[PATCH v3 00/14] arch,mm: cleanup Kconfig entries for ARCH_FORCE_MAX_ORDER

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

Hi,

Several architectures have ARCH_FORCE_MAX_ORDER in their Kconfig and
they all have wrong and misleading prompt and help text for this option.

Besides, some define insane limits for possible values of
ARCH_FORCE_MAX_ORDER, some carefully define ranges only for a subset of
possible configurations, some make this option configurable by users for no
good reason.

This set updates the prompt and help text everywhere and does its best to
update actual definitions of ranges where applicable.

kbuild generated a bunch of false positives because it assigns -1 to
ARCH_FORCE_MAX_ORDER, hopefully this will be fixed soon.

v3:
* fix rebase fallouts (Zi Yan)

v2: https://lore.kernel.org/all/20230324052233.2654090-1-rppt@kernel.org
* arm64: show prompt for ARCH_FORCE_MAX_ORDER only if EXPERT (Catalin)
* Add Acked- and Reviewed-by tags (thanks Geert, Kirill and Max)

v1: https://lore.kernel.org/all/20230323092156.2545741-1-rppt@kernel.org

Mike Rapoport (IBM) (14):
  arm: reword ARCH_FORCE_MAX_ORDER prompt and help text
  arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER
  arm64: reword ARCH_FORCE_MAX_ORDER prompt and help text
  csky: drop ARCH_FORCE_MAX_ORDER
  ia64: don't allow users to override ARCH_FORCE_MAX_ORDER
  m68k: reword ARCH_FORCE_MAX_ORDER prompt and help text
  nios2: reword ARCH_FORCE_MAX_ORDER prompt and help text
  nios2: drop ranges for definition of ARCH_FORCE_MAX_ORDER
  powerpc: reword ARCH_FORCE_MAX_ORDER prompt and help text
  powerpc: drop ranges for definition of ARCH_FORCE_MAX_ORDER
  sh: reword ARCH_FORCE_MAX_ORDER prompt and help text
  sh: drop ranges for definition of ARCH_FORCE_MAX_ORDER
  sparc: reword ARCH_FORCE_MAX_ORDER prompt and help text
  xtensa: reword ARCH_FORCE_MAX_ORDER prompt and help text

 arch/arm/Kconfig      | 16 +++++++++-------
 arch/arm64/Kconfig    | 26 ++++++++++++--------------
 arch/csky/Kconfig     |  4 ----
 arch/ia64/Kconfig     |  3 +--
 arch/m68k/Kconfig.cpu | 16 +++++++++-------
 arch/nios2/Kconfig    | 17 +++++++++--------
 arch/powerpc/Kconfig  | 22 +++++++++-------------
 arch/sh/mm/Kconfig    | 19 +++++++++----------
 arch/sparc/Kconfig    | 16 +++++++++-------
 arch/xtensa/Kconfig   | 16 +++++++++-------
 10 files changed, 76 insertions(+), 79 deletions(-)


base-commit: 51551d71edbc998fd8c8afa7312db3d270f5998e
-- 
2.35.1

*** BLURB HERE ***

Mike Rapoport (IBM) (14):
  arm: reword ARCH_FORCE_MAX_ORDER prompt and help text
  arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER
  arm64: reword ARCH_FORCE_MAX_ORDER prompt and help text
  csky: drop ARCH_FORCE_MAX_ORDER
  ia64: don't allow users to override ARCH_FORCE_MAX_ORDER
  m68k: reword ARCH_FORCE_MAX_ORDER prompt and help text
  nios2: reword ARCH_FORCE_MAX_ORDER prompt and help text
  nios2: drop ranges for definition of ARCH_FORCE_MAX_ORDER
  powerpc: reword ARCH_FORCE_MAX_ORDER prompt and help text
  powerpc: drop ranges for definition of ARCH_FORCE_MAX_ORDER
  sh: reword ARCH_FORCE_MAX_ORDER prompt and help text
  sh: drop ranges for definition of ARCH_FORCE_MAX_ORDER
  sparc: reword ARCH_FORCE_MAX_ORDER prompt and help text
  xtensa: reword ARCH_FORCE_MAX_ORDER prompt and help text

