Re: [PATCH V8] mm/debug: Add tests validating architecture page table helpers

[Anshuman Khandual <anshuman.khandual@xxxxxxx>]

On 11/06/2019 01:06 AM, Gerald Schaefer wrote:
> On Mon, 28 Oct 2019 10:59:22 +0530
> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
>> This adds tests which will validate architecture page table helpers and
>> other accessors in their compliance with expected generic MM semantics.
>> This will help various architectures in validating changes to existing
>> page table helpers or addition of new ones.
>>
>> This test covers basic page table entry transformations including but not
>> limited to old, young, dirty, clean, write, write protect etc at various
>> level along with populating intermediate entries with next page table page
>> and validating them.
>>
>> Test page table pages are allocated from system memory with required size
>> and alignments. The mapped pfns at page table levels are derived from a
>> real pfn representing a valid kernel text symbol. This test gets called
>> right after page_alloc_init_late().
>>
>> This gets build and run when CONFIG_DEBUG_VM_PGTABLE is selected along with
>> CONFIG_VM_DEBUG. Architectures willing to subscribe this test also need to
>> select CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE which for now is limited to x86 and
>> arm64. Going forward, other architectures too can enable this after fixing
>> build or runtime problems (if any) with their page table helpers.
> 
> I've prepared a couple of commits to our arch code to make this work on s390,
> they will go upstream in the next merge window. After that, we can add s390
> to the supported architectures.

Thats good.

> 
> We had some issues, e.g. because we do not report large entries as bad in
> pxd_bad(), do not check for folded page tables in pxd_free(), or assume
> that primitives like pmd_mkdirty() will only be called after pmd_mkhuge().
> None of those should have any impact on current code, but your test module
> revealed that we do not behave like other architectures in some aspects,
> and it's good to find and fix such things to prevent possible future issues.

Right and those s390 fixes are the testimony for the usefulness of this test.

> 
> Thanks a lot for the effort!
> 
> Regards,
> Gerald
> 
>