linux-kernel - [PATCH v2 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap

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Date:   Fri, 19 Jul 2019 12:29:55 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     <linux-mm@...ck.org>
CC:     <linux-kernel@...r.kernel.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...lanox.com>,
        John Hubbard <jhubbard@...dia.com>, <stable@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v2 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap_one

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@...dia.com>
Cc: "Jérôme Glisse" <jglisse@...hat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Christoph Hellwig <hch@....de>
Cc: Jason Gunthorpe <jgg@...lanox.com>
Cc: John Hubbard <jhubbard@...dia.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-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