On Thu, 15 Jul 2021 11:42:21 +0200 David Hildenbrand [off-list ref] wrote:

quoted

I'd propose to add Cc: [off-list ref] since I actually had
the fun to try to debug something like this a couple of years ago:
6cdb18ad98a4 ("mm/vmstat: fix overflow in mod_zone_page_state()")

Good point, and thinking again what can go wrong, I tend to agree. We 
are trying to keep zones contiguous and it could happen that we end up 
with something like ZONE_DMA here (via default_kernel_zone_for_pfn()) 
and would consequently online something to ZONE_DMA that doesn't belong 
there, resulting in crashes.

@Andrew can you add  Cc: [off-list ref] and

"As we will search for a fitting zone using the wrong pfn, we might end 
up onlining memory to one of the special kernel zones, such as ZONE_DMA, 
which can end badly as the onlined memory does not satisfy properties of 
these zones."

Yep, all done.