linux-kernel - Re: [PATCH 3/3] mm/memcg: Unify swap and memsw page counters

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Date:   Mon, 24 Aug 2020 12:02:48 -0400
From:   Waiman Long <longman@...hat.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Chris Down <chris@...isdown.name>,
        Roman Gushchin <guro@...com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH 3/3] mm/memcg: Unify swap and memsw page counters

On 8/20/20 11:46 AM, Shakeel Butt wrote:
> On Thu, Aug 20, 2020 at 6:04 AM Waiman Long <longman@...hat.com> wrote:
>> The swap page counter is v2 only while memsw is v1 only. As v1 and v2
>> controllers cannot be active at the same time, there is no point to keep
>> both swap and memsw page counters in mem_cgroup. The previous patch has
>> made sure that memsw page counter is updated and accessed only when in
>> v1 code paths. So it is now safe to alias the v1 memsw page counter to v2
>> swap page counter. This saves 14 long's in the size of mem_cgroup. This
>> is a saving of 112 bytes for 64-bit archs.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   include/linux/memcontrol.h | 3 +--
>>   mm/memcontrol.c            | 8 +++++---
>>   2 files changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index d0b036123c6a..d2a819d7db70 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -216,10 +216,9 @@ struct mem_cgroup {
>>
>>          /* Accounted resources */
>>          struct page_counter memory;
>> -       struct page_counter swap;
>> +       struct page_counter swap;       /* memsw (memory+swap) for v1 */
>>
>>          /* Legacy consumer-oriented counters */
>> -       struct page_counter memsw;
>>          struct page_counter kmem;
>>          struct page_counter tcpmem;
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index d219dca5239f..04c3794cdc98 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -68,6 +68,11 @@
>>
>>   #include <trace/events/vmscan.h>
>>
>> +/*
>> + * The v1 memsw page counter is aliased to the v2 swap page counter.
>> + */
>> +#define memsw  swap
>> +
> Personally I would prefer a union instead of #define.

Yes, that is also what I am thinking about in the v2.

Cheers,
Longman