From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S933509AbeAKKmn (ORCPT <rfc822;ralf@linux-mips.org> + 1 other);
        Thu, 11 Jan 2018 05:42:43 -0500
Received: from mx2.suse.de ([195.135.220.15]:36935 "EHLO mx2.suse.de"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S933142AbeAKKml (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 11 Jan 2018 05:42:41 -0500
Date: Thu, 11 Jan 2018 11:42:39 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Shakeel Butt <shakeelb@google.com>
Subject: Re: [PATCH v4] mm/memcg: try harder to decrease
 [memory,memsw].limit_in_bytes
Message-ID: <20180111104239.GZ1732@dhcp22.suse.cz>
References: <20180109152622.31ca558acb0cc25a1b14f38c@linux-foundation.org>
 <20180110124317.28887-1-aryabinin@virtuozzo.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180110124317.28887-1-aryabinin@virtuozzo.com>
User-Agent: Mutt/1.9.2 (2017-12-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Return-Path: <linux-kernel-owner@vger.kernel.org>

On Wed 10-01-18 15:43:17, Andrey Ryabinin wrote:
[...]
> @@ -2506,15 +2480,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
>  		if (!ret)
>  			break;
>  
> -		try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, !memsw);
> -
> -		curusage = page_counter_read(counter);
> -		/* Usage is reduced ? */
> -		if (curusage >= oldusage)
> -			retry_count--;
> -		else
> -			oldusage = curusage;
> -	} while (retry_count);
> +		usage = page_counter_read(counter);
> +		if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> +						GFP_KERNEL, !memsw)) {

If the usage drops below limit in the meantime then you get underflow
and reclaim the whole memcg. I do not think this is a good idea. This
can also lead to over reclaim. Why don't you simply stick with the
original SWAP_CLUSTER_MAX (aka 1 for try_to_free_mem_cgroup_pages)?

> +			ret = -EBUSY;
> +			break;
> +		}
> +	} while (true);

-- 
Michal Hocko
SUSE Labs