Re: [RFC PATCH 1/3] mm/numa: add exclusive node pool and numa=standby boot parameter

[Gregory Price <gourry@gourry.net>]

On Thu, Jun 11, 2026 at 12:00:17PM +0300, Mike Rapoport wrote:
> > 1) Can we do dynamic addition of nodes?
> > 
> >    Not Trivially
> > 
> >    Some services utilize num_possible_nodes() as a static value to
> >    calculate the amount of resources to use at runtime (bpf, md/raid5).
> > 
> >    Example: futex_init uses num_possible_nodes() as part of its
> >             hashsize calculation during __init.
> 
> AFAIU, we don't add the additional nodes for generic hotplug memory but
> rather for exclusive use of by drivers/applications that are aware of these
> nodes.

The intent is to use for "non-generic" hotplug (see the whole private
node series [1]), which would eventually still use the hotplug mechanism
just not for generic memory.

[1] https://lore.kernel.org/linux-mm/20260222084842.1824063-1-gourry@gourry.net/

> Wouldn't adding them to possible nodes actually skew the calculation of the
> resources by the services utilizing num_possible_nodes()?
> 
> With the futex_init() example, won't be hashsize scaled down two much
> because we've added these special nodes to the possible mask?
>

The result is the same as BIOS reserving nodes with PXM entries that
don't get used.  The CXL ACPI Tables do this for CXL Fixed Memory
Windows that may never be hotplugged.

So really i think you're pointing out that futex_init() here probably
shouldn't be using num_possible_nodes?

~Gregory