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Date:   Mon, 29 Apr 2019 09:15:49 -0400
From:   Michal Hocko <mhocko@kernel.org>
To:     Jiri Slaby <jslaby@suse.cz>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        cgroups@vger.kernel.org,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Subject: Re: [PATCH] memcg: make it work on sparse non-0-node systems
Message-ID: <20190429131549.GL21837@dhcp22.suse.cz>
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On Mon 29-04-19 13:55:26, Jiri Slaby wrote:
> On 29. 04. 19, 13:30, Michal Hocko wrote:
> > On Mon 29-04-19 12:59:39, Jiri Slaby wrote:
> > [...]
> >>  static inline bool list_lru_memcg_aware(struct list_lru *lru)
> >>  {
> >> -	/*
> >> -	 * This needs node 0 to be always present, even
> >> -	 * in the systems supporting sparse numa ids.
> >> -	 */
> >> -	return !!lru->node[0].memcg_lrus;
> >> +	return !!lru->node[first_online_node].memcg_lrus;
> >>  }
> >>  
> >>  static inline struct list_lru_one *
> > 
> > How come this doesn't blow up later - e.g. in memcg_destroy_list_lru
> > path which does iterate over all existing nodes thus including the
> > node 0.
> 
> If the node is not disabled (i.e. is N_POSSIBLE), lru->node is allocated
> for that node too. It will also have memcg_lrus properly set.
> 
> If it is disabled, it will never be iterated.
> 
> Well, I could have used first_node. But I am not sure, if the first
> POSSIBLE node is also ONLINE during boot?

I dunno. I would have to think about this much more. The whole
expectation that node 0 is always around is simply broken. But also
list_lru_memcg_aware looks very suspicious. We should have a flag or
something rather than what we have now.

I am still not sure I have completely understood the problem though.
I will try to get to this during the week but Vladimir should be much
better fit to judge here.
-- 
Michal Hocko
SUSE Labs