linux-kernel - Re: [PATCH v2, 3/4] mm, memcg: Prevent memory.oom

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Date:   Mon, 6 Mar 2023 18:53:15 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Yue Zhao <findns94@...il.com>
Cc:     akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        hannes@...xchg.org, shakeelb@...gle.com, muchun.song@...ux.dev,
        willy@...radead.org, linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, tangyeechou@...il.com
Subject: Re: [PATCH v2, 3/4] mm, memcg: Prevent memory.oom_control load/store
 tearing

On Mon 06-03-23 23:41:37, Yue Zhao wrote:
> The knob for cgroup v1 memory controller: memory.oom_control
> 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.
> But it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> doing anything funky.
> 
> The access of memcg->oom_kill_disable 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@...il.com>
> ---
>  mm/memcontrol.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index dca895c66a9b..26605b2f51b1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4515,7 +4515,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_seq(sf);
>  
> -	seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
> +	seq_printf(sf, "oom_kill_disable %d\n", READ_ONCE(memcg->oom_kill_disable));
>  	seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
>  	seq_printf(sf, "oom_kill %lu\n",
>  		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
> @@ -4531,7 +4531,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
>  	if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
>  		return -EINVAL;
>  
> -	memcg->oom_kill_disable = val;
> +	WRITE_ONCE(memcg->oom_kill_disable, val);
>  	if (!val)
>  		memcg_oom_recover(memcg);

Any specific reasons you haven't covered other accesses
(mem_cgroup_css_alloc, mem_cgroup_oom, mem_cgroup_oom_synchronize)?
>  
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs