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

[David Hildenbrand <david@redhat.com>]

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.

-- 

Thanks,

David / dhildenb