vma, start, end - start, NULL);
 	madvise_userfault_dontneed(vma, prev, start, end);
+	zap_page_range(vma, start, end - start, NULL);
 	return 0;
 }