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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers
Date:   Tue, 28 Jan 2020 00:48:30 -0500
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References: <012158b7-a40e-050f-cd1b-d6ce7faf042f@arm.com>
Cc:     Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dan Williams <dan.j.williams@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        Mark Rutland <Mark.Rutland@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Steven Price <Steven.Price@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Kees Cook <keescook@chromium.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Matthew Wilcox <willy@infradead.org>,
        Sri Krishna chowdary <schowdary@nvidia.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        "David S. Miller" <davem@davemloft.net>,
        Vineet Gupta <vgupta@synopsys.com>,
        James Hogan <jhogan@kernel.org>,
        Paul Burton <paul.burton@mips.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Gerald Schaefer <gerald.schaefer@de.ibm.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Ingo Molnar <mingo@kernel.org>,
        linux-snps-arc@lists.infradead.org, linux-mips@vger.kernel.org,
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        x86@kernel.org, linux-kernel@vger.kernel.org
In-Reply-To: <012158b7-a40e-050f-cd1b-d6ce7faf042f@arm.com>
To:     Anshuman Khandual <Anshuman.Khandual@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jan 27, 2020, at 11:58 PM, Anshuman Khandual <Anshuman.Khandual@arm.com=
> wrote:
>=20
> As I had mentioned before, the test attempts to formalize page table helpe=
r semantics
> as expected from generic MM code paths and intend to catch deviations when=
 enabled on
> a given platform. How else should we test semantics errors otherwise ? The=
re are past
> examples of usefulness for this procedure on arm64 and on s390. I am wonde=
ring how
> else to prove the usefulness of a debug feature if these references are no=
t enough.

Not saying it will not be useful. As you mentioned it actually found a bug o=
r two in the past. The problem is that there is always a cost to maintain so=
mething like this, and nobody knew how things could be broken even for the i=
solated code you mentioned in the future given how complicated the kernel co=
de base is. I am not so positive that many developers would enable this debu=
g feature and use it on a regular basis from the information you gave so far=
.=20

On the other hand, it might just be good at maintaining this thing out of tr=
ee by yourself anyway, because if there isn=E2=80=99t going to be used by ma=
ny developers, few people is going to contribute to this and even noticed wh=
en it is broken. What=E2=80=99s the point of getting this merged apart from b=
eing getting some meaningless credits?=