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"?

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