Date:   Mon, 6 Mar 2023 18:47:04 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Yue Zhao <findns94@gmail.com>
Cc:     akpm@linux-foundation.org, roman.gushchin@linux.dev,
        hannes@cmpxchg.org, shakeelb@google.com, muchun.song@linux.dev,
        willy@infradead.org, linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, tangyeechou@gmail.com
Subject: Re: [PATCH v2, 1/4] mm, memcg: Prevent memory.oom.group load/store
 tearing
Message-ID: <ZAYnGK8mXjxmarDm@dhcp22.suse.cz>
References: <20230306154138.3775-1-findns94@gmail.com>
 <20230306154138.3775-2-findns94@gmail.com>
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On Mon 06-03-23 23:41:35, Yue Zhao wrote:
> The knob for cgroup v2 memory controller: memory.oom.group
> is not protected by any locking so it can be modified while it is used.
> This is not an actual problem because races are unlikely (the knob is
> usually configured long before any workloads hits actual memcg oom)
> but it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> doing anything funky.
> 
> The access of memcg->oom_group is lockless, so it can be
> concurrently set at the same time as we are trying to read it.
> 
> Signed-off-by: Yue Zhao <findns94@gmail.com>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
>  mm/memcontrol.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5abffe6f8389..06821e5f7604 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2067,7 +2067,7 @@ struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
>  	 * highest-level memory cgroup with oom.group set.
>  	 */
>  	for (; memcg; memcg = parent_mem_cgroup(memcg)) {
> -		if (memcg->oom_group)
> +		if (READ_ONCE(memcg->oom_group))
>  			oom_group = memcg;
>  
>  		if (memcg == oom_domain)
> @@ -6623,7 +6623,7 @@ static int memory_oom_group_show(struct seq_file *m, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
>  
> -	seq_printf(m, "%d\n", memcg->oom_group);
> +	seq_printf(m, "%d\n", READ_ONCE(memcg->oom_group));
>  
>  	return 0;
>  }
> @@ -6645,7 +6645,7 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
>  	if (oom_group != 0 && oom_group != 1)
>  		return -EINVAL;
>  
> -	memcg->oom_group = oom_group;
> +	WRITE_ONCE(memcg->oom_group, oom_group);
>  
>  	return nbytes;
>  }
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs