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

[David Hildenbrand <david@redhat.com>]

On 18.06.19 10:32, Wei Yang wrote:
> On Tue, Jun 18, 2019 at 09:49:48AM +0200, Oscar Salvador wrote:
>> On Tue, Jun 18, 2019 at 08:55:37AM +0800, 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.
>>>
>>> BTW, current online_pages works because it leverages nid in memory_block.
>>> But the granularity of node id should be mem_section wide.
>>
>> I forgot to ask this before, but why do you mention online_pages here?
>> IMHO, it does not add any value to the changelog, and it does not have much
>> to do with the matter.
>>
> 
> Since to me it is a little confused why we don't set the node info but still
> could online memory to the correct node. It turns out we leverage the
> information in memblock.

I'd also drop the comment here.

> 
>> online_pages() works with memblock granularity and not section granularity.
>> That memblock is just a hot-added range of memory, worth of either 1 section or multiple
>> sections, depending on the arch or on the size of the current memory.
>> And we assume that each hot-added memory all belongs to the same node.
>>
> 
> So I am not clear about the granularity of node id. section based or memblock
> based. Or we have two cases:
> 
> * for initial memory, section wide
> * for hot-add memory, mem_block wide

It's all a big mess. Right now, you can offline initial memory with
mixed nodes. Also on my list of many ugly things to clean up.

(I even remember that we can have mixed nodes within a section, but I
haven't figured out yet how that is supposed to work in some scenarios)

-- 

Thanks,

David / dhildenb