[PATCH] mm: do not access page->mapping directly on page_endio

[Minchan Kim]

With rw_page, page_endio is used for completing IO on a page
and it propagates write error to the address space if the IO
fails. The problem is it accesses page->mapping directly which
might be okay for file-backed pages but it shouldn't for
anonymous page. Otherwise, it can corrupt one of field from
anon_vma under us and system goes panic randomly.

Cc: Matthew Wilcox <wi...@infradead.org>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Minchan Kim <minc...@kernel.org>
---
 mm/filemap.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 2ba46f410c7c..1944c631e3e6 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1008,9 +1008,12 @@ void page_endio(struct page *page, bool is_write, int 
err)
                unlock_page(page);
        } else {
                if (err) {
+                       struct address_space *mapping;
+
                        SetPageError(page);
-                       if (page->mapping)
-                               mapping_set_error(page->mapping, err);
+                       mapping = page_mapping(page);
+                       if (mapping)
+                               mapping_set_error(mapping, err);
                }
                end_page_writeback(page);
        }
-- 
2.7.4