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Date:   Mon, 30 Nov 2020 12:19:56 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Dmitry Osipenko <digetx@gmail.com>
Cc:     Alex Shi <alex.shi@linux.alibaba.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Roman Gushchin <guro@fb.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        linux-kernel@vger.kernel.org,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] mm/memcg: bail out early when !memcg in
 mem_cgroup_lruvec
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On Mon, 30 Nov 2020 22:44:11 +0300 Dmitry Osipenko <digetx@gmail.com> wrote:

> > From: Alex Shi <alex.shi@linux.alibaba.com>
> > Date: Wed, 25 Nov 2020 14:06:33 +0800
> > Subject: [PATCH v2] mm/memcg: bail out early when !memcg in mem_cgroup_lruvec
> > 
> > Sometime, we use NULL memcg in mem_cgroup_lruvec(memcg, pgdat)
> > so we could get out early in the situation to avoid useless checking.
> > 
> > Polished as Andrew Morton's suggestion.
> > 
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -610,20 +610,17 @@ mem_cgroup_nodeinfo(struct mem_cgroup *memcg, int nid)
> >  static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
> >  					       struct pglist_data *pgdat)
> >  {
> > -	struct mem_cgroup_per_node *mz;
> >  	struct lruvec *lruvec;
> >  
> > -	if (mem_cgroup_disabled()) {
> > +	if (mem_cgroup_disabled() || !memcg) {
> >  		lruvec = &pgdat->__lruvec;
> > -		goto out;
> > -	}
> > +	} else {
> > +		struct mem_cgroup_per_node *mz;
> >  
> > -	if (!memcg)
> > -		memcg = root_mem_cgroup;
> > +		mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> > +		lruvec = &mz->lruvec;
> > +	}
> >  
> > -	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> > -	lruvec = &mz->lruvec;
> > -out:
> >  	/*
> >  	 * Since a node can be onlined after the mem_cgroup was created,
> >  	 * we have to be prepared to initialize lruvec->pgdat here;
> > 
> 
> Hi,
> 
> This patch causes a hard lock on one of my ARM32 devices using today's
> linux-next, please fix.

Thanks.

This is unexpected.  I assume you've confirmed that reverting this
change from linux-next fixes things?