mm/Kconfig

   1 config SELECT_MEMORY_MODEL
   2         def_bool y
   3         depends on ARCH_SELECT_MEMORY_MODEL
   4
   5 choice
   6         prompt "Memory model"
   7         depends on SELECT_MEMORY_MODEL
   8         default DISCONTIGMEM_MANUAL if ARCH_DISCONTIGMEM_DEFAULT
   9         default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT
  10         default FLATMEM_MANUAL
  11
  12 config FLATMEM_MANUAL
  13         bool "Flat Memory"
  14         depends on !(ARCH_DISCONTIGMEM_ENABLE || ARCH_SPARSEMEM_ENABLE) || ARCH_FLATMEM_ENABLE
  15         help
  16           This option allows you to change some of the ways that
  17           Linux manages its memory internally.  Most users will
  18           only have one option here: FLATMEM.  This is normal
  19           and a correct option.
  20
  21           Some users of more advanced features like NUMA and
  22           memory hotplug may have different options here.
  23           DISCONTIGMEM is a more mature, better tested system,
  24           but is incompatible with memory hotplug and may suffer
  25           decreased performance over SPARSEMEM.  If unsure between
  26           "Sparse Memory" and "Discontiguous Memory", choose
  27           "Discontiguous Memory".
  28
  29           If unsure, choose this option (Flat Memory) over any other.
  30
  31 config DISCONTIGMEM_MANUAL
  32         bool "Discontiguous Memory"
  33         depends on ARCH_DISCONTIGMEM_ENABLE
  34         help
  35           This option provides enhanced support for discontiguous
  36           memory systems, over FLATMEM.  These systems have holes
  37           in their physical address spaces, and this option provides
  38           more efficient handling of these holes.  However, the vast
  39           majority of hardware has quite flat address spaces, and
  40           can have degraded performance from the extra overhead that
  41           this option imposes.
  42
  43           Many NUMA configurations will have this as the only option.
  44
  45           If unsure, choose "Flat Memory" over this option.
  46
  47 config SPARSEMEM_MANUAL
  48         bool "Sparse Memory"
  49         depends on ARCH_SPARSEMEM_ENABLE
  50         help
  51           This will be the only option for some systems, including
  52           memory hotplug systems.  This is normal.
  53
  54           For many other systems, this will be an alternative to
  55           "Discontiguous Memory".  This option provides some potential
  56           performance benefits, along with decreased code complexity,
  57           but it is newer, and more experimental.
  58
  59           If unsure, choose "Discontiguous Memory" or "Flat Memory"
  60           over this option.
  61
  62 endchoice
  63
  64 config DISCONTIGMEM
  65         def_bool y
  66         depends on (!SELECT_MEMORY_MODEL && ARCH_DISCONTIGMEM_ENABLE) || DISCONTIGMEM_MANUAL
  67
  68 config SPARSEMEM
  69         def_bool y
  70         depends on (!SELECT_MEMORY_MODEL && ARCH_SPARSEMEM_ENABLE) || SPARSEMEM_MANUAL
  71
  72 config FLATMEM
  73         def_bool y
  74         depends on (!DISCONTIGMEM && !SPARSEMEM) || FLATMEM_MANUAL
  75
  76 config FLAT_NODE_MEM_MAP
  77         def_bool y
  78         depends on !SPARSEMEM
  79
  80 #
  81 # Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
  82 # to represent different areas of memory.  This variable allows
  83 # those dependencies to exist individually.
  84 #
  85 config NEED_MULTIPLE_NODES
  86         def_bool y
  87         depends on DISCONTIGMEM || NUMA
  88
  89 config HAVE_MEMORY_PRESENT
  90         def_bool y
  91         depends on ARCH_HAVE_MEMORY_PRESENT || SPARSEMEM
  92
  93 #
  94 # SPARSEMEM_EXTREME (which is the default) does some bootmem
  95 # allocations when memory_present() is called.  If this cannot
  96 # be done on your architecture, select this option.  However,
  97 # statically allocating the mem_section[] array can potentially
  98 # consume vast quantities of .bss, so be careful.
  99 #
 100 # This option will also potentially produce smaller runtime code
 101 # with gcc 3.4 and later.