 arch/arm/Kconfig      | 16 +++++++++-------
 arch/arm64/Kconfig    | 26 ++++++++++++--------------
 arch/csky/Kconfig     |  4 ----
 arch/ia64/Kconfig     |  3 +--
 arch/m68k/Kconfig.cpu | 16 +++++++++-------
 arch/nios2/Kconfig    | 17 +++++++++--------
 arch/powerpc/Kconfig  | 22 +++++++++-------------
 arch/sh/mm/Kconfig    | 19 +++++++++----------
 arch/sparc/Kconfig    | 16 +++++++++-------
 arch/xtensa/Kconfig   | 16 +++++++++-------
 10 files changed, 76 insertions(+), 79 deletions(-)


base-commit: 51551d71edbc998fd8c8afa7312db3d270f5998e
-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/arm/Kconfig | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 929e646e84b9..0b15384c62e6 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig

@@ -1354,17 +1354,19 @@ config ARM_MODULE_PLTS
 	  configurations. If unsure, say y.
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
+	int "Order of maximal physically contiguous allocations"
 	default "11" if SOC_AM33XX
 	default "8" if SA1111
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
+
+	  Don't change if unsure.
 
 config ALIGNMENT_TRAP
 	def_bool CPU_CP15_MMU

-- 
2.35.1

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

Acked-by: Kirill A. Shutemov <redacted>
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

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/arm64/Kconfig | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7324032af859..cc11cdcf5a00 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -1487,24 +1487,24 @@ config XEN
 # 16K |       27          |      14      |       13        |         11         |
 # 64K |       29          |      16      |       13        |         13         |
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order" if EXPERT && (ARM64_4K_PAGES || ARM64_16K_PAGES)
+	int "Order of maximal physically contiguous allocations" if EXPERT && (ARM64_4K_PAGES || ARM64_16K_PAGES)
 	default "13" if ARM64_64K_PAGES
 	default "11" if ARM64_16K_PAGES
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
 
-	  We make sure that we can allocate up to a HugePage size for each configuration.
-	  Hence we have :
-		MAX_ORDER = PMD_SHIFT - PAGE_SHIFT  => PAGE_SHIFT - 3
+	  The maximal size of allocation cannot exceed the size of the
+	  section, so the value of MAX_ORDER should satisfy
 
-	  However for 4K, we choose a higher default value, 10 as opposed to 9, giving us
-	  4M allocations matching the default size used by generic code.
+	    MAX_ORDER + PAGE_SHIFT <= SECTION_SIZE_BITS
+
+	  Don't change if unsure.
 
 config UNMAP_KERNEL_AT_EL0
 	bool "Unmap kernel when running in userspace (aka \"KAISER\")" if EXPERT

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The default value of ARCH_FORCE_MAX_ORDER matches the generic default
defined in the MM code, the architecture does not support huge pages, so
there is no need to keep ARCH_FORCE_MAX_ORDER option available.

Drop it.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/csky/Kconfig | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/csky/Kconfig b/arch/csky/Kconfig
index c694fac43bed..00379a843c37 100644
--- a/arch/csky/Kconfig
+++ b/arch/csky/Kconfig

@@ -332,10 +332,6 @@ config HIGHMEM
 	select KMAP_LOCAL
 	default y
 
-config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
-	default "10"
-
 config DRAM_BASE
 	hex "DRAM start addr (the same with memory-section in dts)"
 	default 0x0

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

It is enough to keep default values for base and huge pages without
letting users to override ARCH_FORCE_MAX_ORDER.

Drop the prompt to make the option unvisible in *config.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/ia64/Kconfig | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
index 0d2f41fa56ee..b61437cae162 100644
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig

@@ -202,8 +202,7 @@ config IA64_CYCLONE
 	  If you're unsure, answer N.
 
 config ARCH_FORCE_MAX_ORDER
-	int "MAX_ORDER (10 - 16)"  if !HUGETLB_PAGE
-	range 10 16  if !HUGETLB_PAGE
+	int
 	default "16" if HUGETLB_PAGE
 	default "10"
 

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/m68k/Kconfig.cpu | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
index c9df6572133f..e530bc8f240f 100644
--- a/arch/m68k/Kconfig.cpu
+++ b/arch/m68k/Kconfig.cpu

@@ -398,21 +398,23 @@ config SINGLE_MEMORY_CHUNK
 	  Say N if not sure.
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order" if ADVANCED
+	int "Order of maximal physically contiguous allocations" if ADVANCED
 	depends on !SINGLE_MEMORY_CHUNK
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
 
 	  For systems that have holes in their physical address space this
 	  value also defines the minimal size of the hole that allows
 	  freeing unused memory map.
 
+	  Don't change if unsure.
+
 config 060_WRITETHROUGH
 	bool "Use write-through caching for 68060 supervisor accesses"
 	depends on ADVANCED && M68060

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/nios2/Kconfig | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/nios2/Kconfig b/arch/nios2/Kconfig
index 89708b95978c..fcaa6bbda3fc 100644
--- a/arch/nios2/Kconfig
+++ b/arch/nios2/Kconfig

@@ -45,16 +45,18 @@ menu "Kernel features"
 source "kernel/Kconfig.hz"
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
+	int "Order of maximal physically contiguous allocations"
 	range 8 19
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
+
+	  Don't change if unsure.
 
 endmenu
 

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

nios2 defines range for ARCH_FORCE_MAX_ORDER allowing MAX_ORDER
up to 19, which implies maximal contiguous allocation size of 2^19
pages or 2GiB.

Drop bogus definition of ranges for ARCH_FORCE_MAX_ORDER and leave it a
simple integer with sensible default.

Users that *really* need to change the value of ARCH_FORCE_MAX_ORDER
will be able to do so but they won't be mislead by the bogus ranges.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/nios2/Kconfig | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/nios2/Kconfig b/arch/nios2/Kconfig
index fcaa6bbda3fc..e5936417d3cd 100644
--- a/arch/nios2/Kconfig
+++ b/arch/nios2/Kconfig

@@ -46,7 +46,6 @@ source "kernel/Kconfig.hz"
 
 config ARCH_FORCE_MAX_ORDER
 	int "Order of maximal physically contiguous allocations"
-	range 8 19
 	default "10"
 	help
 	  The kernel page allocator limits the size of maximal physically

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/powerpc/Kconfig | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 24d56536b269..c0095bf795ca 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -896,7 +896,7 @@ config DATA_SHIFT
 	  8M pages will be pinned.
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
+	int "Order of maximal physically contiguous allocations"
 	range 7 8 if PPC64 && PPC_64K_PAGES
 	default "8" if PPC64 && PPC_64K_PAGES
 	range 12 12 if PPC64 && !PPC_64K_PAGES

@@ -910,17 +910,19 @@ config ARCH_FORCE_MAX_ORDER
 	range 10 63
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
 
 	  The page size is not necessarily 4KB.  For example, on 64-bit
 	  systems, 64KB pages can be enabled via CONFIG_PPC_64K_PAGES.  Keep
 	  this in mind when choosing a value for this option.
 
+	  Don't change if unsure.
+
 config PPC_SUBPAGE_PROT
 	bool "Support setting protections for 4k subpages (subpage_prot syscall)"
 	default n

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

PowerPC defines ranges for ARCH_FORCE_MAX_ORDER some of which are
insanely allowing MAX_ORDER up to 63, which implies maximal contiguous
allocation size of 2^63 pages.

Drop bogus definitions of ranges for ARCH_FORCE_MAX_ORDER and leave it a
simple integer with sensible defaults.

