Re: [PATCH] Revert arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER

From: Mike Rapoport <rppt@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Justin M. Forbes" <jforbes@fedoraproject.org>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, jmforbes@linuxtx.org,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] Revert arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER
Date: Sat, 29 Apr 2023 22:01:07 +0300	[thread overview]
Message-ID: <ZE1pcwi95nPdlKzN@kernel.org> (raw)
In-Reply-To: <ZEv76qfIiJcUvdql@arm.com>

On Fri, Apr 28, 2023 at 06:01:30PM +0100, Catalin Marinas wrote:
> + Mike and Andrew
> 
> On Fri, Apr 28, 2023 at 10:36:45AM -0500, Justin M. Forbes wrote:
> > While the ARCH_FORCE_MAX_ORDER changes clarified the descriptions quite
> > a bit, the aarch64 specific change moved this config to sit behind
> > CONFIG_EXPERT. This becomes problematic when distros are setting this to
> > a non default value already. Pushing it behind EXPERT where it was not
> > before will silently change the configuration for users building with
> > oldconfig.  If distros patch out if EXPERT downstream, it still creates
> > problems for users testing out upstream patches, or trying to bisect to
> > find the root of problem, as the configuration will change unexpectedly,
> > possibly leading to different behavior and false results.
> > 
> > Whem I asked about reverting the EXPERT, dependency, I was asked to add

Nit: When

> > the ranges back.
> > 
> > This essentially reverts commit 34affcd7577a232803f729d1870ba475f294e4ea
> > 
> > Signed-off-by: Justin M. Forbes <jforbes@fedoraproject.org>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > ---
> >  arch/arm64/Kconfig | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index b1201d25a8a4..dae18ac01e94 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -1516,9 +1516,11 @@ config XEN
> >  # 16K |       27          |      14      |       13        |         11         |
> >  # 64K |       29          |      16      |       13        |         13         |
> >  config ARCH_FORCE_MAX_ORDER
> > -	int "Order of maximal physically contiguous allocations" if EXPERT && (ARM64_4K_PAGES || ARM64_16K_PAGES)
> > +	int "Order of maximal physically contiguous allocations" if ARM64_4K_PAGES || ARM64_16K_PAGES
> >  	default "13" if ARM64_64K_PAGES
> > +	range 11 13 if ARM64_16K_PAGES
> >  	default "11" if ARM64_16K_PAGES
> > +	range 10 15 if ARM64_4K_PAGES
> >  	default "10"
> >  	help
> >  	  The kernel page allocator limits the size of maximal physically
> 
> The revert looks fine to me:
> 
> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> 
> For the record, the original discussion:
> 
> Link: https://lore.kernel.org/r/CAFxkdAr5C7ggZ+WdvDbsfmwuXujT_z_x3qcUnhnCn-WrAurvgA@mail.gmail.com

I'm not really happy about this revert because MAX_ORDER is not something
that should be changed easily.
But since hiding it behind EXPERT would silently change lots of existing
builds, I won't object.

Still, I never got the answer _why_ Fedora/RHEL configs use non-default
value. Quite possible something else needs to be fixed rather than having
overgrown MAX_ORDER.

-- 
Sincerely yours,
Mike.

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