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

[Vineet Gupta]

Hi Anshuman,

On 11/4/19 4:00 PM, Anshuman Khandual wrote:
> On 10/28/2019 10:59 AM, Anshuman Khandual 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.
>>
>> Folks interested in making sure that a given platform's page table helpers
>> conform to expected generic MM semantics should enable the above config
>> which will just trigger this test during boot. Any non conformity here will
>> be reported as an warning which would need to be fixed. This test will help
>> catch any changes to the agreed upon semantics expected from generic MM and
>> enable platforms to accommodate it thereafter.
>>

I tried enabling this on ARC and ran into a build issue

../mm/debug_vm_pgtable.c: In function ‘pmd_basic_tests’:
../mm/debug_vm_pgtable.c:73:14: error: implicit declaration of function ‘pfn_pmd’;
did you mean ‘pfn_pte’? [-Werror=implicit-function-declaration]
  pmd_t pmd = pfn_pmd(pfn, prot);
              ^~~~~~~

The reason being THP was not enabled (although ARC supports THP) - for the
combination below
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
# CONFIG_TRANSPARENT_HUGEPAGE is not set

I think you need to use latter for guarding pmd_basic_tests()

Other than that the tests pass for !THP and THP too. So once fixed, you could
enable that for ARC as well
Thx for doing this.

-Vineet