 102 #
 103 config SPARSEMEM_STATIC
 104         bool
 105
 106 #
 107 # Architecture platforms which require a two level mem_section in SPARSEMEM
 108 # must select this option. This is usually for architecture platforms with
 109 # an extremely sparse physical address space.
 110 #
 111 config SPARSEMEM_EXTREME
 112         def_bool y
 113         depends on SPARSEMEM && !SPARSEMEM_STATIC
 114
 115 config SPARSEMEM_VMEMMAP_ENABLE
 116         bool
 117
 118 config SPARSEMEM_ALLOC_MEM_MAP_TOGETHER
 119         def_bool y
 120         depends on SPARSEMEM && X86_64
 121
 122 config SPARSEMEM_VMEMMAP
 123         bool "Sparse Memory virtual memmap"
 124         depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
 125         default y
 126         help
 127          SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
 128          pfn_to_page and page_to_pfn operations.  This is the most
 129          efficient option when sufficient kernel resources are available.
 130
 131 config HAVE_MEMBLOCK
 132         bool
 133
 134 config HAVE_MEMBLOCK_NODE_MAP
 135         bool
 136
 137 config HAVE_MEMBLOCK_PHYS_MAP
 138         bool
 139
 140 config HAVE_GENERIC_RCU_GUP
 141         bool
 142
 143 config ARCH_DISCARD_MEMBLOCK
 144         bool
 145
 146 config NO_BOOTMEM
 147         bool
 148
 149 config MEMORY_ISOLATION
 150         bool
 151
 152 config MOVABLE_NODE
 153         bool "Enable to assign a node which has only movable memory"
 154         depends on HAVE_MEMBLOCK
 155         depends on NO_BOOTMEM
 156         depends on X86_64
 157         depends on NUMA
 158         default n
 159         help
 160           Allow a node to have only movable memory.  Pages used by the kernel,
 161           such as direct mapping pages cannot be migrated.  So the corresponding
 162           memory device cannot be hotplugged.  This option allows the following
 163           two things:
 164           - When the system is booting, node full of hotpluggable memory can
 165           be arranged to have only movable memory so that the whole node can
 166           be hot-removed. (need movable_node boot option specified).
 167           - After the system is up, the option allows users to online all the
 168           memory of a node as movable memory so that the whole node can be
 169           hot-removed.
 170
 171           Users who don't use the memory hotplug feature are fine with this
 172           option on since they don't specify movable_node boot option or they
 173           don't online memory as movable.
 174
 175           Say Y here if you want to hotplug a whole node.
 176           Say N here if you want kernel to use memory on all nodes evenly.
 177
 178 #
 179 # Only be set on architectures that have completely implemented memory hotplug
 180 # feature. If you are not sure, don't touch it.
 181 #
 182 config HAVE_BOOTMEM_INFO_NODE
 183         def_bool n
 184
 185 # eventually, we can have this option just 'select SPARSEMEM'
 186 config MEMORY_HOTPLUG
 187         bool "Allow for memory hot-add"
 188         depends on SPARSEMEM || X86_64_ACPI_NUMA
 189         depends on ARCH_ENABLE_MEMORY_HOTPLUG
 190         depends on (IA64 || X86 || PPC_BOOK3S_64 || SUPERH || S390)
 191
 192 config MEMORY_HOTPLUG_SPARSE
 193         def_bool y
 194         depends on SPARSEMEM && MEMORY_HOTPLUG
 195
 196 config MEMORY_HOTREMOVE
 197         bool "Allow for memory hot remove"
 198         select MEMORY_ISOLATION
 199         select HAVE_BOOTMEM_INFO_NODE if (X86_64 || PPC64)
 200         depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
 201         depends on MIGRATION
 202
 203 #
 204 # If we have space for more page flags then we can enable additional
 205 # optimizations and functionality.
 206 #
 207 # Regular Sparsemem takes page flag bits for the sectionid if it does not
 208 # use a virtual memmap. Disable extended page flags for 32 bit platforms
 209 # that require the use of a sectionid in the page flags.
