[PATCH RESEND -mm 2/2] mm/mempolicy.c: add comment in queue_pages_hugetlb()

[Naoya Horiguchi]

We have a race where we try to migrate an invalid page, resulting in
hitting VM_BUG_ON_PAGE in isolate_huge_page().
queue_pages_hugetlb() is OK to fail, so let's check !PageHeadHuge to keep
invalid hugepage from queuing.

Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
 mm/mempolicy.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git v3.14-rc7-mmotm-2014-03-18-16-37.orig/mm/mempolicy.c v3.14-rc7-mmotm-2014-03-18-16-37/mm/mempolicy.c
index 9d2ef4111a4c..ae6e2d9dc855 100644
--- v3.14-rc7-mmotm-2014-03-18-16-37.orig/mm/mempolicy.c
+++ v3.14-rc7-mmotm-2014-03-18-16-37/mm/mempolicy.c
@@ -530,6 +530,17 @@ static int queue_pages_hugetlb(pte_t *pte, unsigned long addr,
 	if (!pte_present(entry))
 		return 0;
 	page = pte_page(entry);
+
+	/*
+	 * Trinity found that page could be a non-hugepage. This is an
+	 * unexpected behavior, but it's not clear how this problem happens.
+	 * So let's simply skip such corner case. Page migration can often
+	 * fail for various reasons, so it's ok to just skip the address
+	 * unsuitable to hugepage migration.
+	 */
+	if (!PageHeadHuge(page))
+		return 0;
+
 	nid = page_to_nid(page);
 	if (node_isset(nid, *qp->nmask) == !!(flags & MPOL_MF_INVERT))
 		return 0;
-- 
1.8.5.3

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