linux-kernel - Re: [PATCH -V9 14/15] hugetlb/cgroup: migrate hugetlb cgroup info from oldpage to new page during migration

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Date:	Thu, 14 Jun 2012 12:04:54 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, rientjes@...gle.com, akpm@...ux-foundation.org,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V9 14/15] hugetlb/cgroup: migrate hugetlb cgroup info
 from oldpage to new page during migration

On Wed 13-06-12 15:57:33, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
> 
> With HugeTLB pages, hugetlb cgroup is uncharged in compound page destructor.  Since
> we are holding a hugepage reference, we can be sure that old page won't
> get uncharged till the last put_page().
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

Reviewed-by: Michal Hocko <mhocko@...e.cz>

One question below
[...]
> +void hugetlb_cgroup_migrate(struct page *oldhpage, struct page *newhpage)
> +{
> +	struct hugetlb_cgroup *h_cg;
> +
> +	if (hugetlb_cgroup_disabled())
> +		return;
> +
> +	VM_BUG_ON(!PageHuge(oldhpage));
> +	spin_lock(&hugetlb_lock);
> +	h_cg = hugetlb_cgroup_from_page(oldhpage);
> +	set_hugetlb_cgroup(oldhpage, NULL);
> +	cgroup_exclude_rmdir(&h_cg->css);
> +
> +	/* move the h_cg details to new cgroup */
> +	set_hugetlb_cgroup(newhpage, h_cg);
> +	spin_unlock(&hugetlb_lock);
> +	cgroup_release_and_wakeup_rmdir(&h_cg->css);
> +	return;
> +}
> +

The changelog says that the old page won't get uncharged - which means
that the the cgroup cannot go away (even if we raced with the move
parent, hugetlb_lock makes sure we either see old or new cgroup) so why
do we need to play with css ref. counting?
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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