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Date: Tue, 10 Sep 2019 04:45:30 +0000
From: Christophe Leroy <christophe.leroy@....fr>
To: Anshuman Khandual <anshuman.khandual@....com>,
"Kirill A. Shutemov" <kirill@...temov.name>
Cc: Mark Rutland <mark.rutland@....com>, linux-ia64@...r.kernel.org,
linux-sh@...r.kernel.org,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
James Hogan <jhogan@...nel.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
Dave Hansen <dave.hansen@...el.com>,
Paul Mackerras <paulus@...ba.org>, sparclinux@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
linux-s390@...r.kernel.org, x86@...nel.org,
Russell King - ARM Linux <linux@...linux.org.uk>,
Matthew Wilcox <willy@...radead.org>,
Steven Price <Steven.Price@....com>,
Jason Gunthorpe <jgg@...pe.ca>,
Vlastimil Babka <vbabka@...e.cz>,
linux-snps-arc@...ts.infradead.org,
Kees Cook <keescook@...omium.org>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
Mark Brown <broonie@...nel.org>,
Dan Williams <dan.j.williams@...el.com>,
Gerald Schaefer <gerald.schaefer@...ibm.com>,
linux-arm-kernel@...ts.infradead.org,
Sri Krishna chowdary <schowdary@...dia.com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-mips@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Paul Burton <paul.burton@...s.com>,
Vineet Gupta <vgupta@...opsys.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linuxppc-dev@...ts.ozlabs.org,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture
page table helpers
On 09/10/2019 03:56 AM, Anshuman Khandual wrote:
>
>
> On 09/09/2019 08:43 PM, Kirill A. Shutemov wrote:
>> 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);
>
> Agreed.
>
> As I had mentioned before this would have been really the cleanest approach.
>
>>
>> It *should* be more portable.
>
> Not really, because not all the platforms have __pxx() definitions right now.
> Going with these will clearly cause build failures on affected platforms. Lets
> examine __pud() for instance. It is defined only on these platforms.
>
> arch/arm64/include/asm/pgtable-types.h: #define __pud(x) ((pud_t) { (x) } )
> arch/mips/include/asm/pgtable-64.h: #define __pud(x) ((pud_t) { (x) })
> arch/powerpc/include/asm/pgtable-be-types.h: #define __pud(x) ((pud_t) { cpu_to_be64(x) })
> arch/powerpc/include/asm/pgtable-types.h: #define __pud(x) ((pud_t) { (x) })
> arch/s390/include/asm/page.h: #define __pud(x) ((pud_t) { (x) } )
> arch/sparc/include/asm/page_64.h: #define __pud(x) ((pud_t) { (x) } )
> arch/sparc/include/asm/page_64.h: #define __pud(x) (x)
> arch/x86/include/asm/pgtable.h: #define __pud(x) native_make_pud(x)
You missed:
arch/x86/include/asm/paravirt.h:static inline pud_t __pud(pudval_t val)
include/asm-generic/pgtable-nop4d-hack.h:#define __pud(x)
((pud_t) { __pgd(x) })
include/asm-generic/pgtable-nopud.h:#define __pud(x)
((pud_t) { __p4d(x) })
>
> Similarly for __pmd()
>
> arch/alpha/include/asm/page.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/arm/include/asm/page-nommu.h: #define __pmd(x) (x)
> arch/arm/include/asm/pgtable-2level-types.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/arm/include/asm/pgtable-2level-types.h: #define __pmd(x) (x)
> arch/arm/include/asm/pgtable-3level-types.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/arm/include/asm/pgtable-3level-types.h: #define __pmd(x) (x)
> arch/arm64/include/asm/pgtable-types.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/m68k/include/asm/page.h: #define __pmd(x) ((pmd_t) { { (x) }, })
> arch/mips/include/asm/pgtable-64.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/nds32/include/asm/page.h: #define __pmd(x) (x)
> arch/parisc/include/asm/page.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/parisc/include/asm/page.h: #define __pmd(x) (x)
> arch/powerpc/include/asm/pgtable-be-types.h: #define __pmd(x) ((pmd_t) { cpu_to_be64(x) })
