memcg: increment static branch right after limit set

{

	int ret = -EINVAL;

#ifdef CONFIG_MEMCG_KMEM

	bool must_inc_static_branch = false;

	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);

	/*

	 * For simplicity, we won't allow this to be disabled.  It also can't

			res_counter_set_limit(&memcg->kmem, RESOURCE_MAX);

			goto out;

		}

		must_inc_static_branch = true;

		static_key_slow_inc(&memcg_kmem_enabled_key);

		/*

		 * setting the active bit after the inc will guarantee no one

		 * starts accounting before all call sites are patched

		 */

		memcg_kmem_set_active(memcg);

		/*

		 * kmem charges can outlive the cgroup. In the case of slab

		 * pages, for instance, a page contain objects from various

out:

	mutex_unlock(&set_limit_mutex);

	mutex_unlock(&memcg_create_mutex);

	/*

	 * We are by now familiar with the fact that we can't inc the static

	 * branch inside cgroup_lock. See disarm functions for details. A

	 * worker here is overkill, but also wrong: After the limit is set, we

	 * must start accounting right away. Since this operation can't fail,

	 * we can safely defer it to here - no rollback will be needed.

	 *

	 * The boolean used to control this is also safe, because

	 * KMEM_ACCOUNTED_ACTIVATED guarantees that only one process will be

	 * able to set it to true;

	 */

	if (must_inc_static_branch) {

		static_key_slow_inc(&memcg_kmem_enabled_key);

		/*

		 * setting the active bit after the inc will guarantee no one

		 * starts accounting before all call sites are patched

		 */

		memcg_kmem_set_active(memcg);

	}

#endif

	return ret;

}