linux-kernel - Re: [PATCH] mm/memcontrol: Don't increase effective low/min if no protection needed

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Date:   Tue, 11 Oct 2022 17:39:41 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Tejun Heo <tj@...nel.org>,
        Chris Down <chris@...isdown.name>
Subject: Re: [PATCH] mm/memcontrol: Don't increase effective low/min if no
 protection needed

On Tue 11-10-22 10:30:15, Waiman Long wrote:
> Since commit bc50bcc6e00b ("mm: memcontrol: clean up and document
> effective low/min calculations"), the effective low/min protections can
> be non-zero even if the corresponding memory.low/min values are 0. That
> can surprise users to see MEMCG_LOW events even when the memory.low
> value is not set. One example is the LTP's memcontrol04 test which fails
> because it detects some MEMCG_LOW events for a cgroup with a memory.min
> value of 0.

Is this with memory_recursiveprot mount option?

> Fix this by updating effective_protection() to not returning a non-zero
> low/min protection values if the corresponding memory.low/min values
> or those of its parent are 0.
> 
> Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  mm/memcontrol.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b69979c9ced5..893d4d5e518a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6660,6 +6660,9 @@ static unsigned long effective_protection(unsigned long usage,
>  	unsigned long protected;
>  	unsigned long ep;
>  
> +	if (!setting || !parent_effective)
> +		return 0UL;	/* No protection is needed */
> +

This will break the above memory_recursiveprot AFAICS.

>  	protected = min(usage, setting);
>  	/*
>  	 * If all cgroups at this level combined claim and use more
> -- 
> 2.31.1

-- 
Michal Hocko
SUSE Labs