Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 11 Mar 2019 13:17:48 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guroan@gmail.com>
Cc:     linux-mm@kvack.org, kernel-team@fb.com,
        linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH 2/5] mm: prepare to premature release of per-node
 lruvec_stat_cpu
Message-ID: <20190311171748.GB10823@cmpxchg.org>
References: <20190307230033.31975-1-guro@fb.com>
 <20190307230033.31975-3-guro@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190307230033.31975-3-guro@fb.com>
User-Agent: Mutt/1.11.3 (2019-02-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@fb.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.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.