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

From: Gerald Schaefer <[email-masked]>
Date: Tue, 5 Nov 2019 20:36:38 +0100

On Mon, 28 Oct 2019 10:59:22 +0530
Anshuman Khandual <[email-masked]> 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.

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.

Thanks a lot for the effort!

Regards,
Gerald