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Date:   Mon, 3 Dec 2018 12:56:46 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Xunlei Pang <xlpang@linux.alibaba.com>
Cc:     Roman Gushchin <guro@fb.com>, Johannes Weiner <hannes@cmpxchg.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/3] mm/vmscan: Enable kswapd to reclaim low-protected
 memory
Message-ID: <20181203115646.GP31738@dhcp22.suse.cz>
References: <20181203080119.18989-1-xlpang@linux.alibaba.com>
 <20181203080119.18989-2-xlpang@linux.alibaba.com>
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On Mon 03-12-18 16:01:18, Xunlei Pang wrote:
> There may be cgroup memory overcommitment, it will become
> even common in the future.
> 
> Let's enable kswapd to reclaim low-protected memory in case
> of memory pressure, to mitigate the global direct reclaim
> pressures which could cause jitters to the response time of
> lantency-sensitive groups.

Please be more descriptive about the problem you are trying to handle
here. I haven't actually read the patch but let me emphasise that the
low limit protection is important isolation tool. And allowing kswapd to
reclaim protected memcgs is going to break the semantic as it has been
introduced and designed.

> 
> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
> ---
>  mm/vmscan.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 62ac0c488624..3d412eb91f73 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3531,6 +3531,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  
>  	count_vm_event(PAGEOUTRUN);
>  
> +retry:
>  	do {
>  		unsigned long nr_reclaimed = sc.nr_reclaimed;
>  		bool raise_priority = true;
> @@ -3622,6 +3623,13 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>  			sc.priority--;
>  	} while (sc.priority >= 1);
>  
> +	if (!sc.nr_reclaimed && sc.memcg_low_skipped) {
> +		sc.priority = DEF_PRIORITY;
> +		sc.memcg_low_reclaim = 1;
> +		sc.memcg_low_skipped = 0;
> +		goto retry;
> +	}
> +
>  	if (!sc.nr_reclaimed)
>  		pgdat->kswapd_failures++;
>  
> -- 
> 2.13.5 (Apple Git-94)
> 

-- 
Michal Hocko
SUSE Labs