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_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 #
 153 # Only be set on architectures that have completely implemented memory hotplug
 154 # feature. If you are not sure, don't touch it.
 155 #
 156 config HAVE_BOOTMEM_INFO_NODE
 157         def_bool n
 158
 159 # eventually, we can have this option just 'select SPARSEMEM'
 160 config MEMORY_HOTPLUG
 161         bool "Allow for memory hot-add"
 162         depends on SPARSEMEM || X86_64_ACPI_NUMA
 163         depends on ARCH_ENABLE_MEMORY_HOTPLUG
 164
 165 config MEMORY_HOTPLUG_SPARSE
 166         def_bool y
 167         depends on SPARSEMEM && MEMORY_HOTPLUG
 168
 169 config MEMORY_HOTPLUG_DEFAULT_ONLINE
 170         bool "Online the newly added memory blocks by default"
 171         default n
 172         depends on MEMORY_HOTPLUG
 173         help
 174           This option sets the default policy setting for memory hotplug
 175           onlining policy (/sys/devices/system/memory/auto_online_blocks) which
 176           determines what happens to newly added memory regions. Policy setting
 177           can always be changed at runtime.
 178           See Documentation/memory-hotplug.txt for more information.
 179
 180           Say Y here if you want all hot-plugged memory blocks to appear in
 181           'online' state by default.
 182           Say N here if you want the default policy to keep all hot-plugged
 183           memory blocks in 'offline' state.
 184
 185 config MEMORY_HOTREMOVE
 186         bool "Allow for memory hot remove"
 187         select MEMORY_ISOLATION
 188         select HAVE_BOOTMEM_INFO_NODE if (X86_64 || PPC64)
 189         depends on MEMORY_HOTPLUG && ARCH_ENABLE_MEMORY_HOTREMOVE
 190         depends on MIGRATION
 191
 192 # Heavily threaded applications may benefit from splitting the mm-wide
 193 # page_table_lock, so that faults on different parts of the user address
 194 # space can be handled with less contention: split it at this NR_CPUS.
 195 # Default to 4 for wider testing, though 8 might be more appropriate.
 196 # ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
 197 # PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
 198 # DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page.
 199 #
 200 config SPLIT_PTLOCK_CPUS
 201         int
 202         default "999999" if !MMU
 203         default "999999" if ARM && !CPU_CACHE_VIPT
 204         default "999999" if PARISC && !PA20
 205         default "4"
 206
 207 config ARCH_ENABLE_SPLIT_PMD_PTLOCK
 208         bool
 209
 210 #
 211 # support for memory balloon
 212 config MEMORY_BALLOON
 213         bool
 214
 215 #
 216 # support for memory balloon compaction
 217 config BALLOON_COMPACTION
 218         bool "Allow for balloon memory compaction/migration"
 219         def_bool y
 220         depends on COMPACTION && MEMORY_BALLOON
 221         help
 222           Memory fragmentation introduced by ballooning might reduce
 223           significantly the number of 2MB contiguous memory blocks that can be
 224           used within a guest, thus imposing performance penalties associated
 225           with the reduced number of transparent huge pages that could be used
 226           by the guest workload. Allowing the compaction & migration for memory
 227           pages enlisted as being part of memory balloon devices avoids the
 228           scenario aforementioned and helps improving memory defragmentation.
 229
 230 #
 231 # support for memory compaction
 232 config COMPACTION
 233         bool "Allow for memory compaction"
 234         def_bool y
 235         select MIGRATION
 236         depends on MMU
 237         help
 238           Compaction is the only memory management component to form
 239           high order (larger physically contiguous) memory blocks
 240           reliably. The page allocator relies on compaction heavily and
 241           the lack of the feature can lead to unexpected OOM killer
 242           invocations for high order memory requests. You shouldn't
 243           disable this option unless there really is a strong reason for
 244           it and then we would be really interested to hear about that at
 245           linux-mm@kvack.org.
