mm: improve comment on page->mapping

The comment on page->mapping is terse, and out of date (it does not
mention the possibility of PAGE_MAPPING_MOVABLE).  Instead, point the
interested reader to page-flags.h where there is a much better comment.

Link: http://lkml.kernel.org/r/20171220155552.15884-5-willy@infradead.orgSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -50,15 +50,9 @@ struct page {
 	unsigned long flags;		/* Atomic flags, some possibly
 					 * updated asynchronously */
 	union {
-		struct address_space *mapping;	/* If low bit clear, points to
-						 * inode address_space, or NULL.
-						 * If page mapped as anonymous
-						 * memory, low bit is set, and
-						 * it points to anon_vma object
-						 * or KSM private structure. See
-						 * PAGE_MAPPING_ANON and
-						 * PAGE_MAPPING_KSM.
-						 */
+		/* See page-flags.h for the definition of PAGE_MAPPING_FLAGS */
+		struct address_space *mapping;
+
 		void *s_mem;			/* slab first object */
 		atomic_t compound_mapcount;	/* first tail page */
 		/* page_deferred_list().next	 -- second tail page */