[PATCH v1] mm: hugetlb: call huge_pte_alloc() only if ptep is null

[Naoya Horiguchi]

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@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [2.6.36+]
---
 mm/hugetlb.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git next-20151123/mm/hugetlb.c next-20151123_patched/mm/hugetlb.c
index 1101ccd..6ad5e91 100644
--- next-20151123/mm/hugetlb.c
+++ next-20151123_patched/mm/hugetlb.c
@@ -3696,12 +3696,12 @@ 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);
 
-- 
1.7.1

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