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

[David Hildenbrand <david@redhat.com>]

On 19.06.19 11:04, Michal Hocko wrote:
> 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.

Yes and no. Storing it per section clearly doesn't allow storing node
information on smaller granularity, like storing in page->flags does.

So no, it is not only an implementation detail.

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.
Depends on how performance-critical pfn_to_nid() is. I can't tell.

-- 

Thanks,

David / dhildenb