memcg: increment static branch right after limit set

We were deferring the kmemcg static branch increment to a later time, due
to a nasty dependency between the cpu_hotplug lock, taken by the jump
label update, and the cgroup_lock.

Now we no longer take the cgroup lock, and we can save ourselves the
trouble.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 46cdaef78b01f75114051b730704ccc850c49fca..f4f41c36e70325c00a56e37cba22f80e7384336d 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4974,8 +4974,6 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
 {
        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
@@ -5004,7 +5002,13 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
                        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
@@ -5017,27 +5021,6 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
 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;
 }