mm: memcg: fix obsolete code comments

This patch fixes/removes some obsolete comments in the code related
to the kernel memory accounting:

 - kmem_cache->memcg_params.memcg_caches has been removed by commit
   9855609 ("mm: memcg/slab: use a single set of kmem_caches for
   all accounted allocations")

 - memcg->kmemcg_id is not used as a gate for kmem accounting since
   commit 0b8f73e ("mm: memcontrol: clean up alloc, online,
   offline, free functions")

Link: https://lkml.kernel.org/r/20201110184615.311974-1-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -296,7 +296,6 @@ 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;
@@ -1562,9 +1561,8 @@ static inline void memcg_kmem_uncharge(struct mem_cgroup *memcg,
 }
 
 /*
- * 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)
 {

mm/memcontrol.c

@@ -3703,12 +3703,6 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 
 	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
-	 * guarantee no one starts accounting before all call sites are
-	 * patched.
-	 */
 	memcg->kmemcg_id = memcg_id;
 	memcg->kmem_state = KMEM_ONLINE;