Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout

[Mike Rapoport <rppt@linux.ibm.com>]

On Thu, Feb 25, 2021 at 07:38:44PM +0100, Vlastimil Babka wrote:
> On 2/25/21 7:05 PM, Mike Rapoport wrote:
> >> 
> >> What if two zones are adjacent? I.e. if the hole was at a boundary between two
> >> zones.
> > 
> > What do you mean by "adjacent zones"? If there is a hole near the zone
> > boundary, zone span would be clamped to exclude the hole.
> 
> Yeah, zone span should exclude those pages, but you still somehow handle them?
> That's how I read "pages that are not spanned by any node will get links to the
> adjacent zone/node."
> So is it always a unique zone/node can be determined?
> 
> Let's say we have:
> 
> <memory on node 0>
> ---- pageblock boundary ----
> <more memory on node 0>
> <a hole>
> <memory on node 1>
> ---- pageblock boundary ----
> 
> Now I hope such configurations don't really exist :) But if we simulated them in
> QEMU, what would be the linkage in struct pages in that hole?

I don't think such configuration is possible in practice but it can be
forced with e.g memmap="2M hole at 4G - 1M".

The hole in your example the hole will get node1 for node and zone that
spans the beginning of node1 for zone.
 
-- 
Sincerely yours,
Mike.