[PATCH 1/2 mmotm] mm/migrate: check-before-clear PageSwapCache

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

With page flag sanitization patchset, an invalid usage of ClearPageSwapCache()
is detected in migration_page_copy().
migrate_page_copy() is shared by both normal and hugepage (both thp and hugetlb)
code path, so let's check PageSwapCache() and clear it if it's set to avoid
misuse of the invalid clear operation.

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

diff --git a/mm/migrate.c b/mm/migrate.c
index 25fd7f6291de..5fa399d20435 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -537,7 +537,8 @@ void migrate_page_copy(struct page *newpage, struct page *page)
 	 * Please do not reorder this without considering how mm/ksm.c's
 	 * get_ksm_page() depends upon ksm_migrate_page() and PageSwapCache().
 	 */
-	ClearPageSwapCache(page);
+	if (PageSwapCache(page))
+		ClearPageSwapCache(page);
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
 
-- 
2.1.0

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