- * Usage count, *USE WRAPPER FUNCTION*
- * when manual accounting. See page_ref.h
+ * Count of ptes mapped in mms, to show when
+ * page is mapped & limit reverse map searches.
+ *
+ * Extra information about page type may be
+ * stored here for pages that are never mapped,
+ * in which case the value MUST BE <= -2.
+ * See page-flags.h for more details.