Re: [RFC] mm/pgtable/debug: Add test validating architecture page table helpers

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jul 25, 2019 at 12:25:23PM +0530, Anshuman Khandual wrote:
> This adds a test module which will validate architecture page table helpers
> and accessors regarding compliance with generic MM semantics expectations.
> This will help various architectures in validating changes to the existing
> page table helpers or addition of new ones.

I think this is a really good idea.

>  lib/Kconfig.debug       |  14 +++
>  lib/Makefile            |   1 +
>  lib/test_arch_pgtable.c | 290 ++++++++++++++++++++++++++++++++++++++++++++++++

Is this the right place for it?  I worry that lib/ is going to get overloaded
with test code, and this feels more like mm/ test code.

> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE
> +static void pmd_basic_tests(void)
> +{
> +	pmd_t pmd;
> +
> +	pmd = mk_pmd(page, prot);

But 'page' isn't necessarily PMD-aligned.  I don't think we can rely on
architectures doing the right thing if asked to make a PMD for a randomly
aligned page.

How about finding the physical address of something like kernel_init(),
and using the corresponding pte/pmd/pud/p4d/pgd that encompasses that
address?  It's also better to pass in the pfn/page rather than using global
variables to communicate to the test functions.

> +	/*
> +	 * A huge page does not point to next level page table
> +	 * entry. Hence this must qualify as pmd_bad().
> +	 */
> +	WARN_ON(!pmd_bad(pmd_mkhuge(pmd)));

I didn't know that rule.  This is helpful because it gives us somewhere
to document all these tricksy little rules.

> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +static void pud_basic_tests(void)

Is this the right ifdef?