+ /*
+ * THP can not break up slab pages so avoid taking
+ * compound_lock() and skip the tail page refcounting (in
+ * _mapcount) too. Slab performs non-atomic bit ops on
+ * page->flags for better performance. In particular
+ * slab_unlock() in slub used to be a hot path. It is still
+ * hot on arches that do not support
+ * this_cpu_cmpxchg_double().
+ *
+ * If "page" is part of a slab or hugetlbfs page it cannot be
+ * splitted and the head page cannot change from under us. And
+ * if "page" is part of a THP page under splitting, if the
+ * head page pointed by the THP tail isn't a THP head anymore,
+ * we'll find PageTail clear after smp_rmb() and we'll treat
+ * it as a single page.
+ */
+ if (!__compound_tail_refcounted(page_head)) {
+ /*
+ * If "page" is a THP tail, we must read the tail page
+ * flags after the head page flags. The
+ * split_huge_page side enforces write memory barriers
+ * between clearing PageTail and before the head page
+ * can be freed and reallocated.
+ */
+ smp_rmb();
+ if (likely(PageTail(page))) {