mm: add !pte_present() check on existing hugetlb_entry callbacks

The age table walker doesn't check non-present hugetlb entry in common
path, so hugetlb_entry() callbacks must check it.  The reason for this
behavior is that some callers want to handle it in its own way.

[ I think that reason is bogus, btw - it should just do what the regular
  code does, which is to call the "pte_hole()" function for such hugetlb
  entries  - Linus]

However, some callers don't check it now, which causes unpredictable
result, for example when we have a race between migrating hugepage and
reading /proc/pid/numa_maps.  This patch fixes it by adding !pte_present
checks on buggy callbacks.

This bug exists for years and got visible by introducing hugepage
migration.

ChangeLog v2:
- fix if condition (check !pte_present() instead of pte_present())

Reported-by: Sasha Levin <[email protected]>
Signed-off-by: Naoya Horiguchi <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: <[email protected]> [3.12+]
Signed-off-by: Andrew Morton <[email protected]>
[ Backported to 3.15.  Signed-off-by: Josh Boyer <[email protected]> ]
Signed-off-by: Linus Torvalds <[email protected]>

mm/mempolicy.c

@@ -526,9 +526,13 @@ static void queue_pages_hugetlb_pmd_range(struct vm_area_struct *vma,
 	int nid;
 	struct page *page;
 	spinlock_t *ptl;
+	pte_t entry;
 
 	ptl = huge_pte_lock(hstate_vma(vma), vma->vm_mm, (pte_t *)pmd);
-	page = pte_page(huge_ptep_get((pte_t *)pmd));
+	entry = huge_ptep_get((pte_t *)pmd);
+	if (!pte_present(entry))
+		goto unlock;
+	page = pte_page(entry);
 	nid = page_to_nid(page);
 	if (node_isset(nid, *nodes) == !!(flags & MPOL_MF_INVERT))
 		goto unlock;