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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers
Date:   Fri, 6 Mar 2020 19:34:18 -0500
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References: <61250cdc-f80b-2e50-5168-2ec67ec6f1e6@arm.com>
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,
        Christophe Leroy <christophe.leroy@c-s.fr>
In-Reply-To: <61250cdc-f80b-2e50-5168-2ec67ec6f1e6@arm.com>
To:     Anshuman Khandual <Anshuman.Khandual@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> w=
rote:
>=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 sor=
t
> of 'static' tests. set_pte_at() is platform provided and could/might trigg=
er
> these flushes or some other platform specific synchronization stuff. Just

Why is that important for this debugging option?

> wondering is there specific reason with respect to the soft lock up proble=
m
> 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.=