linux-kernel - Re: [PATCH -V8 14/16] hugetlb/cgroup: add charge/uncharge calls for HugeTLB alloc/free

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Date:	Sat, 09 Jun 2012 18:39:06 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	dhillf@...il.com, rientjes@...gle.com, mhocko@...e.cz,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V8 14/16] hugetlb/cgroup: add charge/uncharge calls for HugeTLB alloc/free

Johannes Weiner <hannes@...xchg.org> writes:

> On Sat, Jun 09, 2012 at 02:29:59PM +0530, Aneesh Kumar K.V wrote:
>> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
>> 
>> This adds necessary charge/uncharge calls in the HugeTLB code.  We do
>> hugetlb cgroup charge in page alloc and uncharge in compound page destructor.
>> 
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
>> ---
>>  mm/hugetlb.c        |   16 +++++++++++++++-
>>  mm/hugetlb_cgroup.c |    7 +------
>>  2 files changed, 16 insertions(+), 7 deletions(-)
>> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index bf79131..4ca92a9 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -628,6 +628,8 @@ static void free_huge_page(struct page *page)
>>  	BUG_ON(page_mapcount(page));
>>  
>>  	spin_lock(&hugetlb_lock);
>> +	hugetlb_cgroup_uncharge_page(hstate_index(h),
>> +				     pages_per_huge_page(h), page);
>
> hugetlb_cgroup_uncharge_page() takes the hugetlb_lock, no?

Yes, But this patch also modifies it to not take the lock, because we
hold spin_lock just below in the call site. I didn't want to drop the
lock and take it again.

>
> It's quite hard to review code that is split up like this.  Please
> always keep the introduction of new functions in the same patch that
> adds the callsite(s).

One of the reason I split the charge/uncharge routines and the callers
in separate patches is to make it easier for review. Irrespective of
the call site charge/uncharge routines should be correct with respect
to locking and other details. What I did in this patch is a small
optimization of avoiding dropping and taking the lock again. May be the
right approach would have been to name it __hugetlb_cgroup_uncharge_page
and make sure the hugetlb_cgroup_uncharge_page still takes spin_lock.
But then we don't have any callers for that.

-aneesh



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