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

[Michal Hocko <mhocko@suse.com>]

On Tue 18-06-19 10:40:06, David Hildenbrand wrote:
> 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)

Yes, that is indeed the case. See 4aa9fc2a435abe95a1e8d7f8c7b3d6356514b37a.
How to fix this? Well, I do not think we can. Section based granularity
simply doesn't agree with the reality and so we have to live with that.
There is a long way to remove all those section size assumptions from
the code though.

-- 
Michal Hocko
SUSE Labs