Patch "mm: hugetlb: call huge_pte_alloc() only if ptep is null" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

mm: hugetlb: call huge_pte_alloc() only if ptep is null

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-hugetlb-call-huge_pte_alloc-only-if-ptep-is-null.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Naoya Horiguchi <[email protected]>
Date: Fri, 11 Dec 2015 13:40:49 -0800
Subject: mm: hugetlb: call huge_pte_alloc() only if ptep is null

From: Naoya Horiguchi <[email protected]>

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.)

---
 mm/hugetlb.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

@@ -3590,12 +3590,12 @@ int hugetlb_fault(struct mm_struct *mm,
 		} 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);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/mm-hugetlb-call-huge_pte_alloc-only-if-ptep-is-null.patch queue-4.3/mm-hugetlbfs-fix-bugs-in-fallocate-hole-punch-of-areas-with-holes.patch queue-4.3/mm-hugetlb-fix-hugepage-memory-leak-caused-by-wrong-reserve-count.patch