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

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Date:   Fri, 6 Mar 2020 20:10:52 -0500
From:   Qian Cai <cai@....pw>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Linux Memory Management List <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 Mar 6, 2020, at 7:56 PM, Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
> 
> 
> 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()

No, barrier() is not enough.

> does as compared to WRITE_ONCE() that avoids the soft lock up, just trying
> to understand.

I surely can spend hours to figure which exact things in set_pte_at() is necessary for
pte_clear() not to stuck, and then propose a solution and possible need to retest on
multiple arches. I am not sure if that is a good use of my time just to saving
a few TLB/cache flush on a debug kernel?