[RFC] arm64: mm: update max_pfn after memory hotplug

[Chris Goldsworthy]

On arm64 we set max_pfn at boot in arch/arm64/mm/init.c. If you
hotplug in memory after booting up, max_pfn is not updated. This
breaks diagnostic functions executed from user space like
read_page_owner():

https://elixir.bootlin.com/linux/v5.14.7/source/mm/page_owner.c#L472

or kpageflags_read() (see how get_max_dump_pfn() is used):

https://elixir.bootlin.com/linux/v5.14.7/source/fs/proc/page.c#L47

Thus, this patch updates max_pfn and max_low_pfn in arm64's
arch_add_memory() function, mirroring what is updatated during boot:

https://elixir.bootlin.com/linux/v5.14.7/source/arch/arm64/mm/init.c#L448

We would appreciate feedback on any other variables that should be
updated when hotplugging in memory - note that we're mirroring x86, in
that max_pfn is only ever incremented when calling arch_add_memory():

https://elixir.bootlin.com/linux/v5.14.7/source/arch/x86/mm/init_64.c#L958

Sudarshan Rajagopalan (1):
  arm64: mm: update max_pfn after memory hotplug

 arch/arm64/mm/mmu.c | 5 +++++
 1 file changed, 5 insertions(+)

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2.7.4