linux-kernel - Re: [PATCH] memcg: cleanup racy sum avoidance code

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Date:   Mon, 2 Aug 2021 08:57:56 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>, Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: cleanup racy sum avoidance code

On Tue 27-07-21 18:22:43, Shakeel Butt wrote:
> We used to have per-cpu memcg and lruvec stats and the readers have to
> traverse and sum the stats from each cpu. This summing was racy and may
> expose transient negative values. So, an explicit check was added to
> avoid such scenarios. Now these stats are moved to rstat infrastructure
> and are no more per-cpu, so we can remove the fixup for transient
> negative values.
> 
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>

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

> ---
>  include/linux/memcontrol.h | 15 ++-------------
>  1 file changed, 2 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 7028d8e4a3d7..5f2a39a43d47 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -991,30 +991,19 @@ static inline void mod_memcg_state(struct mem_cgroup *memcg,
>  
>  static inline unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
>  {
> -	long x = READ_ONCE(memcg->vmstats.state[idx]);
> -#ifdef CONFIG_SMP
> -	if (x < 0)
> -		x = 0;
> -#endif
> -	return x;
> +	return READ_ONCE(memcg->vmstats.state[idx]);
>  }
>  
>  static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
>  					      enum node_stat_item idx)
>  {
>  	struct mem_cgroup_per_node *pn;
> -	long x;
>  
>  	if (mem_cgroup_disabled())
>  		return node_page_state(lruvec_pgdat(lruvec), idx);
>  
>  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> -	x = READ_ONCE(pn->lruvec_stats.state[idx]);
> -#ifdef CONFIG_SMP
> -	if (x < 0)
> -		x = 0;
> -#endif
> -	return x;
> +	return READ_ONCE(pn->lruvec_stats.state[idx]);
>  }
>  
>  static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> -- 
> 2.32.0.432.gabb21c7263-goog

-- 
Michal Hocko
SUSE Labs