mm: kmem: make memcg keep a reference to the original objcg

Keep a reference to the original objcg object for the entire life of a
memcg structure.

This allows to simplify the synchronization on the kernel memory
allocation paths: pinning a (live) memcg will also pin the corresponding
objcg.

The memory overhead of this change is minimal because object cgroups
usually outlive their corresponding memory cgroups even without this
change, so it's only an additional pointer per memcg.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Roman Gushchin (Cruise) <[email protected]>
Tested-by: Naresh Kamboju <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

include/linux/memcontrol.h

@@ -289,7 +289,13 @@ struct mem_cgroup {
 
 #ifdef CONFIG_MEMCG_KMEM
 	int kmemcg_id;
-	struct obj_cgroup __rcu *objcg;
+	/*
+	 * memcg->objcg is wiped out as a part of the objcg repaprenting
+	 * process. memcg->orig_objcg preserves a pointer (and a reference)
+	 * to the original objcg until the end of live of memcg.
+	 */
+	struct obj_cgroup __rcu	*objcg;
+	struct obj_cgroup	*orig_objcg;
 	/* list of inherited objcgs, protected by objcg_lock */
 	struct list_head objcg_list;
 #endif

mm/memcontrol.c

@@ -3899,6 +3899,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 
 	objcg->memcg = memcg;
 	rcu_assign_pointer(memcg->objcg, objcg);
+	obj_cgroup_get(objcg);
+	memcg->orig_objcg = objcg;
 
 	static_branch_enable(&memcg_kmem_online_key);
 
@@ -5406,6 +5408,9 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 {
 	int node;
 
+	if (memcg->orig_objcg)
+		obj_cgroup_put(memcg->orig_objcg);
+
 	for_each_node(node)
 		free_mem_cgroup_per_node_info(memcg, node);
 	kfree(memcg->vmstats);