Users that *really* need to change the value of ARCH_FORCE_MAX_ORDER
will be able to do so but they won't be mislead by the bogus ranges.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/powerpc/Kconfig | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c0095bf795ca..419be4a71004 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -897,17 +897,11 @@ config DATA_SHIFT
 
 config ARCH_FORCE_MAX_ORDER
 	int "Order of maximal physically contiguous allocations"
-	range 7 8 if PPC64 && PPC_64K_PAGES
 	default "8" if PPC64 && PPC_64K_PAGES
-	range 12 12 if PPC64 && !PPC_64K_PAGES
 	default "12" if PPC64 && !PPC_64K_PAGES
-	range 8 63 if PPC32 && PPC_16K_PAGES
 	default "8" if PPC32 && PPC_16K_PAGES
-	range 6 63 if PPC32 && PPC_64K_PAGES
 	default "6" if PPC32 && PPC_64K_PAGES
-	range 4 63 if PPC32 && PPC_256K_PAGES
 	default "4" if PPC32 && PPC_256K_PAGES
-	range 10 63
 	default "10"
 	help
 	  The kernel page allocator limits the size of maximal physically

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/sh/mm/Kconfig | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/sh/mm/Kconfig b/arch/sh/mm/Kconfig
index 40271090bd7d..238b2772751e 100644
--- a/arch/sh/mm/Kconfig
+++ b/arch/sh/mm/Kconfig

@@ -19,7 +19,7 @@ config PAGE_OFFSET
 	default "0x00000000"
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
+	int "Order of maximal physically contiguous allocations"
 	range 8 63 if PAGE_SIZE_16KB
 	default "8" if PAGE_SIZE_16KB
 	range 6 63 if PAGE_SIZE_64KB

@@ -28,16 +28,18 @@ config ARCH_FORCE_MAX_ORDER
 	default "13" if !MMU
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
 
 	  The page size is not necessarily 4KB. Keep this in mind when
 	  choosing a value for this option.
 
+	  Don't change if unsure.
+
 config MEMORY_START
 	hex "Physical memory start address"
 	default "0x08000000"

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

sh defines insane ranges for ARCH_FORCE_MAX_ORDER allowing MAX_ORDER
up to 63, which implies maximal contiguous allocation size of 2^63
pages.

Drop bogus definitions of ranges for ARCH_FORCE_MAX_ORDER and leave it a
simple integer with sensible defaults.

Users that *really* need to change the value of ARCH_FORCE_MAX_ORDER
will be able to do so but they won't be mislead by the bogus ranges.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/sh/mm/Kconfig | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/sh/mm/Kconfig b/arch/sh/mm/Kconfig
index 238b2772751e..511c17aede4a 100644
--- a/arch/sh/mm/Kconfig
+++ b/arch/sh/mm/Kconfig

@@ -20,11 +20,8 @@ config PAGE_OFFSET
 
 config ARCH_FORCE_MAX_ORDER
 	int "Order of maximal physically contiguous allocations"
-	range 8 63 if PAGE_SIZE_16KB
 	default "8" if PAGE_SIZE_16KB
-	range 6 63 if PAGE_SIZE_64KB
 	default "6" if PAGE_SIZE_64KB
-	range 10 63
 	default "13" if !MMU
 	default "10"
 	help

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/sparc/Kconfig | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index e3242bf5a8df..959e43a1aaca 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig

@@ -270,15 +270,17 @@ config ARCH_SPARSEMEM_DEFAULT
 	def_bool y if SPARC64
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
+	int "Order of maximal physically contiguous allocations"
 	default "12"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
+
+	  Don't change if unsure.
 
 if SPARC64 || COMPILE_TEST
 source "kernel/power/Kconfig"

-- 
2.35.1

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
 arch/xtensa/Kconfig | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index 3eee334ba873..3c6e5471f025 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig

@@ -772,15 +772,17 @@ config HIGHMEM
 	  If unsure, say Y.
 
