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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Dan Williams <dan.j.williams@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Michal Hocko <mhocko@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Mark Brown <broonie@kernel.org>,
	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>,
	Matthew Wilcox <willy@infradead.org>,
	Sri Krishna chowdary <schowdary@nvidia.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Paul Mackerras <paulus@samba.org>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	"David S. Miller" <davem@davemloft.net>,
	Vineet Gupta <vgupta@synopsys.com>,
	James Hogan <jhogan@kernel.org>,
	Paul Burton <paul.burton@mips.com>,
	Ralf Baechle <ralf@linux-mips.org>,
	linux-snps-arc@lists.infradead.org, linux-mips@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
	linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
	x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture page table helpers
Date: Mon, 9 Sep 2019 18:13:44 +0300
Message-ID: <20190909151344.ghfypjbgxyosjdk3@box> (raw)
In-Reply-To: <3d5de35f-8192-1c75-50a9-03e66e3b8e5c@arm.com>

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@arm.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@arm.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

  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-03  8:01 [PATCH 0/1] mm/debug: Add tests for architecture exported " Anshuman Khandual
2019-09-03  8:01 ` [PATCH 1/1] mm/pgtable/debug: Add test validating architecture " Anshuman Khandual
2019-09-03 11:13   ` kbuild test robot
2019-09-04  6:14     ` Anshuman Khandual
2019-09-04 14:19   ` Kirill A. Shutemov
2019-09-05  8:18     ` Anshuman Khandual
2019-09-05  8:59       ` Kirill A. Shutemov
2019-09-06  7:03         ` Anshuman Khandual
2019-09-04 20:16   ` Gerald Schaefer
2019-09-05  9:18     ` Anshuman Khandual
2019-09-05 17:06       ` Gerald Schaefer
2019-09-06  6:28         ` Anshuman Khandual
2019-09-06 19:03           ` Gerald Schaefer
2019-09-09  6:26             ` Anshuman Khandual
2019-09-09 15:13               ` Kirill A. Shutemov [this message]
2019-09-10  3:56                 ` Anshuman Khandual
2019-09-10  4:45                   ` Christophe Leroy
2019-09-10  5:43                     ` Anshuman Khandual
2019-09-09 16:51               ` Gerald Schaefer
2019-09-04 23:14   ` Dave Hansen

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