 210 #
 211 config PAGEFLAGS_EXTENDED
 212         def_bool y
 213         depends on 64BIT || SPARSEMEM_VMEMMAP || !SPARSEMEM
 214
 215 # Heavily threaded applications may benefit from splitting the mm-wide
 216 # page_table_lock, so that faults on different parts of the user address
 217 # space can be handled with less contention: split it at this NR_CPUS.
 218 # Default to 4 for wider testing, though 8 might be more appropriate.
 219 # ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
 220 # PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
 221 # DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page.
 222 #
 223 config SPLIT_PTLOCK_CPUS
 224         int
 225         default "999999" if !MMU
 226         default "999999" if ARM && !CPU_CACHE_VIPT
 227         default "999999" if PARISC && !PA20
 228         default "4"
 229
 230 config ARCH_ENABLE_SPLIT_PMD_PTLOCK
 231         bool
 232
 233 #
 234 # support for memory balloon
 235 config MEMORY_BALLOON
 236         bool
 237
 238 #
 239 # support for memory balloon compaction
 240 config BALLOON_COMPACTION
 241         bool "Allow for balloon memory compaction/migration"
 242         def_bool y
 243         depends on COMPACTION && MEMORY_BALLOON
 244         help
 245           Memory fragmentation introduced by ballooning might reduce
 246           significantly the number of 2MB contiguous memory blocks that can be
 247           used within a guest, thus imposing performance penalties associated
 248           with the reduced number of transparent huge pages that could be used
 249           by the guest workload. Allowing the compaction & migration for memory
 250           pages enlisted as being part of memory balloon devices avoids the
 251           scenario aforementioned and helps improving memory defragmentation.
 252
 253 #
 254 # support for memory compaction
 255 config COMPACTION
 256         bool "Allow for memory compaction"
 257         def_bool y
 258         select MIGRATION
 259         depends on MMU
 260         help
 261           Allows the compaction of memory for the allocation of huge pages.
 262
 263 #
 264 # support for page migration
 265 #
 266 config MIGRATION
 267         bool "Page migration"
 268         def_bool y
 269         depends on (NUMA || ARCH_ENABLE_MEMORY_HOTREMOVE || COMPACTION || CMA) && MMU
 270         help
 271           Allows the migration of the physical location of pages of processes
 272           while the virtual addresses are not changed. This is useful in
 273           two situations. The first is on NUMA systems to put pages nearer
 274           to the processors accessing. The second is when allocating huge
 275           pages as migration can relocate pages to satisfy a huge page
 276           allocation instead of reclaiming.
 277
 278 config ARCH_ENABLE_HUGEPAGE_MIGRATION
 279         bool
 280
 281 config PHYS_ADDR_T_64BIT
 282         def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT
 283
 284 config ZONE_DMA_FLAG
 285         int
 286         default "0" if !ZONE_DMA
 287         default "1"
 288
 289 config BOUNCE
 290         bool "Enable bounce buffers"
 291         default y
 292         depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
 293         help
 294           Enable bounce buffers for devices that cannot access
 295           the full range of memory available to the CPU. Enabled
 296           by default when ZONE_DMA or HIGHMEM is selected, but you
 297           may say n to override this.
 298
 299 # On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
 300 # have more than 4GB of memory, but we don't currently use the IOTLB to present
 301 # a 32-bit address to OHCI.  So we need to use a bounce pool instead.
 302 #
 303 # We also use the bounce pool to provide stable page writes for jbd.  jbd
 304 # initiates buffer writeback without locking the page or setting PG_writeback,
 305 # and fixing that behavior (a second time; jbd2 doesn't have this problem) is
 306 # a major rework effort.  Instead, use the bounce buffer to snapshot pages
 307 # (until jbd goes away).  The only jbd user is ext3.
 308 config NEED_BOUNCE_POOL
 309         bool
 310         default y if (TILE && USB_OHCI_HCD) || (BLK_DEV_INTEGRITY && JBD)
 311
 312 config NR_QUICK
 313         int
 314         depends on QUICKLIST
 315         default "2" if AVR32
 316         default "1"
 317
 318 config VIRT_TO_BUS
 319         bool
 320         help
 321           An architecture should select this if it implements the
 322           deprecated interface virt_to_bus().  All new architectures
 323           should probably not select this.
