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Subject: Re: [PATCH V14] mm/debug: Add tests validating architecture page
 table helpers
To:     Qian Cai <cai@lca.pw>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Vineet Gupta <vgupta@synopsys.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        linux-snps-arc@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        linux-riscv@lists.infradead.org, x86@kernel.org,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1581909460-19148-1-git-send-email-anshuman.khandual@arm.com>
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <7c707b7f-ce3d-993b-8042-44fdc1ed28bf@c-s.fr>
Date:   Wed, 26 Feb 2020 15:45:44 +0100
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Le 26/02/2020 à 15:09, Qian Cai a écrit :
> On Mon, 2020-02-17 at 08:47 +0530, Anshuman Khandual wrote:
>> This adds tests which will validate architecture page table helpers and
>> other accessors in their compliance with expected generic MM semantics.
>> This will help various architectures in validating changes to existing
>> page table helpers or addition of new ones.
>>
>> This test covers basic page table entry transformations including but not
>> limited to old, young, dirty, clean, write, write protect etc at various
>> level along with populating intermediate entries with next page table page
>> and validating them.
>>
>> Test page table pages are allocated from system memory with required size
>> and alignments. The mapped pfns at page table levels are derived from a
>> real pfn representing a valid kernel text symbol. This test gets called
>> inside kernel_init() right after async_synchronize_full().
>>
>> This test gets built and run when CONFIG_DEBUG_VM_PGTABLE is selected. Any
>> architecture, which is willing to subscribe this test will need to select
>> ARCH_HAS_DEBUG_VM_PGTABLE. For now this is limited to arc, arm64, x86, s390
>> and ppc32 platforms where the test is known to build and run successfully.
>> Going forward, other architectures too can subscribe the test after fixing
>> any build or runtime problems with their page table helpers. Meanwhile for
>> better platform coverage, the test can also be enabled with CONFIG_EXPERT
>> even without ARCH_HAS_DEBUG_VM_PGTABLE.
>>
>> Folks interested in making sure that a given platform's page table helpers
>> conform to expected generic MM semantics should enable the above config
>> which will just trigger this test during boot. Any non conformity here will
>> be reported as an warning which would need to be fixed. This test will help
>> catch any changes to the agreed upon semantics expected from generic MM and
>> enable platforms to accommodate it thereafter.
> 
> How useful is this that straightly crash the powerpc?
> 
> [   23.263425][    T1] debug_vm_pgtable: debug_vm_pgtable: Validating
> architecture page table helpers
> [   23.263625][    T1] ------------[ cut here ]------------
> [   23.263649][    T1] kernel BUG at arch/powerpc/mm/pgtable.c:274!

The problem on PPC64 is known and has to be investigated and fixed.

Christophe