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Subject: Re: [PATCH V12] mm/debug: Add tests validating architecture page
 table helpers
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200129103640.GA668562@arrakis.emea.arm.com>
Date:   Wed, 29 Jan 2020 06:09:56 -0500
Cc:     Mark Rutland <Mark.Rutland@arm.com>, linux-ia64@vger.kernel.org,
        linux-sh@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        James Hogan <jhogan@kernel.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Michal Hocko <mhocko@kernel.org>,
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        Michael Ellerman <mpe@ellerman.id.au>,
        Vlastimil Babka <vbabka@suse.cz>,
        the arch/x86 maintainers <x86@kernel.org>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Steven Price <Steven.Price@arm.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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        Masahiro Yamada <yamada.masahiro@socionext.com>,
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        "Kirill A . Shutemov" <kirill@shutemov.name>,
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        Dave Hansen <dave.hansen@intel.com>,
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        linux-mips@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>,
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        Vineet Gupta <vgupta@synopsys.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
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        "David S. Miller" <davem@davemloft.net>
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To:     Catalin Marinas <catalin.marinas@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 29, 2020, at 5:36 AM, Catalin Marinas <catalin.marinas@arm.com> =
wrote:
>=20
> On Tue, Jan 28, 2020 at 02:07:10PM -0500, Qian Cai wrote:
>> On Jan 28, 2020, at 12:47 PM, Catalin Marinas =
<catalin.marinas@arm.com> wrote:
>>> The primary goal here is not finding regressions but having clearly
>>> defined semantics of the page table accessors across architectures. =
x86
>>> and arm64 are a good starting point and other architectures will be
>>> enabled as they are aligned to the same semantics.
>>=20
>> This still does not answer the fundamental question. If this test is
>> simply inefficient to find bugs,
>=20
> Who said this is inefficient (other than you)?

Inefficient of finding bugs. It said only found a bug or two in its =
lifetime?

>=20
>> who wants to spend time to use it regularly?=20
>=20
> Arch maintainers, mm maintainers introducing new macros or assuming
> certain new semantics of the existing macros.
>=20
>> If this is just one off test that may get running once in a few years
>> (when introducing a new arch), how does it justify the ongoing cost =
to
>> maintain it?
>=20
> You are really missing the point. It's not only for a new arch but
> changes to existing arch code. And if the arch code churn in this area
> is relatively small, I'd expect a similarly small cost of maintaining
> this test.
>=20
> If you only turn DEBUG_VM on once every few years, don't generalise =
this
> to the rest of the kernel developers (as others pointed out, this test
> is default y if DEBUG_VM).

Quite the opposite, I am running DEBUG_VM almost daily for regression
workload while I felt strongly this thing does not add any value mixing =
there.

So, I would suggest to decouple this away from DEBUG_VM, and clearly
document that this test is not something intended for automated =
regression
workloads, so those people don=E2=80=99t need to waste time running =
this.

>=20
> Anyway, I think that's a pointless discussion, so not going to reply
> further (unless you have technical content to add).
>=20
> --=20
> Catalin