 324
 325
 326 config MMU_NOTIFIER
 327         bool
 328         select SRCU
 329
 330 config KSM
 331         bool "Enable KSM for page merging"
 332         depends on MMU
 333         help
 334           Enable Kernel Samepage Merging: KSM periodically scans those areas
 335           of an application's address space that an app has advised may be
 336           mergeable.  When it finds pages of identical content, it replaces
 337           the many instances by a single page with that content, so
 338           saving memory until one or another app needs to modify the content.
 339           Recommended for use with KVM, or with other duplicative applications.
 340           See Documentation/vm/ksm.txt for more information: KSM is inactive
 341           until a program has madvised that an area is MADV_MERGEABLE, and
 342           root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).
 343
 344 config DEFAULT_MMAP_MIN_ADDR
 345         int "Low address space to protect from user allocation"
 346         depends on MMU
 347         default 4096
 348         help
 349           This is the portion of low virtual memory which should be protected
 350           from userspace allocation.  Keeping a user from writing to low pages
 351           can help reduce the impact of kernel NULL pointer bugs.
 352
 353           For most ia64, ppc64 and x86 users with lots of address space
 354           a value of 65536 is reasonable and should cause no problems.
 355           On arm and other archs it should not be higher than 32768.
 356           Programs which use vm86 functionality or have some need to map
 357           this low address space will need CAP_SYS_RAWIO or disable this
 358           protection by setting the value to 0.
 359
 360           This value can be changed after boot using the
 361           /proc/sys/vm/mmap_min_addr tunable.
 362
 363 config ARCH_SUPPORTS_MEMORY_FAILURE
 364         bool
 365
 366 config MEMORY_FAILURE
 367         depends on MMU
 368         depends on ARCH_SUPPORTS_MEMORY_FAILURE
 369         bool "Enable recovery from hardware memory errors"
 370         select MEMORY_ISOLATION
 371         help
 372           Enables code to recover from some memory failures on systems
 373           with MCA recovery. This allows a system to continue running
 374           even when some of its memory has uncorrected errors. This requires
 375           special hardware support and typically ECC memory.
 376
 377 config HWPOISON_INJECT
 378         tristate "HWPoison pages injector"
 379         depends on MEMORY_FAILURE && DEBUG_KERNEL && PROC_FS
 380         select PROC_PAGE_MONITOR
 381
 382 config NOMMU_INITIAL_TRIM_EXCESS
 383         int "Turn on mmap() excess space trimming before booting"
 384         depends on !MMU
 385         default 1
 386         help
 387           The NOMMU mmap() frequently needs to allocate large contiguous chunks
 388           of memory on which to store mappings, but it can only ask the system
 389           allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently
 390           more than it requires.  To deal with this, mmap() is able to trim off
 391           the excess and return it to the allocator.
 392
 393           If trimming is enabled, the excess is trimmed off and returned to the
 394           system allocator, which can cause extra fragmentation, particularly
 395           if there are a lot of transient processes.
 396
 397           If trimming is disabled, the excess is kept, but not used, which for
 398           long-term mappings means that the space is wasted.
 399
 400           Trimming can be dynamically controlled through a sysctl option
 401           (/proc/sys/vm/nr_trim_pages) which specifies the minimum number of
 402           excess pages there must be before trimming should occur, or zero if
 403           no trimming is to occur.
 404
 405           This option specifies the initial value of this option.  The default
 406           of 1 says that all excess pages should be trimmed.
 407
 408           See Documentation/nommu-mmap.txt for more information.
 409
 410 config TRANSPARENT_HUGEPAGE
 411         bool "Transparent Hugepage Support"
 412         depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
 413         select COMPACTION
 414         help
 415           Transparent Hugepages allows the kernel to use huge pages and
 416           huge tlb transparently to the applications whenever possible.
 417           This feature can improve computing performance to certain
 418           applications by speeding up page faults during memory
 419           allocation, by reducing the number of tlb misses and by speeding
 420           up the pagetable walking.
