mm: update comments for struct page.mapping

struct page.mapping can be NULL or points to one object of type
address_space, anon_vma or KSM private structure.

Link: http://lkml.kernel.org/r/1506485067-15954-1-git-send-email-changbin.du@intel.com
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -48,8 +48,10 @@ struct page {
 						 * inode address_space, or NULL.
 						 * If page mapped as anonymous
 						 * memory, low bit is set, and
-						 * it points to anon_vma object:
-						 * see PAGE_MAPPING_ANON below.
+						 * it points to anon_vma object
+						 * or KSM private structure. See
+						 * PAGE_MAPPING_ANON and
+						 * PAGE_MAPPING_KSM.
 						 */
 		void *s_mem;			/* slab first object */
 		atomic_t compound_mapcount;	/* first tail page */