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

[Anshuman Khandual <anshuman.khandual@xxxxxxx>]

On 08/09/2019 07:22 PM, Matthew Wilcox wrote:
> 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.

As Mark pointed out earlier that might end up being just a synthetic PFN
which might not even exist on a given system.