[PATCH v2 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap_one

From: Ralph Campbell <rcampbell@nvidia.com>
To: <linux-mm@kvack.org>
Cc: linux-kernel@vger.kernel.org,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Jason Gunthorpe" <jgg@mellanox.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	stable@vger.kernel.org,
	"Andrew Morton" <akpm@linux-foundation.org>
Subject: [PATCH v2 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap_one
Date: Fri, 19 Jul 2019 12:29:55 -0700	[thread overview]
Message-ID: <20190719192955.30462-4-rcampbell@nvidia.com> (raw)
In-Reply-To: <20190719192955.30462-1-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 | 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

