From: Justin Forbes <jforbes@fedoraproject.org>
To: Mike Rapoport <rppt@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Arnd Bergmann <arnd@arndb.de>,
Catalin Marinas <catalin.marinas@arm.com>,
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,
linux-csky@vger.kernel.org, linux-ia64@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-m68k@lists.linux-m68k.org,
linux-mm@kvack.org, linux-sh@vger.kernel.org,
linux-xtensa@linux-xtensa.org, linuxppc-dev@lists.ozlabs.org,
sparclinux@vger.kernel.org
Subject: Re: [PATCH v3 02/14] arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER
Date: Tue, 4 Apr 2023 06:50:01 -0500 [thread overview]
Message-ID: <CAFbkSA38eTA_iJ3ttBvQ8G4Rjj8qB12GxY7Z=qmZ8wm+0tZieA@mail.gmail.com> (raw)
In-Reply-To: <ZCvQGJzdED+An8an@kernel.org>
On Tue, Apr 4, 2023 at 2:22 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Wed, Mar 29, 2023 at 10:55:37AM -0500, Justin Forbes wrote:
> > On Sat, Mar 25, 2023 at 1:09 AM Mike Rapoport <rppt@kernel.org> wrote:
> > >
> > > From: "Mike Rapoport (IBM)" <rppt@kernel.org>
> > >
> > > It is not a good idea to change fundamental parameters of core memory
> > > management. Having predefined ranges suggests that the values within
> > > those ranges are sensible, but one has to *really* understand
> > > implications of changing MAX_ORDER before actually amending it and
> > > ranges don't help here.
> > >
> > > Drop ranges in definition of ARCH_FORCE_MAX_ORDER and make its prompt
> > > visible only if EXPERT=y
> >
> > I do not like suddenly hiding this behind EXPERT for a couple of
> > reasons. Most importantly, it will silently change the config for
> > users building with an old kernel config. If a user has for instance
> > "13" set and building with 4K pages, as is the current configuration
> > for Fedora and RHEL aarch64 builds, an oldconfig build will now set it
> > to 10 with no indication that it is doing so. And while I think that
> > 10 is a fine default for many aarch64 users, there are valid reasons
> > for choosing other values. Putting this behind expert makes it much
> > less obvious that this is an option.
>
> That's the idea of EXPERT, no?
>
> This option was intended to allow allocation of huge pages for
> architectures that had PMD_ORDER > MAX_ORDER and not to allow user to
> select size of maximal physically contiguous allocation.
>
> Changes to MAX_ORDER fundamentally change the behaviour of core mm and
> unless users *really* know what they are doing there is no reason to choose
> non-default values so hiding this option behind EXPERT seems totally
> appropriate to me.
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 just
with the MAX_ORDER, but a whole bunch more as well. If everyone were
already running 10, this might be less of a problem. At least Fedora
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 upstream
kernels or testing a patch. In those cases, people tend to use
pristine upstream sources without distro patches to verify, and they
tend to use their existing configs. With this change, their MAX_ORDER
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 regression
at all, but a config change they couldn't see.
>
> > Justin
> >
> > > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > Reviewed-by: Zi Yan <ziy@nvidia.com>
> > > Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
> > > ---
> > > arch/arm64/Kconfig | 4 +---
> > > 1 file changed, 1 insertion(+), 3 deletions(-)
> > >
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index e60baf7859d1..7324032af859 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -1487,11 +1487,9 @@ config XEN
> > > # 16K | 27 | 14 | 13 | 11 |
> > > # 64K | 29 | 16 | 13 | 13 |
> > > config ARCH_FORCE_MAX_ORDER
> > > - int "Maximum zone order" if ARM64_4K_PAGES || ARM64_16K_PAGES
> > > + int "Maximum zone order" if EXPERT && (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 memory allocator divides physically contiguous memory
> > > --
> > > 2.35.1
> > >
> > >
>
> --
> Sincerely yours,
> Mike.
>
next prev parent reply other threads:[~2023-04-04 11:58 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-25 6:08 [PATCH v3 00/14] arch,mm: cleanup Kconfig entries for ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 01/14] arm: reword ARCH_FORCE_MAX_ORDER prompt and help text Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 02/14] arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:35 ` Kefeng Wang
2023-03-29 15:55 ` Justin Forbes
2023-04-04 7:22 ` Mike Rapoport
2023-04-04 11:50 ` Justin Forbes [this message]
2023-04-12 17:27 ` Catalin Marinas
2023-04-18 22:05 ` Andrew Morton
2023-04-19 11:05 ` Catalin Marinas
2023-04-19 11:27 ` Justin Forbes
2023-04-25 16:09 ` Justin Forbes
2023-04-27 13:40 ` Catalin Marinas
2023-03-25 6:08 ` [PATCH v3 03/14] arm64: reword ARCH_FORCE_MAX_ORDER prompt and help text Mike Rapoport
2023-03-25 6:35 ` Kefeng Wang
2023-03-25 6:08 ` [PATCH v3 04/14] csky: drop ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 05/14] ia64: don't allow users to override ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:38 ` Kefeng Wang
2023-04-19 8:56 ` Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 06/14] m68k: reword ARCH_FORCE_MAX_ORDER prompt and help text Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 07/14] nios2: " Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 08/14] nios2: drop ranges for definition of ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 09/14] powerpc: reword ARCH_FORCE_MAX_ORDER prompt and help text Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 10/14] powerpc: drop ranges for definition of ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 11/14] sh: reword ARCH_FORCE_MAX_ORDER prompt and help text Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 12/14] sh: drop ranges for definition of ARCH_FORCE_MAX_ORDER Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 13/14] sparc: reword ARCH_FORCE_MAX_ORDER prompt and help text Mike Rapoport
2023-03-25 6:08 ` [PATCH v3 14/14] xtensa: " Mike Rapoport
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