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

[Michal Hocko <mhocko@suse.com>]

On Wed 19-06-19 10:51:47, David Hildenbrand wrote:
> On 19.06.19 09:53, Oscar Salvador wrote:
> > 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.
> > 
> 
> I'd love to see NODE_NOT_IN_PAGE_FLAGS go.

NODE_NOT_IN_PAGE_FLAGS is an implementation detail on where the
information is stored. I cannot say how much it is really needed now but
I can see there will be a demand for it in a longer term because
page->flags space is scarce and very interesting storage. So I do not
see it go away I am afraid.
-- 
Michal Hocko
SUSE Labs