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

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Date:   Thu, 25 Jul 2019 22:38:58 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        x86@...nel.org, Dave Hansen <dave.hansen@...el.com>,
        linux-kernel@...r.kernel.org, Steven Price <Steven.Price@....com>,
        linux-mm@...ck.org, Mark Brown <Mark.Brown@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC] mm/pgtable/debug: Add test validating architecture page
 table helpers

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?
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

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