Re: [PATCH] memcg: use ID in page_cgroup

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Fri, 20 Aug 2010 19:01:32 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> 
> I have an idea to remove page_cgroup->page pointer, 8bytes reduction per page.
> But it will be after this work.
Another off topic. I think we can reduce the size of mem_cgroup by packing
some boolean members into one "unsinged long flags".

> @@ -300,12 +300,13 @@ static atomic_t mem_cgroup_num;
>  #define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
>  static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
>  
> -/* Must be called under rcu_read_lock */
> -static struct mem_cgroup *id_to_memcg(unsigned short id)
> +/* Must be called under rcu_read_lock, set safe==true if under lock */
Do you mean, "Set safe==true if we can ensure by some locks that the id can be
safely dereferenced without rcu_read_lock", right ?

> +static struct mem_cgroup *id_to_memcg(unsigned short id, bool safe)
>  {
>  	struct mem_cgroup *ret;
>  	/* see mem_cgroup_free() */
> -	ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
> +	ret = rcu_dereference_check(mem_cgroups[id],
> +				rch_read_lock_held() || safe);
>  	if (likely(ret && ret->valid))
>  		return ret;
>  	return NULL;

(snip)
> @@ -723,6 +729,11 @@ static inline bool mem_cgroup_is_root(st
>  	return (mem == root_mem_cgroup);
>  }
>  
> +static inline bool mem_cgroup_is_rootid(unsigned short id)
> +{
> +	return (id == 1);
> +}
> +
It might be better to add

	BUG_ON(newid->id != 1)

in cgroup.c::cgroup_init_idr().


Thanks,
Daisuke Nishimura.