[PATCH rfc 3/5] mm: kmem: make memcg keep a reference to the original objcg

[Roman Gushchin <roman.gushchin@linux.dev>]

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.

Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
---
 include/linux/memcontrol.h | 8 +++++++-
 mm/memcontrol.c            | 5 +++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 84425bfe4124..412ff0e8694d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -299,7 +299,13 @@ 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
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7f33a503d600..4815f897758c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3803,6 +3803,8 @@ 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);
 
@@ -5297,6 +5299,9 @@ 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);
-- 
2.42.0