Re: [PATCH v4 0/1] numa: add 'spm' option for Specific Purpose Memory

[Gregory Price <gourry@gourry.net>]

On Tue, Dec 30, 2025 at 09:15:34PM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/30/25 15:06, Gregory Price wrote:
> 
> And given we don't have a boot memory device, the idea was to set it for the
> Node, where it means "all boot memory is SPM". And we only allow one type of
> boot memory (one memory backend) per node in QEMU.
> 
> The tricky question is what happens with memory hotplug (DIMMs etc) on such
> a node. I'd argue that it's simply not SPM.
>

...

+++ .../docs/whatever

+ Don't do that.

:]

> > 
> > ---
> > 
> > bit of an aside - but at LPC we also talked about SPM NUMA nodes:
> > https://lore.kernel.org/linux-mm/20251112192936.2574429-1-gourry@gourry.net/
> > 
> > Would be cool to be able to detect this in the drivers and have hotplug
> > automatically mark a node SPM unless a driver overrides it.
> > (MHP flag? Sorry David :P)
> 
> :)
> 
> If it's a per-node thing, MHP flags feel a bit like "too late". It should be
> configured earlier for the node somehow.
> 

just a clarification, the flag would be an override to have mhp mark a
node N_MEMORY instead of N_SPM.

As it stands right now, a node is "online with memory" if N_MEMORY is
set for that node.

https://elixir.bootlin.com/linux/v6.14-rc6/source/mm/memory_hotplug.c#L717

I imagine hotplugged N_SPM would operate the same.

So mhp code would look like

if (node_data->is_spm && !override)
	node_set_state(node, N_SPM)
else
	node_set_state(node, N_MEMORY)

Basically would allow SPM nodes to operate the same as they did before
when hotplugged to retain existing behavior.

(Sorry i'm think waaaaaaaaaaaaay far ahead here)

~Gregory