linux-kernel - Re: [RFC][PATCH] Reduce size of swap

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Date:	Fri, 06 Feb 2009 10:07:40 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] Reduce size of swap_cgroup by CSS ID

> +/*
> + * A helper function to get mem_cgroup from ID. must be called under
> + * rcu_read_lock(). Because css_tryget() is called under this, css_put
> + * should be called later.
> + */
> +static struct mem_cgroup *mem_cgroup_lookup_get(unsigned short id)
> +{
> +	struct cgroup_subsys_state *css;
> +
> +	/* ID 0 is unused ID */
> +	if (!id)
> +		return NULL;
> +	css = css_lookup(&mem_cgroup_subsys, id);
> +	if (css && css_tryget(css))
> +		return container_of(css, struct mem_cgroup, css);
> +	return NULL;
> +}

the returned mem_cgroup needn't be protected by rcu_read_lock(), so I
think this is better:
	rcu_read_lock();
	css = css_lookup(&mem_cgroup_subsys, id);
	rcu_read_unlock();
and no lock is needed when calling mem_cgroup_lookup_get().

>   * Returns old value at success, NULL at failure.
>   * (Of course, old value can be NULL.)
>   */
> -struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
> +unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)

kernel-doc needs to be updated

>   * lookup_swap_cgroup - lookup mem_cgroup tied to swap entry
>   * @ent: swap entry to be looked up.
>   *
> - * Returns pointer to mem_cgroup at success. NULL at failure.
> + * Returns CSS ID of mem_cgroup at success. NULL at failure.

s/NULL/0/

>   */
> -struct mem_cgroup *lookup_swap_cgroup(swp_entry_t ent)
> +unsigned short lookup_swap_cgroup(swp_entry_t ent)



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