struct {
union {
- atomic_t _mapcount; /* Count of ptes mapped in mms,
- * to show when page is mapped
- * & limit reverse map searches.
- */
+ /*
+ * Count of ptes mapped in
+ * mms, to show when page is
+ * mapped & limit reverse map
+ * searches.
+ *
+ * Used also for tail pages
+ * refcounting instead of
+ * _count. Tail pages cannot
+ * be mapped and keeping the
+ * tail page _count zero at
+ * all times guarantees
+ * get_page_unless_zero() will
+ * never succeed on tail
+ * pages.
+ */
+ atomic_t _mapcount;
struct {
unsigned inuse:16;