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Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page
 table helpers
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <a45834bc-e6f2-ac21-de9e-1aff67d12797@arm.com>
Date:   Fri, 6 Mar 2020 20:10:52 -0500
Cc:     Linux Memory Management List <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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        Christophe Leroy <christophe.leroy@c-s.fr>
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To:     Anshuman Khandual <anshuman.khandual@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Mar 6, 2020, at 7:56 PM, Anshuman Khandual =
<anshuman.khandual@arm.com> wrote:
>=20
>=20
>=20
> On 03/07/2020 06:04 AM, Qian Cai wrote:
>>=20
>>=20
>>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual =
<Anshuman.Khandual@arm.com> wrote:
>>>=20
>>> 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
>>=20
>> Why is that important for this debugging option?
>=20
> 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 ?
>=20
>>=20
>>> 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() ?
>>=20
>> Looks at the s390 version of set_pte_at(), it has this comment,
>> vmaddr);
>>=20
>> /*
>> * Certain architectures need to do special things when PTEs
>> * within a page table are directly modified.  Thus, the following
>> * hook is made available.
>> */
>>=20
>> I can only guess that powerpc  could be the same here.
>=20
> 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?=