Re: [PATCH] vmap(): don't allow invalid pages

[Matthew Wilcox]

On Wed, Jan 19, 2022 at 01:28:14PM +0000, Robin Murphy wrote:
> Is it page_to_pfn() guaranteed to work without blowing up if page is invalid
> in the first place? Looking at the CONFIG_SPARSEMEM case I'm not sure that's
> true...

Something that all the ARM people weighing in on this don't understand
is that basically nobody uses SPARSEMEM without SPARSEMEM_VMEMMAP.
So all this complicated code to do page_to_pfn() is never tested.
Real users all do a simple subtraction and so the simple pfn_valid()
works fine.