 config ARCH_FORCE_MAX_ORDER
-	int "Maximum zone order"
+	int "Order of maximal physically contiguous allocations"
 	default "10"
 	help
-	  The kernel memory allocator divides physically contiguous memory
-	  blocks into "zones", where each zone is a power of two number of
-	  pages.  This option selects the largest power of two that the kernel
-	  keeps in the memory allocator.  If you need to allocate very large
-	  blocks of physically contiguous memory, then you may need to
-	  increase this value.
+	  The kernel page allocator limits the size of maximal physically
+	  contiguous allocations. The limit is called MAX_ORDER and it
+	  defines the maximal power of two of number of pages that can be
+	  allocated as a single contiguous block. This option allows
+	  overriding the default setting when ability to allocate very
+	  large blocks of physically contiguous memory is required.
+
+	  Don't change if unsure.
 
 endmenu
 

-- 
2.35.1

On 2023/3/25 14:08, Mike Rapoport 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

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>

Reviewed-by: Kefeng Wang <redacted>

On 2023/3/25 14:08, Mike Rapoport wrote:

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
describe this configuration option.

Update both to actually describe what this option does.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>

Reviewed-by: Kefeng Wang <redacted>

On 2023/3/25 14:08, Mike Rapoport wrote:

It seems that we could drop the following part?

diff --git a/arch/ia64/include/asm/sparsemem.h 

b/arch/ia64/include/asm/sparsemem.h
index a58f8b466d96..18187551b183 100644

--- a/arch/ia64/include/asm/sparsemem.h
+++ b/arch/ia64/include/asm/sparsemem.h

@@ -11,11 +11,6 @@

  #define SECTION_SIZE_BITS      (30)
  #define MAX_PHYSMEM_BITS       (50)
-#ifdef CONFIG_ARCH_FORCE_MAX_ORDER
-#if (CONFIG_ARCH_FORCE_MAX_ORDER + PAGE_SHIFT > SECTION_SIZE_BITS)
-#undef SECTION_SIZE_BITS
-#define SECTION_SIZE_BITS (CONFIG_ARCH_FORCE_MAX_ORDER + PAGE_SHIFT)
-#endif
  #endif

On Sat, Mar 25, 2023 at 1:09 AM Mike Rapoport [off-list ref] 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.

Justin

On Wed, Mar 29, 2023 at 10:55:37AM -0500, Justin Forbes wrote:

On Sat, Mar 25, 2023 at 1:09 AM Mike Rapoport [off-list ref] wrote:

quoted

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.
 

Justin

quoted

Acked-by: Kirill A. Shutemov <redacted>
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.

On Tue, Apr 4, 2023 at 2:22 AM Mike Rapoport [off-list ref] wrote:

On Wed, Mar 29, 2023 at 10:55:37AM -0500, Justin Forbes wrote:

quoted

On Sat, Mar 25, 2023 at 1:09 AM Mike Rapoport [off-list ref] wrote:

quoted

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.

quoted

Justin

quoted

Acked-by: Kirill A. Shutemov <redacted>
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.

On Tue, Apr 04, 2023 at 06:50:01AM -0500, Justin Forbes wrote:

On Tue, Apr 4, 2023 at 2:22 AM Mike Rapoport [off-list ref] wrote:

quoted

On Wed, Mar 29, 2023 at 10:55:37AM -0500, Justin Forbes wrote:

quoted

On Sat, Mar 25, 2023 at 1:09 AM Mike Rapoport [off-list ref] wrote:

quoted

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.

If we remove EXPERT (as prior to this patch), I'd rather keep the ranges
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 sure
we got to any conclusion there).

-- 
Catalin

On Wed, 12 Apr 2023 18:27:08 +0100 Catalin Marinas [off-list ref] wrote:

quoted

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.

If we remove EXPERT (as prior to this patch), I'd rather keep the ranges
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 sure
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.

On Sat, Mar 25, 2023 at 02:38:15PM +0800, Kefeng Wang wrote:

On 2023/3/25 14:08, Mike Rapoport wrote:

quoted

From: "Mike Rapoport (IBM)" <rppt@kernel.org>

It is enough to keep default values for base and huge pages without
letting users to override ARCH_FORCE_MAX_ORDER.

