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

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Message-ID: <20190725225658.GH30641@bombadil.infradead.org>
Date:   Thu, 25 Jul 2019 15:56:58 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
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 10:58:12PM +0100, Russell King - ARM Linux admin wrote:
> On Thu, Jul 25, 2019 at 02:42:22PM -0700, Matthew Wilcox wrote:
> > On Thu, Jul 25, 2019 at 10:38:58PM +0100, Russell King - ARM Linux admin wrote:
> > > On Thu, Jul 25, 2019 at 07:39:21AM -0700, Matthew Wilcox 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(),
> > > > 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.
> > 
> > This test doesn't go so far as to insert the PTE/PMD/PUD/... into the
> > page tables.  It merely needs an appropriately aligned PFN to create a
> > PTE/PMD/PUD/... from.
> 
> Well, in any case,
> 
> c085ac68 t kernel_init
> 
> so I'm not sure that would be an improvement.

I said "the corresponding pte/pmd/pud/p4d/pgd that encompasses that address"

So for a PTE, you'd use PFN 0xc085a000, for a PMD, you'd use PFN 0xc0000000
and for a PGD, you'd use PFN 0 (assuming 9 bits per level of table).