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Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page
 table helpers
To:     Christophe Leroy <christophe.leroy@c-s.fr>, Qian Cai <cai@lca.pw>
Cc:     linux-mm@kvack.org, 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
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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <2d950d8c-4b23-741e-591f-e22e857c0755@arm.com>
Date:   Tue, 10 Mar 2020 08:01:51 +0530
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On 03/07/2020 12:35 PM, Christophe Leroy wrote:
> 
> 
> Le 07/03/2020 à 01:56, Anshuman Khandual a écrit :
>>
>>
>> On 03/07/2020 06:04 AM, Qian Cai wrote:
>>>
>>>
>>>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> wrote:
>>>>
>>>> Hmm, set_pte_at() function is not preferred here for these tests. The idea
>>>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort
>>>> of 'static' tests. set_pte_at() is platform provided and could/might trigger
>>>> these flushes or some other platform specific synchronization stuff. Just
>>>
>>> Why is that important for this debugging option?
>>
>> Primarily reason is to avoid TLB/cache flush instructions on the system
>> during these tests that only involve transforming different page table
>> level entries through helpers. Unless really necessary, why should it
>> emit any TLB/cache flush instructions ?
> 
> What's the problem with thoses flushes ?
> 
>>
>>>
>>>> wondering is there specific reason with respect to the soft lock up problem
>>>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ?
>>>
>>> Looks at the s390 version of set_pte_at(), it has this comment,
>>> vmaddr);
>>>
>>> /*
>>>   * Certain architectures need to do special things when PTEs
>>>   * within a page table are directly modified.  Thus, the following
>>>   * hook is made available.
>>>   */
>>>
>>> I can only guess that powerpc  could be the same here.
>>
>> This comment is present in multiple platforms while defining set_pte_at().
>> Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at()
>> does as compared to WRITE_ONCE() that avoids the soft lock up, just trying
>> to understand.
>>
> 
> 
> Argh ! I didn't realise that you were writing directly into the page tables. When it works, that's only by chance I guess.
> 
> To properly set the page table entries, set_pte_at() has to be used:
> - On powerpc 8xx, with 16k pages, the page table entry must be copied four times. set_pte_at() does it, WRITE_ONCE() doesn't.
> - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't.
> 
> set_pte_at() also does a few other mandatory things, like calling pte_mkpte()
> 
> So, the WRITE_ONCE() must definitely become a set_pte_at()

Sure, will do. These are part of the clear tests that populates a given
entry with a non zero value before clearing and testing it with pxx_none().
In that context, WRITE_ONCE() seemed sufficient. But pte_clear() might be
closely tied with proper page table entry update and hence a preceding
set_pte_at() will be better.

There are still more WRITE_ONCE() for other page table levels during these
clear tests. set_pmd_at() and set_pud_at() are defined on platforms that
support (and enable) THP and PUD based THP respectively. Hence they could
not be used for clear tests as remaining helpers pmd_clear(), pud_clear(),
p4d_clear() and pgd_clear() still need to be validated with or without
THP support and enablement. We should just leave all other WRITE_ONCE()
instances unchanged. Please correct me if I am missing something here.

> 
> Christophe
>