[PATCH v3 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap_one

[Ralph Campbell <rcampbell@nvidia.com>]

When migrating an anonymous private page to a ZONE_DEVICE private page,
the source page->mapping and page->index fields are copied to the
destination ZONE_DEVICE struct page and the page_mapcount() is increased.
This is so rmap_walk() can be used to unmap and migrate the page back to
system memory. However, try_to_unmap_one() computes the subpage pointer
from a swap pte which computes an invalid page pointer and a kernel panic
results such as:

BUG: unable to handle page fault for address: ffffea1fffffffc8

Currently, only single pages can be migrated to device private memory so
no subpage computation is needed and it can be set to "page".

Fixes: a5430dda8a3a1c ("mm/migrate: support un-addressable ZONE_DEVICE page in migration")
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 mm/rmap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index e5dfe2ae6b0d..003377e24232 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1475,7 +1475,15 @@ 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.
+			 *
+			 * The assignment to subpage above was computed from a
+			 * swap PTE which results in an invalid pointer.
+			 * Since only PAGE_SIZE pages can currently be
+			 * migrated, just set it to page. This will need to be
+			 * changed when hugepage migrations to device private
+			 * memory are supported.
 			 */
+			subpage = page;
 			goto discard;
 		}
 
-- 
2.20.1