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

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Message-ID: <87bokpvp4c.fsf@skywalker.in.ibm.com>
Date:	Mon, 11 Jun 2012 21:04:43 +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 -V8 12/16] hugetlb/cgroup: Add support for cgroup removal

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

> 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?

That is correct. Updated the patch and also added the below comment to
the function.
+
+/*
+ * Should be called with hugetlb_lock held.
+ * Since we are holding hugetlb_lock, pages cannot get moved from
+ * active list or uncharged from the cgroup, So no need to get
+ * page reference and test for page active here.
+ */

-aneesh

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