linux-kernel - [PATCH 3.2 57/77] mm: hugetlb: call huge_pte

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Message-ID: <lsq.1450971462.120292654@decadent.org.uk>
Date:	Thu, 24 Dec 2015 15:37:42 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Dave Hansen" <dave.hansen@...el.com>,
	"Joonsoo Kim" <iamjoonsoo.kim@....com>,
	"David Rientjes" <rientjes@...gle.com>,
	"Naoya Horiguchi" <n-horiguchi@...jp.nec.com>,
	"Hugh Dickins" <hughd@...gle.com>, "Mel Gorman" <mgorman@...e.de>,
	"Hillf Danton" <hillf.zj@...baba-inc.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Mike Kravetz" <mike.kravetz@...cle.com>
Subject: [PATCH 3.2 57/77] mm: hugetlb: call huge_pte_alloc() only if ptep
 is null

3.2.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Naoya Horiguchi <n-horiguchi@...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@...jp.nec.com>
Acked-by: Hillf Danton <hillf.zj@...baba-inc.com>
Acked-by: David Rientjes <rientjes@...gle.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/hugetlb.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2818,12 +2818,12 @@ int hugetlb_fault(struct mm_struct *mm,
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
 			return VM_FAULT_HWPOISON_LARGE |
 			       VM_FAULT_SET_HINDEX(h - hstates);
+	} 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;
-
 	/*
 	 * Serialize hugepage allocation and instantiation, so that we don't
 	 * get spurious allocation failures if two CPUs race to instantiate

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