[PATCH] mm: memcg/slab: enable slab memory accounting atomically

[Roman Gushchin]

Many kernel memory accounting paths are guarded by the
memcg_kmem_enabled_key static key. It changes it's state during
the onlining of the first non-root cgroup. However is doesn't
happen atomically: before all call sites will become patched
some charges/uncharges can be skipped, resulting in an unbalanced
charge. The problem is mostly a theoretical issue, unlikely
having a noticeable impact ion the real life.

Before the rework of the slab controller we relied at setting
kmemcg_id after enabling the memcg_kmem_enabled_key static key.
Now we can use the setting of memcg->objcg to enable the slab
memory accounting atomically.

The patch also removes obsolete comments related to already deleted
members of kmem_cache->memcg_params.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
---
 include/linux/memcontrol.h | 6 ++----
 mm/memcontrol.c            | 7 ++++---
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 20108e426f84..01099dfa839c 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -310,7 +310,6 @@ struct mem_cgroup {
 	int			tcpmem_pressure;
 
 #ifdef CONFIG_MEMCG_KMEM
-        /* Index in the kmem_cache->memcg_params.memcg_caches array */
 	int kmemcg_id;
 	enum memcg_kmem_state kmem_state;
 	struct obj_cgroup __rcu *objcg;
@@ -1641,9 +1640,8 @@ static inline void memcg_kmem_uncharge_page(struct page *page, int order)
 }
 
 /*
- * helper for accessing a memcg's index. It will be used as an index in the
- * child cache array in kmem_cache, and also to derive its name. This function
- * will return -1 when this is not a kmem-limited memcg.
+ * A helper for accessing memcg's kmem_id, used for getting
+ * corresponding LRU lists.
  */
 static inline int memcg_cache_id(struct mem_cgroup *memcg)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 69a2893a6455..267cc68fba05 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3675,17 +3675,18 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 		memcg_free_cache_id(memcg_id);
 		return -ENOMEM;
 	}
-	objcg->memcg = memcg;
-	rcu_assign_pointer(memcg->objcg, objcg);
 
 	static_branch_enable(&memcg_kmem_enabled_key);
 
 	/*
 	 * A memory cgroup is considered kmem-online as soon as it gets
-	 * kmemcg_id. Setting the id after enabling static branching will
+	 * objcg. Setting the objcg after enabling static branching will
 	 * guarantee no one starts accounting before all call sites are
 	 * patched.
 	 */
+	objcg->memcg = memcg;
+	rcu_assign_pointer(memcg->objcg, objcg);
+
 	memcg->kmemcg_id = memcg_id;
 	memcg->kmem_state = KMEM_ONLINE;
 
-- 
2.26.2