linux-kernel - Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture page table helpers

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Date:   Thu, 5 Sep 2019 19:06:29 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Steven Price <Steven.Price@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Matthew Wilcox <willy@...radead.org>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Vineet Gupta <vgupta@...opsys.com>,
        James Hogan <jhogan@...nel.org>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-snps-arc@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture
 page table helpers

On Thu, 5 Sep 2019 14:48:14 +0530
Anshuman Khandual <anshuman.khandual@....com> wrote:

> > [...]  
> >> +
> >> +#if !defined(__PAGETABLE_PMD_FOLDED) && !defined(__ARCH_HAS_4LEVEL_HACK)
> >> +static void pud_clear_tests(pud_t *pudp)
> >> +{
> >> +	memset(pudp, RANDOM_NZVALUE, sizeof(pud_t));
> >> +	pud_clear(pudp);
> >> +	WARN_ON(!pud_none(READ_ONCE(*pudp)));
> >> +}  
> > 
> > For pgd/p4d/pud_clear(), we only clear if the page table level is present
> > and not folded. The memset() here overwrites the table type bits, so
> > pud_clear() will not clear anything on s390 and the pud_none() check will
> > fail.
> > Would it be possible to OR a (larger) random value into the table, so that
> > the lower 12 bits would be preserved?  
> 
> So the suggestion is instead of doing memset() on entry with RANDOM_NZVALUE,
> it should OR a large random value preserving lower 12 bits. Hmm, this should
> still do the trick for other platforms, they just need non zero value. So on
> s390, the lower 12 bits on the page table entry already has valid value while
> entering this function which would make sure that pud_clear() really does
> clear the entry ?

Yes, in theory the table entry on s390 would have the type set in the last
4 bits, so preserving those would be enough. If it does not conflict with
others, I would still suggest preserving all 12 bits since those would contain
arch-specific flags in general, just to be sure. For s390, the pte/pmd tests
would also work with the memset, but for consistency I think the same logic
should be used in all pxd_clear_tests.

However, there is another issue on s390 which will make this only work
for pud_clear_tests(), and not for the p4d/pgd_tests. The problem is that
mm_alloc() will only give you a 3-level page table initially on s390.
This means that pudp == p4dp == pgdp, and so the p4d/pgd_tests will
both see the pud level (of course this also affects other tests).

Not sure yet how to fix this, i.e. how to initialize/update the page table
to 5 levels. We can handle 5 level page tables, and it would be good if
all levels could be tested, but using mm_alloc() to establish the page
tables might not work on s390. One option could be to provide an arch-hook
or weak function to allocate/initialize the mm.

IIUC, the (dummy) mm is really only needed to provide an mm->pgd as starting
point, right?

Regards,
Gerald