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

[Russell King - ARM Linux admin]

On Thu, Jul 25, 2019 at 07:39:21AM -0700, Matthew Wilcox wrote:
> 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.

There are architectures (32-bit ARM) where the kernel is mapped using
section mappings, and we don't expect the Linux page table walking to
work for section mappings.

> > +	/*
> > +	 * 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?
> 
> 
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