linux-kernel - [PATCH v1] mm: hugetlb: call huge_pte

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Message-Id: <1448524936-10501-1-git-send-email-n-horiguchi@ah.jp.nec.com>
Date:	Thu, 26 Nov 2015 17:02:16 +0900
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: [PATCH v1] mm: hugetlb: call huge_pte_alloc() only if ptep is null

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@...jp.nec.com>
Cc: <stable@...r.kernel.org> [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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