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

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Date:   Mon, 9 Sep 2019 18:13:44 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Gerald Schaefer <gerald.schaefer@...ibm.com>, 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 Mon, Sep 09, 2019 at 11:56:50AM +0530, Anshuman Khandual wrote:
> 
> 
> On 09/07/2019 12:33 AM, Gerald Schaefer wrote:
> > On Fri, 6 Sep 2019 11:58:59 +0530
> > Anshuman Khandual <anshuman.khandual@....com> wrote:
> > 
> >> On 09/05/2019 10:36 PM, Gerald Schaefer wrote:
> >>> 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.  
> >>
> >> Makes sense but..
> >>
> >> There is a small challenge with this. Modifying individual bits on a given
> >> page table entry from generic code like this test case is bit tricky. That
> >> is because there are not enough helpers to create entries with an absolute
> >> value. This would have been easier if all the platforms provided functions
> >> like __pxx() which is not the case now. Otherwise something like this should
> >> have worked.
> >>
> >>
> >> pud_t pud = READ_ONCE(*pudp);
> >> pud = __pud(pud_val(pud) | RANDOM_VALUE (keeping lower 12 bits 0))
> >> WRITE_ONCE(*pudp, pud);
> >>
> >> But __pud() will fail to build in many platforms.
> > 
> > Hmm, I simply used this on my system to make pud_clear_tests() work, not
> > sure if it works on all archs:
> > 
> > pud_val(*pudp) |= RANDOM_NZVALUE;
> 
> Which compiles on arm64 but then fails on x86 because of the way pmd_val()
> has been defined there.

Use instead

	*pudp = __pud(pud_val(*pudp) | RANDOM_NZVALUE);

It *should* be more portable.

-- 
 Kirill A. Shutemov