userfaultfd: hugetlbfs: reserve count on error in __mcopy_atomic_hugetlb

If __mcopy_atomic_hugetlb exits with an error, put_page will be called
if a huge page was allocated and needs to be freed.  If a reservation
was associated with the huge page, the PagePrivate flag will be set.
Clear PagePrivate before calling put_page/free_huge_page so that the
global reservation count is not incremented.

Link: http://lkml.kernel.org/r/20161216144821.5183-26-aarcange@redhat.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-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 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>

Author: mjkravetz

mm/userfaultfd.c

@@ -301,8 +301,23 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 out_unlock:
 	up_read(&dst_mm->mmap_sem);
 out:
-	if (page)
+	if (page) {
+		/*
+		 * We encountered an error and are about to free a newly
+		 * allocated huge page.  It is possible that there was a
+		 * reservation associated with the page that has been
+		 * consumed.  See the routine restore_reserve_on_error
+		 * for details.  Unfortunately, we can not call
+		 * restore_reserve_on_error now as it would require holding
+		 * mmap_sem.  Clear the PagePrivate flag so that the global
+		 * reserve count will not be incremented in free_huge_page.
+		 * The reservation map will still indicate the reservation
+		 * was consumed and possibly prevent later page allocation.
+		 * This is better than leaking a global reservation.
+		 */
+		ClearPagePrivate(page);
 		put_page(page);
+	}
 	BUG_ON(copied < 0);
 	BUG_ON(err > 0);
 	BUG_ON(!copied && !err);