[PATCH] mm/hmm: Fix bad subpage pointer in try_to_unmap_one

[Ralph Campbell]

When migrating a ZONE device private page from device memory to system
memory, the subpage pointer is initialized from a swap pte which computes
an invalid page pointer. A kernel panic results such as:

BUG: unable to handle page fault for address: ffffea1fffffffc8

Initialize subpage correctly before calling page_remove_rmap().

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/rmap.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index e5dfe2ae6b0d..ec1af8b60423 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1476,6 +1476,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			 * No need to invalidate here it will synchronize on
 			 * against the special swap migration pte.
 			 */
+			subpage = page;
 			goto discard;
 		}
 
-- 
2.20.1