linux-kernel - Re: [PATCH -V8 12/16] hugetlb/cgroup: Add support for cgroup removal

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Date:	Mon, 11 Jun 2012 15:14:11 +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 -V8 12/16] hugetlb/cgroup: Add support for cgroup removal

On Mon 11-06-12 15:10:20, Aneesh Kumar K.V wrote:
> Michal Hocko <mhocko@...e.cz> writes:
[...]
> >> +static int hugetlb_cgroup_move_parent(int idx, struct cgroup *cgroup,
> >> +				      struct page *page)
> >
> > deserves a comment about the locking (needs to be called with
> > hugetlb_lock).
> 
> will do
> 
> >
> >> +{
> >> +	int csize;
> >> +	struct res_counter *counter;
> >> +	struct res_counter *fail_res;
> >> +	struct hugetlb_cgroup *page_hcg;
> >> +	struct hugetlb_cgroup *h_cg   = hugetlb_cgroup_from_cgroup(cgroup);
> >> +	struct hugetlb_cgroup *parent = parent_hugetlb_cgroup(cgroup);
> >> +
> >> +	if (!get_page_unless_zero(page))
> >> +		goto out;
> >> +
> >> +	page_hcg = hugetlb_cgroup_from_page(page);
> >> +	/*
> >> +	 * We can have pages in active list without any cgroup
> >> +	 * ie, hugepage with less than 3 pages. We can safely
> >> +	 * ignore those pages.
> >> +	 */
> >> +	if (!page_hcg || page_hcg != h_cg)
> >> +		goto err_out;
> >
> > How can we have page_hcg != NULL && page_hcg != h_cg?
> 
> pages belonging to other cgroup ?

OK, I've forgot that you are iterating over all active huge pages in
hugetlb_cgroup_pre_destroy. What prevents you from doing the filtering
in the caller? 
I am also wondering why you need to play with the page reference
counting here. You are under hugetlb_lock so the page cannot disappear
in the meantime or am I missing something?
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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