Date:   Mon, 6 Dec 2021 15:21:05 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Nico Pache <npache@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, shakeelb@google.com,
        ktkhai@virtuozzo.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
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On Mon 06-12-21 15:08:10, David Hildenbrand wrote:
> 
> >> But there might be more missing. Onlining a new zone will get more
> >> expensive in setups with a lot of possible nodes (x86-64 shouldn't
> >> really be an issue in that regard).
> > 
> > Honestly, I am not really concerned by platforms with too many nodes
> > without any memory. If they want to shoot their feet then that's their
> > choice. We can optimize for those if they ever prove to be standar.
> >  
> >> If we want stable backports, we'll want something simple upfront.
> > 
> > For stable backports I would be fine by doing your NODE_DATA check in
> > the allocator. In upstream I think we should be aiming for a more robust
> > solution that is also easier to maintain further down the line. Even if
> > that is an investment at this momemnt because the initialization code is
> > a mess.
> > 
> 
> Agreed. I would be curious *why* we decided to dynamically allocate the
> pgdat. is this just a historical coincidence or was there real reason to
> not allocate it for all possible nodes during boot?

I don't know but if I was to guess the most likely explanation would be
that the numa init code was in a similar order as now and it was easier
to simply allocate a pgdat when a new one was onlined.
9af3c2dea3a3 ("[PATCH] pgdat allocation for new node add (call pgdat allocation)")
doesn't really tell much.
-- 
Michal Hocko
SUSE Labs