Commit fcd353a3 authored Dec 14, 2020 by Mike Rapoport Committed by Linus Torvalds Dec 15, 2020

m68k: deprecate DISCONTIGMEM

DISCONTIGMEM was intended to provide more efficient support for systems
with holes in their physical address space that FLATMEM did.

Yet, it's overhead in terms of the memory consumption seems to
overweight the savings on the unused memory map.

For a ARAnyM system with 16 MBytes of FastRAM configured, the memory
usage reported after page allocator initialization is

  Memory: 23828K/30720K available (3206K kernel code, 535K rwdata, 936K rodata, 768K init, 193K bss, 6892K reserved, 0K cma-reserved)

and with DISCONTIGMEM disabled and with relatively large hole in the memory
map it is:

  Memory: 23864K/30720K available (3197K kernel code, 516K rwdata, 936K rodata, 764K init, 179K bss, 6856K reserved, 0K cma-reserved)

Moreover, since m68k already has custom pfn_valid() it is possible to
define HAVE_ARCH_PFN_VALID to enable freeing of unused memory map.  The
minimal size of a hole that can be freed should not be less than
MAX_ORDER_NR_PAGES so to achieve more substantial memory savings let
m68k also define custom FORCE_MAX_ZONEORDER.

With FORCE_MAX_ZONEORDER set to 9 memory usage becomes:

  Memory: 23880K/30720K available (3197K kernel code, 516K rwdata, 936K rodata, 764K init, 179K bss, 6840K reserved, 0K cma-reserved)

Link: https://lkml.kernel.org/r/20201101170454.9567-14-rppt@kernel.org


Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Meelis Roos <mroos@linux.ee>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 4bfc848e

arch/m68k/Kconfig.cpu

+25 −1

Original line number	Diff line number	Diff line
		@@ -20,6 +20,7 @@ choice

		config M68KCLASSIC
		bool "Classic M68K CPU family support"
		select HAVE_ARCH_PFN_VALID

		config COLDFIRE
		bool "Coldfire CPU family support"
		@@ -377,11 +378,34 @@ config SINGLE_MEMORY_CHUNK
		help
		Ignore all but the first contiguous chunk of physical memory for VM
		purposes. This will save a few bytes kernel size and may speed up
		some operations. Say N if not sure.
		some operations.
		When this option os set to N, you may want to lower "Maximum zone
		order" to save memory that could be wasted for unused memory map.
		Say N if not sure.

		config ARCH_DISCONTIGMEM_ENABLE
		depends on BROKEN
		def_bool MMU && !SINGLE_MEMORY_CHUNK

		config FORCE_MAX_ZONEORDER
		int "Maximum zone order" if ADVANCED
		depends on !SINGLE_MEMORY_CHUNK
		default "11"
		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.

		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.

		This config option is actually maximum order plus one. For example,
		a value of 11 means that the largest free memory block is 2^10 pages.

		config 060_WRITETHROUGH
		bool "Use write-through caching for 68060 supervisor accesses"
		depends on ADVANCED && M68060