[PATCH] mm: fix lazyfree bug on check in try_to_unmap_one

[Minchan Kim]

If a page is swapbacked, it means it should be in swapcache
in try_to_unmap_one's path.

If a page is !swapbacked, it mean it shouldn't be in swapcache
in try_to_unmap_one's path.

Check both two cases all at once and if it fails, warn and
return SWAP_FAIL. Such bug never mean we should shut down
the kernel.

Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/rmap.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 35acb83..9925f32 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1413,8 +1413,13 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			 * Store the swap location in the pte.
 			 * See handle_pte_fault() ...
 			 */
-			VM_BUG_ON_PAGE(!PageSwapCache(page) && PageSwapBacked(page),
-				page);
+			if (VM_WARN_ON_ONCE(PageSwapBacked(page) !=
+						PageSwapCache(page))) {
+				ret = SWAP_FAIL;
+				page_vma_mapped_walk_done(&pvmw);
+				break;
+
+			}
 
 			/* MADV_FREE page check */
 			if (!PageSwapBacked(page)) {
-- 
2.7.4