Re: [RFC] mm/pgtable/debug: Add test validating architecture page table helpers

[Matthew Wilcox]

On Thu, Jul 25, 2019 at 10:38:58PM +0100, Russell King - ARM Linux admin wrote:
> On Thu, Jul 25, 2019 at 07:39:21AM -0700, Matthew Wilcox wrote:
> > But 'page' isn't necessarily PMD-aligned.  I don't think we can rely on
> > architectures doing the right thing if asked to make a PMD for a randomly
> > aligned page.
> > 
> > How about finding the physical address of something like kernel_init(),
> > and using the corresponding pte/pmd/pud/p4d/pgd that encompasses that
> > address?  It's also better to pass in the pfn/page rather than using global
> > variables to communicate to the test functions.
> 
> There are architectures (32-bit ARM) where the kernel is mapped using
> section mappings, and we don't expect the Linux page table walking to
> work for section mappings.

This test doesn't go so far as to insert the PTE/PMD/PUD/... into the
page tables.  It merely needs an appropriately aligned PFN to create a
PTE/PMD/PUD/... from.