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

[Mark Rutland]

On Fri, Aug 09, 2019 at 03:16:33AM -0700, 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?

IIUC the issue is that there aren't necessarily known-to-exist PFNs that
are sufficiently aligned -- they may not even exist.

For example, with 64K pages, a PMD covers 512M. The kernel image is
(generally) smaller than 512M, and will be mapped at page granularity.
In that case, any PMD entry for a kernel symbol address will point to
the PTE level table, and that will only necessarily be page-aligned, as
any P?D level table is only necessarily page-aligned.

In the same configuration, you could have less than 512M of total
memory, and none of this memory is necessarily aligned to 512M. So
beyond the PTE level, I don't think you can guarantee a known-to-exist
valid PFN.

I also believe that synthetic PFNs could fail pfn_valid(), so that might
cause us pain too...

Thanks,
Mark.