linux-kernel - Re: [PATCH v4] mm/vmscan: add a fatal signals check in drop_slab

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Message-ID: <20200915133751.GC3736@dhcp22.suse.cz>
Date:   Tue, 15 Sep 2020 15:37:51 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     zangchunxin@...edance.com
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v4] mm/vmscan: add a fatal signals check in drop_slab_node

On Tue 15-09-20 21:27:41, zangchunxin@...edance.com wrote:
> From: Chunxin Zang <zangchunxin@...edance.com>
> 
> On our server, there are about 10k memcg in one machine. They use memory
> very frequently. We have observed that drop_caches can take a
> considerable amount of time, and can't stop it.
> 
> There are two reasons:
> 1. There is somebody constantly generating more objects to reclaim
>    on drop_caches, result the 'freed' always bigger than 10.
> 2. The process has no chance to process signals.
> 
> We can get the following info through 'ps':
> 
>   root:~# ps -aux | grep drop
>   root  357956 ... R    Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches
>   root 1771385 ... R    Aug16 21146421:17 echo 3 > /proc/sys/vm/drop_caches
> 
> Add a bail out on the fatal signals in the main loop so that the
> operation can be terminated by userspace.
> 
> Signed-off-by: Chunxin Zang <zangchunxin@...edance.com>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

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

> ---
> 
> 	changelogs in v4:
> 	changelogs in v3: 
> 	1) Fix some descriptive problems pointed out by Michal Hocko.
> 		v2 named: mm/vmscan: fix infinite loop in drop_slab_node
> 
> 	changelogs in v2: 
> 	1) via check fatal signal break loop.
> 
>  mm/vmscan.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b6d84326bdf2..6b2b5d420510 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -704,6 +704,9 @@ void drop_slab_node(int nid)
>  	do {
>  		struct mem_cgroup *memcg = NULL;
>  
> +		if (signal_pending(current))
> +			return;
> +
>  		freed = 0;
>  		memcg = mem_cgroup_iter(NULL, NULL, NULL);
>  		do {
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs