[PATCH] mm:add VM_BUG_ON() for page_mapcount()

[Wang, Yalin]

This patch add VM_BUG_ON() for slab page,
because _mapcount is an union with slab struct in struct page,
avoid access _mapcount if this page is a slab page.
Also remove the unneeded bracket.

Signed-off-by: Yalin Wang <yalin.wang@xxxxxxxxxxxxxx>
---
 include/linux/mm.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 11b65cf..34124c4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -373,7 +373,8 @@ static inline void reset_page_mapcount(struct page *page)
 
 static inline int page_mapcount(struct page *page)
 {
-	return atomic_read(&(page)->_mapcount) + 1;
+	VM_BUG_ON(PageSlab(page));
+	return atomic_read(&page->_mapcount) + 1;
 }
 
 static inline int page_count(struct page *page)
-- 
2.1.3
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