Drop the prompt to make the option unvisible in *config.

Acked-by: Kirill A. Shutemov <redacted>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
---
  arch/ia64/Kconfig | 3 +--
  1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
index 0d2f41fa56ee..b61437cae162 100644
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig

@@ -202,8 +202,7 @@ config IA64_CYCLONE
  	  If you're unsure, answer N.
  config ARCH_FORCE_MAX_ORDER
-	int "MAX_ORDER (10 - 16)"  if !HUGETLB_PAGE
-	range 10 16  if !HUGETLB_PAGE
+	int
  	default "16" if HUGETLB_PAGE
  	default "10"

It seems that we could drop the following part?

ia64 can have 64k pages, so with MAX_ORDER==16 we'd need at least 32 bits
for section size
 

-- 
Sincerely yours,
Mike.

On Tue, Apr 18, 2023 at 03:05:57PM -0700, Andrew Morton wrote:

On Wed, 12 Apr 2023 18:27:08 +0100 Catalin Marinas [off-list ref] wrote:

quoted

quoted

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.

If we remove EXPERT (as prior to this patch), I'd rather keep the ranges
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 sure
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.

-- 
Catalin

On Wed, Apr 19, 2023 at 6:12 AM Catalin Marinas [off-list ref] wrote:

On Tue, Apr 18, 2023 at 03:05:57PM -0700, Andrew Morton wrote:

quoted

On Wed, 12 Apr 2023 18:27:08 +0100 Catalin Marinas [off-list ref] wrote:

quoted

quoted

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.

If we remove EXPERT (as prior to this patch), I'd rather keep the ranges
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 sure
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

On Tue, Apr 18, 2023 at 5:22 PM Andrew Morton [off-list ref] wrote:

On Wed, 12 Apr 2023 18:27:08 +0100 Catalin Marinas [off-list ref] wrote:

quoted

quoted

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.

If we remove EXPERT (as prior to this patch), I'd rather keep the ranges
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 sure
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.

Well, I am very sorry to see this going in as is.  It will silently
change people building with oldconfig, and anyone not paying attention
will not notice until an issue is hit where "it worked before, and my
config hasn't changed".  If EXPERT is unset, there is no notification,
just a changed behavior.  While it would be easy for me to carry a
patch dropping the if EXPERT, it will not help any users building on
upstream with our configs, whether for their own regular use, or while
trying to debug other issues,  I expect it will result in a reasonable
amount of frustration from users trying to do the right thing and
bisect or test patches upstream.

Justin

On Tue, Apr 25, 2023 at 11:09:58AM -0500, Justin Forbes wrote:

On Tue, Apr 18, 2023 at 5:22 PM Andrew Morton [off-list ref] wrote:

quoted

On Wed, 12 Apr 2023 18:27:08 +0100 Catalin Marinas [off-list ref] wrote:

quoted

quoted

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.

If we remove EXPERT (as prior to this patch), I'd rather keep the ranges
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 sure
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.

Well, I am very sorry to see this going in as is.  It will silently
change people building with oldconfig, and anyone not paying attention
will not notice until an issue is hit where "it worked before, and my
config hasn't changed".  If EXPERT is unset, there is no notification,
just a changed behavior.  While it would be easy for me to carry a
patch dropping the if EXPERT, it will not help any users building on
upstream with our configs, whether for their own regular use, or while
trying to debug other issues,  I expect it will result in a reasonable
amount of frustration from users trying to do the right thing and
bisect or test patches upstream.

As I said in a previous reply, I'm fine with reverting this commit if it
breaks existing configs. It's only that Andrew had already queued it in
his tree but we have time until the final 6.4 kernel is released.

That said, would you mind sending a patch reverting it (if removing
EXPERT, I'd like to keep the ranges)? ;)

Thanks.

-- 
Catalin