linux-kernel - Re: [PATCH 2/5] mm: prepare to premature release of per-node lruvec_stat

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Date:   Mon, 11 Mar 2019 13:17:48 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guroan@...il.com>
Cc:     linux-mm@...ck.org, kernel-team@...com,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...nel.org>, Roman Gushchin <guro@...com>
Subject: Re: [PATCH 2/5] mm: prepare to premature release of per-node
 lruvec_stat_cpu

On Thu, Mar 07, 2019 at 03:00:30PM -0800, Roman Gushchin wrote:
> Similar to the memcg's vmstats_percpu, per-memcg per-node stats
> consists of percpu- and atomic counterparts, and we do expect
> that both coexist during the whole life-cycle of the memcg.
> 
> To prepare for a premature release of percpu per-node data,
> let's pretend that lruvec_stat_cpu is a rcu-protected pointer,
> which can be NULL. This patch adds corresponding checks whenever
> required.
> 
> Signed-off-by: Roman Gushchin <guro@...com>

Acked-by: Johannes Weiner <hannes@...xchg.org>

> @@ -4430,7 +4436,8 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
>  	if (!pn)
>  		return 1;
>  
> -	pn->lruvec_stat_cpu = alloc_percpu(struct lruvec_stat);
> +	rcu_assign_pointer(pn->lruvec_stat_cpu,
> +			   alloc_percpu(struct lruvec_stat));
>  	if (!pn->lruvec_stat_cpu) {

Nitpick: wouldn't this have to use rcu_dereference()? Might be cleaner
to use an intermediate variable and only assign after the NULL check.