 421
 422           If memory constrained on embedded, you may want to say N.
 423
 424 choice
 425         prompt "Transparent Hugepage Support sysfs defaults"
 426         depends on TRANSPARENT_HUGEPAGE
 427         default TRANSPARENT_HUGEPAGE_ALWAYS
 428         help
 429           Selects the sysfs defaults for Transparent Hugepage Support.
 430
 431         config TRANSPARENT_HUGEPAGE_ALWAYS
 432                 bool "always"
 433         help
 434           Enabling Transparent Hugepage always, can increase the
 435           memory footprint of applications without a guaranteed
 436           benefit but it will work automatically for all applications.
 437
 438         config TRANSPARENT_HUGEPAGE_MADVISE
 439                 bool "madvise"
 440         help
 441           Enabling Transparent Hugepage madvise, will only provide a
 442           performance improvement benefit to the applications using
 443           madvise(MADV_HUGEPAGE) but it won't risk to increase the
 444           memory footprint of applications without a guaranteed
 445           benefit.
 446 endchoice
 447
 448 #
 449 # UP and nommu archs use km based percpu allocator
 450 #
 451 config NEED_PER_CPU_KM
 452         depends on !SMP
 453         bool
 454         default y
 455
 456 config CLEANCACHE
 457         bool "Enable cleancache driver to cache clean pages if tmem is present"
 458         default n
 459         help
 460           Cleancache can be thought of as a page-granularity victim cache
 461           for clean pages that the kernel's pageframe replacement algorithm
 462           (PFRA) would like to keep around, but can't since there isn't enough
 463           memory.  So when the PFRA "evicts" a page, it first attempts to use
 464           cleancache code to put the data contained in that page into
 465           "transcendent memory", memory that is not directly accessible or
 466           addressable by the kernel and is of unknown and possibly
 467           time-varying size.  And when a cleancache-enabled
 468           filesystem wishes to access a page in a file on disk, it first
 469           checks cleancache to see if it already contains it; if it does,
 470           the page is copied into the kernel and a disk access is avoided.
 471           When a transcendent memory driver is available (such as zcache or
 472           Xen transcendent memory), a significant I/O reduction
 473           may be achieved.  When none is available, all cleancache calls
 474           are reduced to a single pointer-compare-against-NULL resulting
 475           in a negligible performance hit.
 476
 477           If unsure, say Y to enable cleancache
 478
 479 config FRONTSWAP
 480         bool "Enable frontswap to cache swap pages if tmem is present"
 481         depends on SWAP
 482         default n
 483         help
 484           Frontswap is so named because it can be thought of as the opposite
 485           of a "backing" store for a swap device.  The data is stored into
 486           "transcendent memory", memory that is not directly accessible or
 487           addressable by the kernel and is of unknown and possibly
 488           time-varying size.  When space in transcendent memory is available,
 489           a significant swap I/O reduction may be achieved.  When none is
 490           available, all frontswap calls are reduced to a single pointer-
 491           compare-against-NULL resulting in a negligible performance hit
 492           and swap data is stored as normal on the matching swap device.
 493
 494           If unsure, say Y to enable frontswap.
 495
 496 config CMA
 497         bool "Contiguous Memory Allocator"
 498         depends on HAVE_MEMBLOCK && MMU
 499         select MIGRATION
 500         select MEMORY_ISOLATION
 501         help
 502           This enables the Contiguous Memory Allocator which allows other
 503           subsystems to allocate big physically-contiguous blocks of memory.
 504           CMA reserves a region of memory and allows only movable pages to
 505           be allocated from it. This way, the kernel can use the memory for
 506           pagecache and when a subsystem requests for contiguous area, the
 507           allocated pages are migrated away to serve the contiguous request.
 508
 509           If unsure, say "n".
 510
 511 config CMA_DEBUG
 512         bool "CMA debug messages (DEVELOPMENT)"
 513         depends on DEBUG_KERNEL && CMA
 514         help
 515           Turns on debug messages in CMA.  This produces KERN_DEBUG
 516           messages for every CMA call as well as various messages while
 517           processing calls such as dma_alloc_from_contiguous().
 518           This option does not affect warning and error messages.
 519
 520 config CMA_AREAS
 521         int "Maximum count of the CMA areas"
 522         depends on CMA
 523         default 7
 524         help
 525           CMA allows to create CMA areas for particular purpose, mainly,
 526           used as device private area. This parameter sets the maximum
 527           number of CMA area in the system.
