mm: memcontrol: reuse memory cgroup ID for kmem ID

There are two idrs being used by memory cgroup, one is for kmem ID,
another is for memory cgroup ID. The maximum ID of both is 64Ki. Both
of them can limit the total number of memory cgroups. Actually, we can
reuse memory cgroup ID for kmem ID to simplify the code.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muchun Song <[e-mail]>
Cc: Alex Shi <[e-mail]>
Cc: Anna Schumaker <[e-mail]>
Cc: Chao Yu <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Cc: Fam Zheng <[e-mail]>
Cc: Jaegeuk Kim <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Kari Argillander <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Qi Zheng <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Theodore Ts'o <[e-mail]>
Cc: Trond Myklebust <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Wei Yang <[e-mail]>
Cc: Xiongchun Duan <[e-mail]>
Cc: Yang Shi <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 }
 
 /*
- * This will be used as a shrinker list's index.
- * The main reason for not using cgroup id for this:
- *  this works better in sparse environments, where we have a lot of memcgs,
- *  but only a few kmem-limited.
- */
-static DEFINE_IDA(memcg_cache_ida);
-
-/*
- * MAX_SIZE should be as large as the number of cgrp_ids. Ideally, we could get
- * this constant directly from cgroup, but it is understandable that this is
- * better kept as an internal representation in cgroup.c. In any case, the
- * cgrp_id space is not getting any smaller, and we don't have to necessarily
- * increase ours as well if it increases.
- */
-#define MEMCG_CACHES_MAX_SIZE MEM_CGROUP_ID_MAX
-
-/*
  * A lot of the calls to the cache allocation functions are expected to be
  * inlined by the compiler. Since the calls to memcg_slab_pre_alloc_hook() are
  * conditional to this static branch, we'll have to allow modules that does
···
 static int memcg_online_kmem(struct mem_cgroup *memcg)
 {
 	struct obj_cgroup *objcg;
-	int memcg_id;
 
 	if (cgroup_memory_nokmem)
 		return 0;
···
 	if (unlikely(mem_cgroup_is_root(memcg)))
 		return 0;
 
-	memcg_id = ida_alloc_max(&memcg_cache_ida, MEMCG_CACHES_MAX_SIZE - 1,
-				 GFP_KERNEL);
-	if (memcg_id < 0)
-		return memcg_id;
-
 	objcg = obj_cgroup_alloc();
-	if (!objcg) {
-		ida_free(&memcg_cache_ida, memcg_id);
+	if (!objcg)
 		return -ENOMEM;
-	}
+
 	objcg->memcg = memcg;
 	rcu_assign_pointer(memcg->objcg, objcg);
 
 	static_branch_enable(&memcg_kmem_enabled_key);
 
-	memcg->kmemcg_id = memcg_id;
+	memcg->kmemcg_id = memcg->id.id;
 
 	return 0;
 }
···
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *parent;
-	int kmemcg_id;
 
 	if (cgroup_memory_nokmem)
 		return;
···
 	memcg_reparent_objcgs(memcg, parent);
 
 	/*
-	 * memcg_reparent_list_lrus() can change memcg->kmemcg_id.
-	 * Cache it to local @kmemcg_id.
-	 */
-	kmemcg_id = memcg->kmemcg_id;
-
-	/*
 	 * After we have finished memcg_reparent_objcgs(), all list_lrus
 	 * corresponding to this cgroup are guaranteed to remain empty.
 	 * The ordering is imposed by list_lru_node->lock taken by
 	 * memcg_reparent_list_lrus().
 	 */
 	memcg_reparent_list_lrus(memcg, parent);
-
-	ida_free(&memcg_cache_ida, kmemcg_id);
 }
 #else
 static int memcg_online_kmem(struct mem_cgroup *memcg)