Re: [PATCH v2] mm/sparse: set section nid for hot-add memory

[Oscar Salvador <osalvador@suse.de>]

On Wed, Jun 19, 2019 at 08:23:30AM +0200, Michal Hocko wrote:
> On Tue 18-06-19 08:55:37, Wei Yang wrote:
> > In case of NODE_NOT_IN_PAGE_FLAGS is set, we store section's node id in
> > section_to_node_table[]. While for hot-add memory, this is missed.
> > Without this information, page_to_nid() may not give the right node id.
> 
> Which would mean that NODE_NOT_IN_PAGE_FLAGS doesn't really work with
> the hotpluged memory, right? Any idea why nobody has noticed this
> so far? Is it because NODE_NOT_IN_PAGE_FLAGS is rare and essentially
> unused with the hotplug? page_to_nid providing an incorrect result
> sounds quite serious to me.

The thing is that for NODE_NOT_IN_PAGE_FLAGS to be enabled we need to run out of
space in page->flags to store zone, nid and section. 
Currently, even with the largest values (with pagetable level 5), that is not
possible on x86_64.
It is possible though, that somewhere in the future, when the values get larger
(e.g: we add more zones, NODE_SHIFT grows, or we need more space to store
the section) we finally run out of room for the flags though.

I am not sure about the other arches though, we probably should audit them
and see which ones can fall in there.

-- 
Oscar Salvador
SUSE L3