[PATCH v2 18/21] mm: memcontrol: reuse memory cgroup ID for kmem ID

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

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.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 40 +++-------------------------------------
 1 file changed, 3 insertions(+), 37 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 82b147597138..ae3ad1001824 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -342,23 +342,6 @@ static void memcg_reparent_objcgs(struct mem_cgroup *memcg,
 }
 
 /*
- * 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
@@ -3542,7 +3525,6 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
 static int memcg_online_kmem(struct mem_cgroup *memcg)
 {
 	struct obj_cgroup *objcg;
-	int memcg_id;
 
 	if (cgroup_memory_nokmem)
 		return 0;
@@ -3550,22 +3532,16 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 	if (unlikely(mem_cgroup_is_root(memcg)))
 		return 0;
 
-	memcg_id = ida_simple_get(&memcg_cache_ida, 0, MEMCG_CACHES_MAX_SIZE,
-				  GFP_KERNEL);
-	if (memcg_id < 0)
-		return memcg_id;
-
 	objcg = obj_cgroup_alloc();
-	if (!objcg) {
-		ida_simple_remove(&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;
 }
@@ -3573,7 +3549,6 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *parent;
-	int kmemcg_id;
 
 	if (cgroup_memory_nokmem)
 		return;
@@ -3586,16 +3561,7 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 		parent = root_mem_cgroup;
 
 	memcg_reparent_objcgs(memcg, parent);
-
-	/*
-	 * memcg_reparent_list_lrus() can change memcg->kmemcg_id.
-	 * Cache it to @kmemcg_id.
-	 */
-	kmemcg_id = memcg->kmemcg_id;
-
 	memcg_reparent_list_lrus(memcg, parent);
-
-	ida_simple_remove(&memcg_cache_ida, kmemcg_id);
 }
 #else
 static int memcg_online_kmem(struct mem_cgroup *memcg)
-- 
2.11.0