 528
 529           If unsure, leave the default value "7".
 530
 531 config MEM_SOFT_DIRTY
 532         bool "Track memory changes"
 533         depends on CHECKPOINT_RESTORE && HAVE_ARCH_SOFT_DIRTY && PROC_FS
 534         select PROC_PAGE_MONITOR
 535         help
 536           This option enables memory changes tracking by introducing a
 537           soft-dirty bit on pte-s. This bit it set when someone writes
 538           into a page just as regular dirty bit, but unlike the latter
 539           it can be cleared by hands.
 540
 541           See Documentation/vm/soft-dirty.txt for more details.
 542
 543 config ZSWAP
 544         bool "Compressed cache for swap pages (EXPERIMENTAL)"
 545         depends on FRONTSWAP && CRYPTO=y
 546         select CRYPTO_LZO
 547         select ZPOOL
 548         default n
 549         help
 550           A lightweight compressed cache for swap pages.  It takes
 551           pages that are in the process of being swapped out and attempts to
 552           compress them into a dynamically allocated RAM-based memory pool.
 553           This can result in a significant I/O reduction on swap device and,
 554           in the case where decompressing from RAM is faster that swap device
 555           reads, can also improve workload performance.
 556
 557           This is marked experimental because it is a new feature (as of
 558           v3.11) that interacts heavily with memory reclaim.  While these
 559           interactions don't cause any known issues on simple memory setups,
 560           they have not be fully explored on the large set of potential
 561           configurations and workloads that exist.
 562
 563 config ZPOOL
 564         tristate "Common API for compressed memory storage"
 565         default n
 566         help
 567           Compressed memory storage API.  This allows using either zbud or
 568           zsmalloc.
 569
 570 config ZBUD
 571         tristate "Low density storage for compressed pages"
 572         default n
 573         help
 574           A special purpose allocator for storing compressed pages.
 575           It is designed to store up to two compressed pages per physical
 576           page.  While this design limits storage density, it has simple and
 577           deterministic reclaim properties that make it preferable to a higher
 578           density approach when reclaim will be used.
 579
 580 config ZSMALLOC
 581         tristate "Memory allocator for compressed pages"
 582         depends on MMU
 583         default n
 584         help
 585           zsmalloc is a slab-based memory allocator designed to store
 586           compressed RAM pages.  zsmalloc uses virtual memory mapping
 587           in order to reduce fragmentation.  However, this results in a
 588           non-standard allocator interface where a handle, not a pointer, is
 589           returned by an alloc().  This handle must be mapped in order to
 590           access the allocated space.
 591
 592 config PGTABLE_MAPPING
 593         bool "Use page table mapping to access object in zsmalloc"
 594         depends on ZSMALLOC
 595         help
 596           By default, zsmalloc uses a copy-based object mapping method to
 597           access allocations that span two pages. However, if a particular
 598           architecture (ex, ARM) performs VM mapping faster than copying,
 599           then you should select this. This causes zsmalloc to use page table
 600           mapping rather than copying for object mapping.
 601
 602           You can check speed with zsmalloc benchmark:
 603           https://github.com/spartacus06/zsmapbench
 604
 605 config ZSMALLOC_STAT
 606         bool "Export zsmalloc statistics"
 607         depends on ZSMALLOC
 608         select DEBUG_FS
 609         help
 610           This option enables code in the zsmalloc to collect various
 611           statistics about whats happening in zsmalloc and exports that
 612           information to userspace via debugfs.
 613           If unsure, say N.
 614
 615 config GENERIC_EARLY_IOREMAP
 616         bool
 617
 618 config MAX_STACK_SIZE_MB
 619         int "Maximum user stack size for 32-bit processes (MB)"
 620         default 80
 621         range 8 256 if METAG
 622         range 8 2048
 623         depends on STACK_GROWSUP && (!64BIT || COMPAT)
 624         help
 625           This is the maximum stack size in Megabytes in the VM layout of 32-bit
 626           user processes when the stack grows upwards (currently only on parisc
 627           and metag arch). The stack will be located at the highest memory
 628           address minus the given value, unless the RLIMIT_STACK hard limit is
 629           changed to a smaller value in which case that is used.
 630
 631           A sane initial value is 80 MB.