linux-kernel - [PATCH v4 2/2] mm: rmap: call page_check_address() with sync enabled to avoid racy check

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Date:	Mon, 27 Jun 2016 10:28:49 +0900
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	linux-mm@...ck.org
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: [PATCH v4 2/2] mm: rmap: call page_check_address() with sync enabled to avoid racy check

The previous patch addresses the race between split_huge_pmd_address() and
someone changing the pmd. The fix is only for splitting of normal thp
(i.e. pmd-mapped thp,) and for splitting of pte-mapped thp there still is
the similar race.

For splitting pte-mapped thp, the pte's conversion is done by
try_to_unmap_one(TTU_MIGRATION). This function checks page_check_address() to
get the target pte, but it can return NULL under some race, leading to
VM_BUG_ON() in freeze_page(). Fortunately, page_check_address() already has
an argument to decide whether we do a quick/racy check or not, so let's flip
it when called from freeze_page().

Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
---
 mm/rmap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git v4.7-rc4-mmotm-2016-06-23-16-33/mm/rmap.c v4.7-rc4-mmotm-2016-06-23-16-33_patched/mm/rmap.c
index 256e585..a8bce293 100644
--- v4.7-rc4-mmotm-2016-06-23-16-33/mm/rmap.c
+++ v4.7-rc4-mmotm-2016-06-23-16-33_patched/mm/rmap.c
@@ -1457,7 +1457,8 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			goto out;
 	}

-	pte = page_check_address(page, mm, address, &ptl, 0);
+	pte = page_check_address(page, mm, address, &ptl,
+				 PageTransCompound(page));
 	if (!pte)
 		goto out;

-- 
2.7.0