Subject: Re: [PATCH v5 13/16] mm: memcontrol: reuse memory cgroup ID for kmem
 ID
To:     Muchun Song <songmuchun@bytedance.com>, willy@infradead.org,
        akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org,
        vdavydov.dev@gmail.com, shakeelb@google.com, guro@fb.com,
        shy828301@gmail.com, alexs@kernel.org, richard.weiyang@gmail.com,
        david@fromorbit.com, trond.myklebust@hammerspace.com,
        anna.schumaker@netapp.com, jaegeuk@kernel.org, chao@kernel.org,
        kari.argillander@gmail.com
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-nfs@vger.kernel.org,
        zhengqi.arch@bytedance.com, duanxiongchun@bytedance.com,
        fam.zheng@bytedance.com, smuchun@gmail.com
References: <20211220085649.8196-1-songmuchun@bytedance.com>
 <20211220085649.8196-14-songmuchun@bytedance.com>
From:   =?UTF-8?Q?Mika_Penttil=c3=a4?= <mika.penttila@nextfour.com>
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Date:   Mon, 20 Dec 2021 11:27:03 +0200
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Hi,


On 20.12.2021 10.56, Muchun Song wrote:
> 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@bytedance.com>
> ---
>   include/linux/memcontrol.h |  1 +
>   mm/memcontrol.c            | 46 ++++++++--------------------------------------
>   2 files changed, 9 insertions(+), 38 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 3fc437162add..7b472f805d77 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -56,6 +56,7 @@ struct mem_cgroup_reclaim_cookie {
>   #ifdef CONFIG_MEMCG
>   
>   #define MEM_CGROUP_ID_SHIFT	16
> +#define MEM_CGROUP_ID_MIN	1
>   #define MEM_CGROUP_ID_MAX	USHRT_MAX
>   
>   struct mem_cgroup_id {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 28d6d2564f9d..04f75055f518 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -348,23 +348,6 @@ 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
> @@ -3528,10 +3511,12 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
>   }
>   
>   #ifdef CONFIG_MEMCG_KMEM
> +#define MEM_CGROUP_KMEM_ID_MIN	-1

Maybe

#define MEM_CGROUP_KMEM_ID_MIN 0

to be the first allocated id, so MEM_CGROUP_ID_DIFF is 1, not 2


> +#define MEM_CGROUP_ID_DIFF	(MEM_CGROUP_ID_MIN - MEM_CGROUP_KMEM_ID_MIN)
> +
>   static int memcg_online_kmem(struct mem_cgroup *memcg)
>   {
>   	struct obj_cgroup *objcg;
> -	int memcg_id;
>   
>   	if (cgroup_memory_nokmem)
>   		return 0;
> @@ -3539,22 +3524,16 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
>   	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 - MEM_CGROUP_ID_DIFF;
>   
>   	return 0;
>   }
> @@ -3562,7 +3541,6 @@ 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;
> @@ -3577,20 +3555,12 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
>   	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)
> @@ -5043,7 +5013,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>   		return ERR_PTR(error);
>   
>   	memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
> -				 1, MEM_CGROUP_ID_MAX,
> +				 MEM_CGROUP_ID_MIN, MEM_CGROUP_ID_MAX,
>   				 GFP_KERNEL);
>   	if (memcg->id.id < 0) {
>   		error = memcg->id.id;
> @@ -5071,7 +5041,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>   	spin_lock_init(&memcg->event_list_lock);
>   	memcg->socket_pressure = jiffies;
>   #ifdef CONFIG_MEMCG_KMEM
> -	memcg->kmemcg_id = -1;
> +	memcg->kmemcg_id = MEM_CGROUP_KMEM_ID_MIN;
>   	INIT_LIST_HEAD(&memcg->objcg_list);
>   #endif
>   #ifdef CONFIG_CGROUP_WRITEBACK