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Subject[PATCH v1 2/2] mm: rmap: call page_check_address() with sync enabled to avoid racy check
Date
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@ah.jp.nec.com>
---
mm/rmap.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git v4.6/mm/rmap.c v4.6_patched/mm/rmap.c
index 4282b56..c357fb36 100644
--- v4.6/mm/rmap.c
+++ v4.6_patched/mm/rmap.c
@@ -1424,7 +1424,8 @@ 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
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