linux/mm/Kconfig

config SELECT_MEMORY_MODEL
	def_bool y
	depends on EXPERIMENTAL || ARCH_SELECT_MEMORY_MODEL

choice
	prompt "Memory model"
	depends on SELECT_MEMORY_MODEL
	default DISCONTIGMEM_MANUAL if ARCH_DISCONTIGMEM_DEFAULT
	default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT
	default FLATMEM_MANUAL

config FLATMEM_MANUAL
	bool "Flat Memory"
	depends on !(ARCH_DISCONTIGMEM_ENABLE || ARCH_SPARSEMEM_ENABLE) || ARCH_FLATMEM_ENABLE
	help
	  This option allows you to change some of the ways that
	  Linux manages its memory internally.  Most users will
	  only have one option here: FLATMEM.  This is normal
	  and a correct option.

	  Some users of more advanced features like NUMA and
	  memory hotplug may have different options here.
	  DISCONTIGMEM is an more mature, better tested system,
	  but is incompatible with memory hotplug and may suffer
	  decreased performance over SPARSEMEM.  If unsure between
	  "Sparse Memory" and "Discontiguous Memory", choose
	  "Discontiguous Memory".

	  If unsure, choose this option (Flat Memory) over any other.

config DISCONTIGMEM_MANUAL
	bool "Discontiguous Memory"
	depends on ARCH_DISCONTIGMEM_ENABLE
	help
	  This option provides enhanced support for discontiguous
	  memory systems, over FLATMEM.  These systems have holes
	  in their physical address spaces, and this option provides
	  more efficient handling of these holes.  However, the vast
	  majority of hardware has quite flat address spaces, and
	  can have degraded performance from the extra overhead that
	  this option imposes.

	  Many NUMA configurations will have this as the only option.

	  If unsure, choose "Flat Memory" over this option.

config SPARSEMEM_MANUAL
	bool "Sparse Memory"
	depends on ARCH_SPARSEMEM_ENABLE
	help
	  This will be the only option for some systems, including
	  memory hotplug systems.  This is normal.

	  For many other systems, this will be an alternative to
	  "Discontiguous Memory".  This option provides some potential
	  performance benefits, along with decreased code complexity,
	  but it is newer, and more experimental.

	  If unsure, choose "Discontiguous Memory" or "Flat Memory"
	  over this option.

endchoice

config DISCONTIGMEM
	def_bool y
	depends on (!SELECT_MEMORY_MODEL && ARCH_DISCONTIGMEM_ENABLE) || DISCONTIGMEM_MANUAL

config SPARSEMEM
	def_bool y
	depends on SPARSEMEM_MANUAL

config FLATMEM
	def_bool y
	depends on (!DISCONTIGMEM && !SPARSEMEM) || FLATMEM_MANUAL

config FLAT_NODE_MEM_MAP
	def_bool y
	depends on !SPARSEMEM

#
# Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
# to represent different areas of memory.  This variable allows
# those dependencies to exist individually.
#
config NEED_MULTIPLE_NODES
	def_bool y
	depends on DISCONTIGMEM || NUMA

config HAVE_MEMORY_PRESENT
	def_bool y
	depends on ARCH_HAVE_MEMORY_PRESENT || SPARSEMEM

#
# SPARSEMEM_EXTREME (which is the default) does some bootmem
# allocations when memory_present() is called.  If this cannot
# be done on your architecture, select this option.  However,
# statically allocating the mem_section[] array can potentially
# consume vast quantities of .bss, so be careful.
#
# This option will also potentially produce smaller runtime code
# with gcc 3.4 and later.
#
config SPARSEMEM_STATIC
	bool

#
# Architecture platforms which require a two level mem_section in SPARSEMEM
# must select this option. This is usually for architecture platforms with
# an extremely sparse physical address space.
#
config SPARSEMEM_EXTREME
	def_bool y
	depends on SPARSEMEM && !SPARSEMEM_STATIC

config SPARSEMEM_VMEMMAP_ENABLE
	bool

config SPARSEMEM_VMEMMAP
	bool "Sparse Memory virtual memmap"
	depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
	default y
	help
	 SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
	 pfn_to_page and page_to_pfn operations.  This is the most
	 efficient option when sufficient kernel resources are available.

