Re: [PATCH V5 7/8] memcg: don't account root memcg page stats if only root exists

@@ -6303,6 +6360,49 @@ mem_cgroup_css_online(struct cgroup *cont)
 	}
 
 	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
+	if (!error) {
+		if (!mem_cgroup_in_use()) {
+			/* I'm the first non-root memcg, move global stats to root memcg.
+			 * Memcg creating is serialized by cgroup locks(cgroup_mutex),
+			 * so the mem_cgroup_in_use() checking is safe.
+			 *
+			 * We use global_page_state() to get global page stats, but
+			 * because of the optimized inc/dec functions in SMP while
+			 * updating each zone's stats, We may lose some numbers
+			 * in a stock(zone->pageset->vm_stat_diff) which brings some
+			 * inaccuracy. But places where kernel use these page stats to
+			 * steer next decision e.g. dirty page throttling or writeback
+			 * also use global_page_state(), so here it's enough too.
+			 */
+			spin_lock(&root_mem_cgroup->pcp_counter_lock);
+			root_mem_cgroup->stats_base.count[MEM_CGROUP_STAT_FILE_MAPPED] =
+						global_page_state(NR_FILE_MAPPED);
+			root_mem_cgroup->stats_base.count[MEM_CGROUP_STAT_FILE_DIRTY] =
+						global_page_state(NR_FILE_DIRTY);
+			root_mem_cgroup->stats_base.count[MEM_CGROUP_STAT_WRITEBACK] =
+						global_page_state(NR_WRITEBACK);
+			spin_unlock(&root_mem_cgroup->pcp_counter_lock);
+		}