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:	Fri, 15 Jun 2012 16:20:31 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Michal Hocko <mhocko@...e.cz>
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

Michal Hocko <mhocko@...e.cz> writes:

> 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?

Ok hugetlb_lock should be sufficient here i guess. I will send a patch
on top to remove the exclude_rmdir and release_and_wakeup_rmdir 

-aneesh

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