linux-kernel - Re: [PATCH -V8 11/16] hugetlb/cgroup: Add charge/uncharge routines for hugetlb cgroup

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Date:	Mon, 11 Jun 2012 21:05:30 +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 11/16] hugetlb/cgroup: Add charge/uncharge routines for hugetlb cgroup

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

> On Mon 11-06-12 14:58:45, Aneesh Kumar K.V wrote:
>> Michal Hocko <mhocko@...e.cz> writes:
>> 
>> > On Sat 09-06-12 14:29:56, Aneesh Kumar K.V wrote:
>> >> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
>> >> 
>> >> This patchset add the charge and uncharge routines for hugetlb cgroup.
>> >> This will be used in later patches when we allocate/free HugeTLB
>> >> pages.
>> >
>> > Please describe the locking rules.
>> 
>> All the update happen within hugetlb_lock.
>
> Yes, I figured but it is definitely worth mentioning in the patch
> description.

Done.

>
> [...]
>> >> +void hugetlb_cgroup_commit_charge(int idx, unsigned long nr_pages,
>> >> +				  struct hugetlb_cgroup *h_cg,
>> >> +				  struct page *page)
>> >> +{
>> >> +	if (hugetlb_cgroup_disabled() || !h_cg)
>> >> +		return;
>> >> +
>> >> +	spin_lock(&hugetlb_lock);
>> >> +	if (hugetlb_cgroup_from_page(page)) {
>> >
>> > How can this happen? Is it possible that two CPUs are trying to charge
>> > one page?
>> 
>> That is why I added that. I looked at the alloc_huge_page, and I
>> don't see we would end with same page from different CPUs but then
>> we have similar checks in memcg, where we drop the charge if we find
>> the page cgroup already used.
>
> Yes but memcg is little bit more complicated than hugetlb which has
> which doesn't have to cope with async charges. Hugetlb allocation is
> serialized by hugetlb_lock so only one caller gets the page.
> I do not think the check is required here or add a comment explaining
> how it can happen.
>

updated.

-aneesh

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