Date:   Mon, 6 Dec 2021 14:24:40 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Nico Pache <npache@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, shakeelb@google.com,
        shy828301@gmail.com, guro@fb.com, vbabka@suse.cz,
        vdavydov.dev@gmail.com, raquini@redhat.com
Subject: Re: [RFC PATCH 2/2] mm/vmscan.c: Prevent allocating shrinker_info on
 offlined nodes
Message-ID: <Ya4PGJZL8tSb/Prj@dhcp22.suse.cz>
References: <20211206033338.743270-1-npache@redhat.com>
 <20211206033338.743270-3-npache@redhat.com>
 <Ya3WcYKcej8XEI0W@dhcp22.suse.cz>
 <d9d14beb-ee20-7ebb-e007-fbf58fb28535@redhat.com>
 <24b4455c-aff9-ca9f-e29f-350833e7a0d1@virtuozzo.com>
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On Mon 06-12-21 16:19:12, Kirill Tkhai wrote:
> On 06.12.2021 13:45, David Hildenbrand wrote:
> >> This doesn't seen complete. Slab shrinkers are used in the reclaim
> >> context. Previously offline nodes could be onlined later and this would
> >> lead to NULL ptr because there is no hook to allocate new shrinker
> >> infos. This would be also really impractical because this would have to
> >> update all existing memcgs...
> > 
> > Instead of going through the trouble of updating...
> > 
> > ...  maybe just keep for_each_node() and check if the target node is
> > offline. If it's offline, just allocate from the first online node.
> > After all, we're not using __GFP_THISNODE, so there are no guarantees
> > either way ...
> 
> Hm, can't we add shrinker maps allocation to __try_online_node() in addition
> to this patch? 

Either that or through hotplug notifier (which would be a better
solution). But allocating a new shrinker map for each memcg would have
to be done as has been mentioned earlier.

-- 
Michal Hocko
SUSE Labs