Re: [GIT PULL] arm64 fix for 5.14

[Christoph Hellwig]

On Thu, Aug 26, 2021 at 11:41:34AM -0700, Linus Torvalds wrote:
> "pfn_valid()" is more about whether you can do a "pfn_to_page()" lookup on it.
> 
> II get the feeling that the dma-mapping code should allow pages that
> are PageReserved() to be mapped - they aren't "ram" in the kernel
> sense.
> 
> Perhaps also make sure it's not the zero page (which is
> PageReserved(), but most definitely RAM).
> 
> In a PC world that would be (for example) the legacy PCI space at
> 0xa0000-0xfffff, but I could easily imagine other platforms having
> other situations.

So what would be the correct check for "this is not actually page backed
normal RAM"?