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Subject: Re: [PATCH] memcg: cleanup racy sum avoidance code
To:     Shakeel Butt <shakeelb@google.com>, Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20210728012243.3369123-1-shakeelb@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <8c35e469-b355-63b0-6cd4-ca39c39ddb79@redhat.com>
Date:   Wed, 28 Jul 2021 10:21:18 +0200
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On 28.07.21 03:22, 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@google.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,
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb