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

[Naoya Horiguchi]

Page 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.

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 <sasha.levin@xxxxxxxxxx>
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.12+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

[ Backported to 3.15.  Signed-off-by: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx> ]
---
 fs/proc/task_mmu.c | 3 +++
 mm/mempolicy.c     | 6 +++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 442177b1119a..89620cdb57c9 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1354,6 +1354,9 @@ static int gather_hugetbl_stats(pte_t *pte, unsigned long hmask,
 	if (pte_none(*pte))
 		return 0;
 
+	if (!pte_present(*pte))
+		return 0;
+
 	page = pte_page(*pte);
 	if (!page)
 		return 0;
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 78e1472933ea..30cc47f8ffa0 100644
--- a/mm/mempolicy.c
+++ b/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;
-- 
1.9.3

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