[PATCH 4.2.y-ckt 178/305] mm: hugetlb: call huge_pte_alloc() only if ptep is null

[Kamal Mostafa]

4.2.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Naoya Horiguchi <n-horiguchi at ah.jp.nec.com>

commit 0d777df5d8953293be090d9ab5a355db893e8357 upstream.

Currently at the beginning of hugetlb_fault(), we call huge_pte_offset()
and check whether the obtained *ptep is a migration/hwpoison entry or
not.  And if not, then we get to call huge_pte_alloc().  This is racy
because the *ptep could turn into migration/hwpoison entry after the
huge_pte_offset() check.  This race results in BUG_ON in
huge_pte_alloc().

We don't have to call huge_pte_alloc() when the huge_pte_offset()
returns non-NULL, so let's fix this bug with moving the code into else
block.

Note that the *ptep could turn into a migration/hwpoison entry after
this block, but that's not a problem because we have another
!pte_present check later (we never go into hugetlb_no_page() in that
case.)

Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
Signed-off-by: Naoya Horiguchi <n-horiguchi at ah.jp.nec.com>
Acked-by: Hillf Danton <hillf.zj at alibaba-inc.com>
Acked-by: David Rientjes <rientjes at google.com>
Cc: Hugh Dickins <hughd at google.com>
Cc: Dave Hansen <dave.hansen at intel.com>
Cc: Mel Gorman <mgorman at suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/hugetlb.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f70850e..080ed4b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3350,12 +3350,12 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
 			return VM_FAULT_HWPOISON_LARGE |
 				VM_FAULT_SET_HINDEX(hstate_index(h));
+	} else {
+		ptep = huge_pte_alloc(mm, address, huge_page_size(h));
+		if (!ptep)
+			return VM_FAULT_OOM;
 	}
 
-	ptep = huge_pte_alloc(mm, address, huge_page_size(h));
-	if (!ptep)
-		return VM_FAULT_OOM;
-
 	mapping = vma->vm_file->f_mapping;
 	idx = vma_hugecache_offset(h, vma, address);
 
-- 
1.9.1