linux-kernel - Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers

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Date:   Sat, 7 Mar 2020 06:26:40 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Qian Cai <cai@....pw>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-riscv@...ts.infradead.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christophe Leroy <christophe.leroy@....fr>
Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page
 table helpers



On 03/07/2020 06:04 AM, Qian Cai wrote:
> 
> 
>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@....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 ?

> 
>> 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.