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

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Date:   Mon, 6 Mar 2023 18:55:11 +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, 4/4] mm, memcg: Prevent memory.soft_limit_in_bytes
 load/store tearing

On Mon 06-03-23 23:41:38, Yue Zhao wrote:
> The knob for cgroup v1 memory controller: memory.soft_limit_in_bytes
> 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->soft_limit is lockless,
> so it can be concurrently set at the same time as we are
> trying to read it.

Similar here. mem_cgroup_css_reset and mem_cgroup_css_alloc are not
covered.
> 
> 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 26605b2f51b1..20566f59bbcb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3728,7 +3728,7 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
>  	case RES_FAILCNT:
>  		return counter->failcnt;
>  	case RES_SOFT_LIMIT:
> -		return (u64)memcg->soft_limit * PAGE_SIZE;
> +		return (u64)READ_ONCE(memcg->soft_limit) * PAGE_SIZE;
>  	default:
>  		BUG();
>  	}
> @@ -3870,7 +3870,7 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
>  		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
>  			ret = -EOPNOTSUPP;
>  		} else {
> -			memcg->soft_limit = nr_pages;
> +			WRITE_ONCE(memcg->soft_limit, nr_pages);
>  			ret = 0;
>  		}
>  		break;
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs