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

From: Matthew Wilcox <willy@infradead.org>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Mark Brown <Mark.Brown@arm.com>,
	Steven Price <Steven.Price@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Kees Cook <keescook@chromium.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Sri Krishna chowdary <schowdary@nvidia.com>,
	Dave Hansen <dave.hansen@intel.com>,
	linux-arm-kernel@lists.infradead.org, x86@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC] mm/pgtable/debug: Add test validating architecture page table helpers
Date: Fri, 26 Jul 2019 12:54:57 -0700	[thread overview]
Message-ID: <20190726195457.GI30641@bombadil.infradead.org> (raw)
In-Reply-To: <c3bb0420-584c-de3b-2439-8702bc09595e@arm.com>

On Fri, Jul 26, 2019 at 10:17:11AM +0530, Anshuman Khandual wrote:
> > 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(),
> 
> Physical address corresponding to the symbol in the kernel text segment ?

Yes.  We need the address of something that's definitely memory.
The stack might be in vmalloc space.  We can't allocate memory from the
allocator that's PUD-aligned.  This seems like a reasonable approximation
to something that might work.

> > and using the corresponding pte/pmd/pud/p4d/pgd that encompasses that
> 
> So I guess this will help us use pte/pmd/pud/p4d/pgd entries from a real and
> present mapping rather then making them up for test purpose. Although we are
> not creating real page tables here just wondering if this could some how
> affect these real mapping in anyway from some accessors. The current proposal
> stays clear from anything real - allocates, evaluates and releases.

I think that's a mistake.  As Russell said, the ARM p*d manipulation
functions expect to operate on tables, not on individual entries
constructed on the stack.

So I think the right thing to do here is allocate an mm, then do the
pgd_alloc / p4d_alloc / pud_alloc / pmd_alloc / pte_alloc() steps giving
you real page tables that you can manipulate.

Then destroy them, of course.  And don't access through them.

> >> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> >> +static void pud_basic_tests(void)
> > 
> > Is this the right ifdef?
> 
> IIUC THP at PUD is where the pud_t entries are directly operated upon and the
> corresponding accessors are present only when HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> is enabled. Am I missing something here ?

Maybe I am.  I thought we could end up operating on PUDs for kernel mappings,
even without transparent hugepages turned on.