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From:   Justin Forbes <jforbes@fedoraproject.org>
Date:   Wed, 19 Apr 2023 06:27:31 -0500
Message-ID: <CAFbkSA3yn_4Monrnk2u3CzfJ934Hy15rjAJ85AdFU40nV7KTkQ@mail.gmail.com>
Subject: Re: [PATCH v3 02/14] arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        "David S. Miller" <davem@davemloft.net>,
        Dinh Nguyen <dinguyen@kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Guo Ren <guoren@kernel.org>,
        John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Max Filippov <jcmvbkbc@gmail.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Rich Felker <dalias@libc.org>,
        Russell King <linux@armlinux.org.uk>,
        Will Deacon <will@kernel.org>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Zi Yan <ziy@nvidia.com>, linux-arm-kernel@lists.infradead.org,
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On Wed, Apr 19, 2023 at 6:12=E2=80=AFAM Catalin Marinas <catalin.marinas@ar=
m.com> wrote:
>
> On Tue, Apr 18, 2023 at 03:05:57PM -0700, Andrew Morton wrote:
> > On Wed, 12 Apr 2023 18:27:08 +0100 Catalin Marinas <catalin.marinas@arm=
.com> wrote:
> > > > It sounds nice in theory. In practice. EXPERT hides too much. When =
you
> > > > flip expert, you expose over a 175ish new config options which are
> > > > hidden behind EXPERT.  You don't have to know what you are doing ju=
st
> > > > with the MAX_ORDER, but a whole bunch more as well.  If everyone we=
re
> > > > already running 10, this might be less of a problem. At least Fedor=
a
> > > > and RHEL are running 13 for 4K pages on aarch64. This was not some
> > > > accidental choice, we had to carry a patch to even allow it for a
> > > > while.  If this does go in as is, we will likely just carry a patch=
 to
> > > > remove the "if EXPERT", but that is a bit of a disservice to users =
who
> > > > might be trying to debug something else upstream, bisecting upstrea=
m
> > > > kernels or testing a patch.  In those cases, people tend to use
> > > > pristine upstream sources without distro patches to verify, and the=
y
> > > > tend to use their existing configs. With this change, their MAX_ORD=
ER
> > > > will drop to 10 from 13 silently.   That can look like a different
> > > > issue enough to ruin a bisect or have them give bad feedback on a
> > > > patch because it introduces a "regression" which is not a regressio=
n
> > > > at all, but a config change they couldn't see.
> > >
> > > If we remove EXPERT (as prior to this patch), I'd rather keep the ran=
ges
> > > and avoid having to explain to people why some random MAX_ORDER doesn=
't
> > > build (keeping the range would also make sense for randconfig, not su=
re
> > > we got to any conclusion there).
> >
> > Well this doesn't seem to have got anywhere.  I think I'll send the
> > patchset into Linus for the next merge window as-is.  Please let's take
> > a look at this Kconfig presentation issue during the following -rc
> > cycle.
>
> That's fine by me. I have a slight preference to drop EXPERT and keep
> the ranges in, especially if it affects current distro kernels. Debian
> seems to enable EXPERT already in their arm64 kernel config but I'm not
> sure about the Fedora or other distro kernels. If they don't, we can
> fix/revert this Kconfig entry once the merging window is closed.

Fedora and RHEL do not enable EXPERT already.

Justin