Re: [RFC V2 0/1] mm/debug: Add tests for architecture exported page table helpers

From: Matthew Wilcox
Date: Fri Aug 09 2019 - 09:52:31 EST

On Fri, Aug 09, 2019 at 04:05:07PM +0530, Anshuman Khandual wrote:
> On 08/09/2019 03:46 PM, Matthew Wilcox wrote:
> > On Fri, Aug 09, 2019 at 01:03:17PM +0530, Anshuman Khandual wrote:
> >> Should alloc_gigantic_page() be made available as an interface for general
> >> use in the kernel. The test module here uses very similar implementation from
> >> HugeTLB to allocate a PUD aligned memory block. Similar for mm_alloc() which
> >> needs to be exported through a header.
> >
> > Why are you allocating memory at all instead of just using some
> > known-to-exist PFNs like I suggested?
> We needed PFN to be PUD aligned for pfn_pud() and PMD aligned for mk_pmd().
> Now walking the kernel page table for a known symbol like kernel_init()

I didn't say to walk the kernel page table. I said to call virt_to_pfn()
for a known symbol like kernel_init().

> as you had suggested earlier we might encounter page table page entries at PMD
> and PUD which might not be PMD or PUD aligned respectively. It seemed to me
> that alignment requirement is applicable only for mk_pmd() and pfn_pud()
> which create large mappings at those levels but that requirement does not
> exist for page table pages pointing to next level. Is not that correct ? Or
> I am missing something here ?

Just clear the bottom bits off the PFN until you get a PMD or PUD aligned
PFN. It's really not hard.