Re: [PATCH RFC 1/2] mm/memory_hotplug: no need to init new pgdat with node_start_pfn

Michal Hocko
2020-04-21T12:52:58Z

On Tue 21-04-20 14:35:12, David Hildenbrand wrote:
> On 21.04.20 14:30, Michal Hocko wrote:
> > Sorry for the late reply
> >
> > On Thu 16-04-20 12:47:06, David Hildenbrand wrote:
> >> A hotadded node/pgdat will span no pages at all, until memory is moved to
> >> the zone/node via move_pfn_range_to_zone() -> resize_pgdat_range - e.g.,
> >> when onlining memory blocks. We don't have to initialize the
> >> node_start_pfn to the memory we are adding.
> >
> > You are right that the node is empty at this phase but that is already
> > reflected by zero present pages (hmm, I do not see spanned pages to be
> > set 0 though). What I am missing here is why this is an improvement. The
> > new node is already visible here and I do not see why we hide the
> > information we already know.
>
> "information we already know" - no, not before we online the memory.

Is this really the case? All add_memory_resource users operate on a
physical memory range.

> Before onlining, it's just setting node_start_pfn to *some value* to be
> overwritten in move_pfn_range_to_zone()->resize_pgdat_range().

Yes the value is overwritten but I am not sure this is actually correct
thing to do. I cannot remember why I've chosen to do that. It doesn't
really seem unlikely to online node in a higher physical address.

Btw. one thing that I have in my notes, I was never able to actually
test the no numa node case. Because I have always been testing with node
being allocated during the boot. Do you have any way to trigger this
path?
--
Michal Hocko
SUSE Labs