linux-kernel - Re: [PATCH] mm: memcontrol: reset memory.low on css offline

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Date:	Mon, 29 Feb 2016 19:29:23 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcontrol: reset memory.low on css offline

On Mon 29-02-16 20:16:33, Vladimir Davydov wrote:
> When a cgroup directory is removed, the memory cgroup subsys state does
> not disappear immediately. Instead, it's left hanging around until the
> last reference to it is gone, which implies reclaiming all pages from
> its lruvec.
> 
> In the unified hierarchy, there's the memory.low knob, which can be used
> to set a best-effort protection for a memory cgroup - the reclaimer
> first scans those cgroups whose consumption is above memory.low, and
> only if it fails to reclaim enough pages, it gets to the rest.
> 
> Currently this protection is not reset when the cgroup directory is
> removed. As a result, if a dead memory cgroup has a lot of page cache
> charged to it and a high value of memory.low, it will result in higher
> pressure exerted on live cgroups, and userspace will have no ways to
> detect such consumers and reconfigure memory.low properly.
> 
> To fix this, let's reset memory.low on css offline.

Makes sense to me
 
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memcontrol.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ae8b81c55685..ab7bfe870c7d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4214,6 +4214,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
>  
>  	memcg_offline_kmem(memcg);
>  	wb_memcg_offline(memcg);
> +
> +	memcg->low = 0;
>  }
>  
>  static void mem_cgroup_css_released(struct cgroup_subsys_state *css)
> -- 
> 2.1.4

-- 
Michal Hocko
SUSE Labs