Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

[Reza Arbab]

On Tue, May 23, 2017 at 05:44:23PM -0500, Michael Bringmann wrote:
> On 05/23/2017 04:49 PM, Reza Arbab wrote:
> > On Tue, May 23, 2017 at 03:05:08PM -0500, Michael Bringmann wrote:
> > > On 05/23/2017 10:52 AM, Reza Arbab wrote:
> > > > On Tue, May 23, 2017 at 10:15:44AM -0500, Michael Bringmann wrote:
> > > > > +static void setup_nodes(void)
> > > > > +{
> > > > > +    int i, l = 32 /* MAX_NUMNODES */;
> > > > > +
> > > > > +    for (i = 0; i < l; i++) {
> > > > > +        if (!node_possible(i)) {
> > > > > +            setup_node_data(i, 0, 0);
> > > > > +            node_set(i, node_possible_map);
> > > > > +        }
> > > > > +    }
> > > > > +}
> > > >
> > > > This seems to be a workaround for 3af229f2071f ("powerpc/numa: Reset node_possible_map to only node_online_map").
> > >
> > > They may be related, but that commit is not a replacement.  The above patch ensures that
> > > there are enough of the nodes initialized at startup to allow for memory hot-add into a
> > > node that was not used at boot.  (See 'setup_node_data' function in 'numa.c'.)  That and
> > > recording that the node was initialized.
> >
> > Is it really necessary to preinitialize these empty nodes using setup_node_data()? When you do memory hotadd into a node that was not used at boot, the node data already gets set up by
> >
> > add_memory
> >  add_memory_resource
> >    hotadd_new_pgdat
> >      arch_alloc_nodedata <-- allocs the pg_data_t
> >      ...
> >      free_area_init_node <-- sets NODE_DATA(nid)->node_id, etc.
> >
> > Removing setup_node_data() from that loop leaves only the call to node_set(). If 3af229f2071f (which reduces node_possible_map) was reverted, you wouldn't need to do that either.
>
> With or without 3af229f2071f, we would still need to add something, somewhere to add new
> bits to the 'node_possible_map'.  That is not being done.

Without 3af229f2071f, those bits would already BE set in 
node_possible_map. You wouldn't have to do anything.

--
Reza Arbab