memcg: decrement static keys at real destroy time

We call the destroy function when a cgroup starts to be removed, such as
by a rmdir event.

However, because of our reference counters, some objects are still
inflight.  Right now, we are decrementing the static_keys at destroy()
time, meaning that if we get rid of the last static_key reference, some
objects will still have charges, but the code to properly uncharge them
won't be run.

This becomes a problem specially if it is ever enabled again, because now
new charges will be added to the staled charges making keeping it pretty
much impossible.

We just need to be careful with the static branch activation: since there
is no particular preferred order of their activation, we need to make sure
that we only start using it after all call sites are active.  This is
achieved by having a per-memcg flag that is only updated after
static_key_slow_inc() returns.  At this time, we are sure all sites are
active.

This is made per-memcg, not global, for a reason: it also has the effect
of making socket accounting more consistent.  The first memcg to be
limited will trigger static_key() activation, therefore, accounting.  But
all the others will then be accounted no matter what.  After this patch,
only limited memcgs will have its sockets accounted.

[akpm@linux-foundation.org: move enum sock_flag_bits into sock.h,
                            document enum sock_flag_bits,
                            convert memcg_proto_active() and memcg_proto_activated() to test_bit(),
                            redo tcp_update_limit() comment to 80 cols]
Signed-off-by: Glauber Costa <glommer@parallels.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 29 insertions, 2 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6fbf50977f77..ac35bccadb7b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -417,6 +417,7 @@ void sock_update_memcg(struct sock *sk)
 {
         if (mem_cgroup_sockets_enabled) {
                 struct mem_cgroup *memcg;
+                struct cg_proto *cg_proto;
 
                 BUG_ON(!sk->sk_prot->proto_cgroup);
 
@@ -436,9 +437,10 @@ void sock_update_memcg(struct sock *sk)
 
                 rcu_read_lock();
                 memcg = mem_cgroup_from_task(current);
-                if (!mem_cgroup_is_root(memcg)) {
+                cg_proto = sk->sk_prot->proto_cgroup(memcg);
+                if (!mem_cgroup_is_root(memcg) && memcg_proto_active(cg_proto)) {
                         mem_cgroup_get(memcg);
-                        sk->sk_cgrp = sk->sk_prot->proto_cgroup(memcg);
+                        sk->sk_cgrp = cg_proto;
                 }
                 rcu_read_unlock();
         }
@@ -467,6 +469,19 @@ EXPORT_SYMBOL(tcp_proto_cgroup);
 #endif /* CONFIG_INET */
 #endif /* CONFIG_CGROUP_MEM_RES_CTLR_KMEM */
 
+#if defined(CONFIG_INET) && defined(CONFIG_CGROUP_MEM_RES_CTLR_KMEM)
+static void disarm_sock_keys(struct mem_cgroup *memcg)
+{
+        if (!memcg_proto_activated(&memcg->tcp_mem.cg_proto))
+                return;
+        static_key_slow_dec(&memcg_socket_limit_enabled);
+}
+#else
+static void disarm_sock_keys(struct mem_cgroup *memcg)
+{
+}
+#endif
+
 static void drain_all_stock_async(struct mem_cgroup *memcg);
 
 static struct mem_cgroup_per_zone *
@@ -4712,6 +4727,18 @@ static void free_work(struct work_struct *work)
         int size = sizeof(struct mem_cgroup);
 
         memcg = container_of(work, struct mem_cgroup, work_freeing);
+        /*
+         * We need to make sure that (at least for now), the jump label
+         * destruction code runs outside of the cgroup lock. This is because
+         * get_online_cpus(), which is called from the static_branch update,
+         * can't be called inside the cgroup_lock. cpusets are the ones
+         * enforcing this dependency, so if they ever change, we might as well.
+         *
+         * schedule_work() will guarantee this happens. Be careful if you need
+         * to move this code around, and make sure it is outside
+         * the cgroup_lock.
+         */
+        disarm_sock_keys(memcg);
         if (size < PAGE_SIZE)
                 kfree(memcg);
         else