Date:   Tue, 14 Dec 2021 11:38:47 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Christoph Lameter <cl@gentwo.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Alexey Makhalov <amakhalov@vmware.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Dennis Zhou <dennis@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Oscar Salvador <osalvador@suse.de>, Tejun Heo <tj@kernel.org>,
        Nico Pache <npache@redhat.com>
Subject: Re: [PATCH v2 2/4] mm: handle uninitialized numa nodes gracefully
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On Tue 14-12-21 11:33:41, Christoph Lameter wrote:
> On Tue, 14 Dec 2021, Michal Hocko wrote:
> 
> > This patch takes a different approach (following a lead of [3]) and it
> > pre allocates pgdat for all possible nodes in an arch indipendent code
> > - free_area_init. All uninitialized nodes are treated as memoryless
> > nodes. node_state of the node is not changed because that would lead to
> > other side effects - e.g. sysfs representation of such a node and from
> > past discussions [4] it is known that some tools might have problems
> > digesting that.
> 
> Would it be possible to define a pgdat statically and place it in read
> only memory? Populate with values that ensure that the page allocator
> does not blow up but does a defined fallback.
> 
> Point the pgdat for all nodes not online to that readonly pgdat?
> 
> Maybe that would save some memory. When the node comes online then a real
> pgdat could be allocated.

This is certainly possible but also it is more complex. I aim for as
simple as possible at this stage. The reason I am not concerned about
memory overhead so much (even though the pgdat is a large data
structure) is that these unpopulated nodes are rather rare. We might see
more of them in the future but we are not quite there yet so I do not
think this is a major obstacle for now.

-- 
Michal Hocko
SUSE Labs