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Subject: Re: [PATCH 2/3] mm/memcg: Simplify mem_cgroup_get_max()
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Chris Down <chris@chrisdown.name>,
        Roman Gushchin <guro@fb.com>,
        Yafang Shao <laoar.shao@gmail.com>
References: <20200820130350.3211-1-longman@redhat.com>
 <20200820130350.3211-3-longman@redhat.com>
 <20200820173546.GB912520@cmpxchg.org>
 <a3d4783b-5aee-da40-06c0-ac63e292ccdb@redhat.com>
 <CALvZod6GARMuO8YzMp-1FZaasSZJ8t2b9dUu5tXUcDeuHxA6KA@mail.gmail.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <73bab001-b819-1e6d-e130-c52ae2206f19@redhat.com>
Date:   Sun, 13 Sep 2020 20:49:01 -0400
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On 8/20/20 5:25 PM, Shakeel Butt wrote:
> On Thu, Aug 20, 2020 at 1:29 PM Waiman Long <longman@redhat.com> wrote:
>> On 8/20/20 1:35 PM, Johannes Weiner wrote:
>>> On Thu, Aug 20, 2020 at 09:03:49AM -0400, Waiman Long wrote:
>>>> The mem_cgroup_get_max() function used to get memory+swap max from
>>>> both the v1 memsw and v2 memory+swap page counters & return the maximum
>>>> of these 2 values. This is redundant and it is more efficient to just
>>>> get either the v1 or the v2 values depending on which one is currently
>>>> in use.
>>>>
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>>> ---
>>>>    mm/memcontrol.c | 14 +++++---------
>>>>    1 file changed, 5 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index 26b7a48d3afb..d219dca5239f 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -1633,17 +1633,13 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
>>>>     */
>>>>    unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
>>>>    {
>>>> -    unsigned long max;
>>>> +    unsigned long max = READ_ONCE(memcg->memory.max);
>>>>
>>>> -    max = READ_ONCE(memcg->memory.max);
>>>>       if (mem_cgroup_swappiness(memcg)) {
>>>> -            unsigned long memsw_max;
>>>> -            unsigned long swap_max;
>>>> -
>>>> -            memsw_max = memcg->memsw.max;
>>>> -            swap_max = READ_ONCE(memcg->swap.max);
>>>> -            swap_max = min(swap_max, (unsigned long)total_swap_pages);
>>>> -            max = min(max + swap_max, memsw_max);
>>>> +            if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
>>>> +                    max += READ_ONCE(memcg->swap.max);
>>>> +            else
>>>> +                    max = memcg->memsw.max;
>>> I agree with the premise of the patch, but v1 and v2 have sufficiently
>>> different logic, and the way v1 overrides max from the innermost
>>> branch again also doesn't help in understanding what's going on.
>>>
>>> Can you please split out the v1 and v2 code?
>>>
>>>        if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
>>>                max = READ_ONCE(memcg->memory.max);
>>>                if (mem_cgroup_swappiness(memcg))
>>>                        max += READ_ONCE(memcg->swap.max);
>>>        } else {
>>>                if (mem_cgroup_swappiness(memcg))
>>>                        max = memcg->memsw.max;
>>>                else
>>>                        max = READ_ONCE(memcg->memory.max);
>>>        }
>>>
>>> It's slightly repetitive, but IMO much more readable.
>>>
>> Sure. That makes it even better.
>>
> Can you please also add in the commit message why it is ok to drop
> total_swap_pages comparison from mem_cgroup_get_max()?
>
My bad. I accidentally skipped the total_swap_pages check. Will add it 
back in v2.

Cheers,
Longman