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Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page
 table helpers
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        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>,
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        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <c40d907a-b64b-ae0d-e58f-33dddf0e8edc@c-s.fr>
Date:   Sat, 7 Mar 2020 08:05:09 +0100
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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()

Christophe