linux-kernel - Re: [PATCH v2 2/3] mm/memcg: Simplify mem_cgroup_get

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Date:   Mon, 14 Sep 2020 13:48:25 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Roman Gushchin <guro@...com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 2/3] mm/memcg: Simplify mem_cgroup_get_max()

On Sun 13-09-20 22:44:51, 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@...hat.com>
> ---
>  mm/memcontrol.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8c74f1200261..ca36bed588d1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1633,17 +1633,15 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
>   */
>  unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
>  {
> -	unsigned long 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);
> +	unsigned long max = READ_ONCE(memcg->memory.max);
> +
> +	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> +		if (mem_cgroup_swappiness(memcg))
> +			max += min(READ_ONCE(memcg->swap.max),
> +				   (unsigned long)total_swap_pages);
> +	} else { /* v1 */
> +		if (mem_cgroup_swappiness(memcg))
> +			max = memcg->memsw.max;

I agree that making v1 vs. v2 distinction here makes the code more
obvious. But I do not think your code is correct for v1. In a default
state it would lead to max = PAGE_COUNTER_MAX which is not something
we want, right?

instead you want
		max += min(READ_ONCE(memcg->memsw.max), total_swap_pages);



>  	}
>  	return max;
>  }
> -- 
> 2.18.1
> 

-- 
Michal Hocko
SUSE Labs