On Wed, Feb 21, 2018 at 02:51:19PM +0100, Jonathan Neuschäfer wrote:
[...]

While looking through arch/powerpc/mm, I noticed that there's a
page_is_ram function, which simply uses the memblocks directly, on
PPC32.

Oops, I misread the code here. memblock is used on PPC64.

It seems like a good candidate for the RAM check in
__ioremap_caller, except that there's this code, which apparently
trashes memblock 0 completely on non-CONFIG_NEED_MULTIPLE_NODES:

  https://elixir.bootlin.com/linux/v4.16-rc2/source/arch/powerpc/mm/mem.c#L223


Thanks,
Jonathan Neuschäfer