Re: [RFC][PATCH 2/5] memcg : handle swap cache

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 4 Nov 2008 17:42:01 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> > +#ifdef CONFIG_SWAP
> > +int mem_cgroup_cache_charge_swapin(struct page *page,
> > +			struct mm_struct *mm, gfp_t mask)
> > +{
> > +	int ret = 0;
> > +
> > +	if (mem_cgroup_subsys.disabled)
> > +		return 0;
> > +	if (unlikely(!mm))
> > +		mm = &init_mm;
> > +
> > +	ret = mem_cgroup_charge_common(page, mm, mask,
> > +			MEM_CGROUP_CHARGE_TYPE_SHMEM, NULL);
> > +	/*
> > +	 * The page may be dropped from SwapCache because we don't have
> > +	 * lock_page().This may cause charge-after-uncharge trouble.
> > +	 * Fix it up here. (the caller have refcnt to this page and
> > +	 * page itself is guaranteed not to be freed.)
> > +	 */
> > +	if (ret && !PageSwapCache(page))
> > +		mem_cgroup_uncharge_swapcache(page);
> > +
> Hmm.. after [5/5], mem_cgroup_cache_charge_swapin has 'locked' parameter,
> calls lock_page(if !locked), and checks PageSwapCache under page lock.
> 
> Why not doing it in this patch?
> 

My intention is to guard swap_cgroup by lock_page() against SwapCache.
In Mem+Swap controller. we get "memcg" from information in page->private.
I think we need lock_page(), there. 

But here, we don't refer page->private information. 
I think we don't need lock_page() because there is no inofrmation we depends on.

Thanks,
-Kame

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