# eventually, we can have this option just 'select SPARSEMEM'
config MEMORY_HOTPLUG
	bool "Allow for memory hot-add"
	depends on SPARSEMEM || X86_64_ACPI_NUMA
	depends on HOTPLUG && !(HIBERNATION && !S390) && ARCH_ENABLE_MEMORY_HOTPLUG
	depends on (IA64 || X86 || PPC64 || SUPERH || S390)

comment "Memory hotplug is currently incompatible with Software Suspend"
	depends on SPARSEMEM && HOTPLUG && HIBERNATION && !S390

config MEMORY_HOTPLUG_SPARSE
	def_bool y
	depends on SPARSEMEM && MEMORY_HOTPLUG

config MEMORY_HOTREMOVE
	bool "Allow for memory hot remove"
	depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
	depends on MIGRATION

#
# If we have space for more page flags then we can enable additional
# optimizations and functionality.
#
# Regular Sparsemem takes page flag bits for the sectionid if it does not
# use a virtual memmap. Disable extended page flags for 32 bit platforms
# that require the use of a sectionid in the page flags.
#
config PAGEFLAGS_EXTENDED
	def_bool y
	depends on 64BIT || SPARSEMEM_VMEMMAP || !SPARSEMEM

# Heavily threaded applications may benefit from splitting the mm-wide
# page_table_lock, so that faults on different parts of the user address
# space can be handled with less contention: split it at this NR_CPUS.
# Default to 4 for wider testing, though 8 might be more appropriate.
# ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
# PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
#
config SPLIT_PTLOCK_CPUS
	int
	default "4096" if ARM && !CPU_CACHE_VIPT
	default "4096" if PARISC && !PA20
	default "4"

#
# support for page migration
#
config MIGRATION
	bool "Page migration"
	def_bool y
	depends on NUMA || ARCH_ENABLE_MEMORY_HOTREMOVE
	help
	  Allows the migration of the physical location of pages of processes
	  while the virtual addresses are not changed. This is useful for
	  example on NUMA systems to put pages nearer to the processors accessing
	  the page.

config PHYS_ADDR_T_64BIT
	def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT

config ZONE_DMA_FLAG
	int
	default "0" if !ZONE_DMA
	default "1"

config BOUNCE
	def_bool y
	depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)

config NR_QUICK
	int
	depends on QUICKLIST
	default "2" if SUPERH || AVR32
	default "1"

config VIRT_TO_BUS
	def_bool y
	depends on !ARCH_NO_VIRT_TO_BUS

config HAVE_MLOCK
	bool
	default y if MMU=y

config HAVE_MLOCKED_PAGE_BIT
	bool
	default y if HAVE_MLOCK=y

config MMU_NOTIFIER
	bool

config KSM
	bool "Enable KSM for page merging"
	depends on MMU
	help
	  Enable Kernel Samepage Merging: KSM periodically scans those areas
	  of an application's address space that an app has advised may be
	  mergeable.  When it finds pages of identical content, it replaces
	  the many instances by a single resident page with that content, so
	  saving memory until one or another app needs to modify the content.
	  Recommended for use with KVM, or with other duplicative applications.

config DEFAULT_MMAP_MIN_ADDR
        int "Low address space to protect from user allocation"
        default 4096
        help
	  This is the portion of low virtual memory which should be protected
	  from userspace allocation.  Keeping a user from writing to low pages
	  can help reduce the impact of kernel NULL pointer bugs.

	  For most ia64, ppc64 and x86 users with lots of address space
	  a value of 65536 is reasonable and should cause no problems.
	  On arm and other archs it should not be higher than 32768.
	  Programs which use vm86 functionality or have some need to map
	  this low address space will need CAP_SYS_RAWIO or disable this
	  protection by setting the value to 0.

	  This value can be changed after boot using the
	  /proc/sys/vm/mmap_min_addr tunable.


config NOMMU_INITIAL_TRIM_EXCESS
	int "Turn on mmap() excess space trimming before booting"
	depends on !MMU
	default 1
	help
	  The NOMMU mmap() frequently needs to allocate large contiguous chunks
	  of memory on which to store mappings, but it can only ask the system
	  allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently
	  more than it requires.  To deal with this, mmap() is able to trim off
	  the excess and return it to the allocator.

	  If trimming is enabled, the excess is trimmed off and returned to the
	  system allocator, which can cause extra fragmentation, particularly
	  if there are a lot of transient processes.

	  If trimming is disabled, the excess is kept, but not used, which for
	  long-term mappings means that the space is wasted.

	  Trimming can be dynamically controlled through a sysctl option
	  (/proc/sys/vm/nr_trim_pages) which specifies the minimum number of
	  excess pages there must be before trimming should occur, or zero if
	  no trimming is to occur.

	  This option specifies the initial value of this option.  The default
	  of 1 says that all excess pages should be trimmed.

	  See Documentation/nommu-mmap.txt for more information.