[PATCH] net: set proper memcg for net_init hooks allocations

[Vasily Averin <vvs@openvz.org>]

__register_pernet_operations() executes init hook of registered
pernet_operation structure in all existing net namespaces.

Typically, these hooks are called by a process associated with
the specified net namespace, and all __GFP_ACCOUNTING marked
allocation are accounted for corresponding container/memcg.

However __register_pernet_operations() calls the hooks in the same
context, and as a result all marked allocations are accounted
to one memcg for all processed net namespaces.

This patch adjusts active memcg for each net namespace and helps
to account memory allocated inside ops_init() into the proper memcg.

Signed-off-by: Vasily Averin <vvs@openvz.org>
---
v1: introduced get_mem_cgroup_from_kmem(), which takes the refcount
    for the found memcg, suggested by Shakeel
---
 include/linux/memcontrol.h | 11 +++++++++++
 net/core/net_namespace.c   |  9 +++++++++
 2 files changed, 20 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 0abbd685703b..16b157852a8c 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1768,4 +1768,15 @@ static inline struct mem_cgroup *mem_cgroup_from_obj(void *p)
 
 #endif /* CONFIG_MEMCG_KMEM */
 
+static inline struct mem_cgroup *get_mem_cgroup_from_kmem(void *p)
+{
+	struct mem_cgroup *memcg;
+
+	rcu_read_lock();
+	do {
+		memcg = mem_cgroup_from_obj(p);
+	} while (memcg && !css_tryget(&memcg->css));
+	rcu_read_unlock();
+	return memcg;
+}
 #endif /* _LINUX_MEMCONTROL_H */
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index a5b5bb99c644..56608f56bed6 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -26,6 +26,7 @@
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
 
+#include <linux/sched/mm.h>
 /*
  *	Our network namespace constructor/destructor lists
  */
@@ -1147,7 +1148,15 @@ static int __register_pernet_operations(struct list_head *list,
 		 * setup_net() and cleanup_net() are not possible.
 		 */
 		for_each_net(net) {
+			struct mem_cgroup *old, *memcg;
+
+			memcg = get_mem_cgroup_from_kmem(net);
+			if (!memcg)
+				memcg = root_mem_cgroup;
+			old = set_active_memcg(memcg);
 			error = ops_init(ops, net);
+			set_active_memcg(old);
+			css_put(&memcg->css);
 			if (error)
 				goto out_undo;
 			list_add_tail(&net->exit_list, &net_exit_list);
-- 
2.31.1