> arch/powerpc/include/asm/pgtable-types.h: #define __pmd(x) ((pmd_t) { (x) })
> arch/riscv/include/asm/pgtable-64.h: #define __pmd(x) ((pmd_t) { (x) })
> arch/s390/include/asm/page.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/sh/include/asm/pgtable-3level.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/sparc/include/asm/page_32.h: #define __pmd(x) ((pmd_t) { { (x) }, })
> arch/sparc/include/asm/page_32.h: #define __pmd(x) ((pmd_t) { { (x) }, })
> arch/sparc/include/asm/page_64.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/sparc/include/asm/page_64.h: #define __pmd(x) (x)
> arch/um/include/asm/page.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/um/include/asm/page.h: #define __pmd(x) ((pmd_t) { (x) } )
> arch/x86/include/asm/pgtable.h: #define __pmd(x) native_make_pmd(x)
You missed:
arch/x86/include/asm/paravirt.h:static inline pmd_t __pmd(pmdval_t val)
include/asm-generic/page.h:#define __pmd(x) ((pmd_t) { (x) } )
include/asm-generic/pgtable-nopmd.h:#define __pmd(x)
((pmd_t) { __pud(x) } )
>
> Similarly for __pgd()
>
> arch/alpha/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/alpha/include/asm/page.h: #define __pgd(x) (x)
> arch/arc/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/arc/include/asm/page.h: #define __pgd(x) (x)
> arch/arm/include/asm/pgtable-3level-types.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/arm/include/asm/pgtable-3level-types.h: #define __pgd(x) (x)
> arch/arm64/include/asm/pgtable-types.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/csky/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/hexagon/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/m68k/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/mips/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/nds32/include/asm/page.h: #define __pgd(x) (x)
> arch/nios2/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/openrisc/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/parisc/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/parisc/include/asm/page.h: #define __pgd(x) (x)
> arch/powerpc/include/asm/pgtable-be-types.h: #define __pgd(x) ((pgd_t) { cpu_to_be64(x) })
> arch/powerpc/include/asm/pgtable-types.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/riscv/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/s390/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/sh/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/sparc/include/asm/page_32.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/sparc/include/asm/page_32.h: #define __pgd(x) (x)
> arch/sparc/include/asm/page_64.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/sparc/include/asm/page_64.h: #define __pgd(x) (x)
> arch/um/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
> arch/unicore32/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) })
> arch/unicore32/include/asm/page.h: #define __pgd(x) (x)
> arch/x86/include/asm/pgtable.h: #define __pgd(x) native_make_pgd(x)
> arch/xtensa/include/asm/page.h: #define __pgd(x) ((pgd_t) { (x) } )
You missed:
arch/x86/include/asm/paravirt.h:static inline pgd_t __pgd(pgdval_t val)
include/asm-generic/page.h:#define __pgd(x) ((pgd_t) { (x) } )
>
> Similarly for __p4d()
>
> arch/s390/include/asm/page.h: #define __p4d(x) ((p4d_t) { (x) } )
> arch/x86/include/asm/pgtable.h: #define __p4d(x) native_make_p4d(x)
You missed:
arch/x86/include/asm/paravirt.h:static inline p4d_t __p4d(p4dval_t val)
include/asm-generic/5level-fixup.h:#define __p4d(x)
__pgd(x)
include/asm-generic/pgtable-nop4d.h:#define __p4d(x)
((p4d_t) { __pgd(x) })
>
> The search pattern here has been "#define __pxx(". Unless I am missing something,
> I dont see how we can use these without risking build failures.
>
I guess you missed that arches not defining them fall back on the
definitions in include/asm-generic
Christophe
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