 246
 247 #
 248 # support for page migration
 249 #
 250 config MIGRATION
 251         bool "Page migration"
 252         def_bool y
 253         depends on (NUMA || ARCH_ENABLE_MEMORY_HOTREMOVE || COMPACTION || CMA) && MMU
 254         help
 255           Allows the migration of the physical location of pages of processes
 256           while the virtual addresses are not changed. This is useful in
 257           two situations. The first is on NUMA systems to put pages nearer
 258           to the processors accessing. The second is when allocating huge
 259           pages as migration can relocate pages to satisfy a huge page
 260           allocation instead of reclaiming.
 261
 262 config ARCH_ENABLE_HUGEPAGE_MIGRATION
 263         bool
 264
 265 config ARCH_ENABLE_THP_MIGRATION
 266         bool
 267
 268 config PHYS_ADDR_T_64BIT
 269         def_bool 64BIT
 270
 271 config BOUNCE
 272         bool "Enable bounce buffers"
 273         default y
 274         depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
 275         help
 276           Enable bounce buffers for devices that cannot access
 277           the full range of memory available to the CPU. Enabled
 278           by default when ZONE_DMA or HIGHMEM is selected, but you
 279           may say n to override this.
 280
 281 config NR_QUICK
 282         int
 283         depends on QUICKLIST
 284         default "1"
 285
 286 config VIRT_TO_BUS
 287         bool
 288         help
 289           An architecture should select this if it implements the
 290           deprecated interface virt_to_bus().  All new architectures
 291           should probably not select this.
 292
 293
 294 config MMU_NOTIFIER
 295         bool
 296         select SRCU
 297
 298 config KSM
 299         bool "Enable KSM for page merging"
 300         depends on MMU
 301         help
 302           Enable Kernel Samepage Merging: KSM periodically scans those areas
 303           of an application's address space that an app has advised may be
 304           mergeable.  When it finds pages of identical content, it replaces
 305           the many instances by a single page with that content, so
 306           saving memory until one or another app needs to modify the content.
 307           Recommended for use with KVM, or with other duplicative applications.
 308           See Documentation/vm/ksm.rst for more information: KSM is inactive
 309           until a program has madvised that an area is MADV_MERGEABLE, and
 310           root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).
 311
 312 config DEFAULT_MMAP_MIN_ADDR
 313         int "Low address space to protect from user allocation"
 314         depends on MMU
 315         default 4096
 316         help
 317           This is the portion of low virtual memory which should be protected
 318           from userspace allocation.  Keeping a user from writing to low pages
 319           can help reduce the impact of kernel NULL pointer bugs.
 320
 321           For most ia64, ppc64 and x86 users with lots of address space
 322           a value of 65536 is reasonable and should cause no problems.
 323           On arm and other archs it should not be higher than 32768.
 324           Programs which use vm86 functionality or have some need to map
 325           this low address space will need CAP_SYS_RAWIO or disable this
 326           protection by setting the value to 0.
 327
 328           This value can be changed after boot using the
 329           /proc/sys/vm/mmap_min_addr tunable.
 330
 331 config ARCH_SUPPORTS_MEMORY_FAILURE
 332         bool
 333
 334 config MEMORY_FAILURE
 335         depends on MMU
 336         depends on ARCH_SUPPORTS_MEMORY_FAILURE
 337         bool "Enable recovery from hardware memory errors"
 338         select MEMORY_ISOLATION
 339         select RAS
 340         help
 341           Enables code to recover from some memory failures on systems
 342           with MCA recovery. This allows a system to continue running
 343           even when some of its memory has uncorrected errors. This requires
 344           special hardware support and typically ECC memory.
 345
 346 config HWPOISON_INJECT
 347         tristate "HWPoison pages injector"
 348         depends on MEMORY_FAILURE && DEBUG_KERNEL && PROC_FS
 349         select PROC_PAGE_MONITOR
 350
 351 config NOMMU_INITIAL_TRIM_EXCESS
 352         int "Turn on mmap() excess space trimming before booting"
 353         depends on !MMU
 354         default 1
 355         help
 356           The NOMMU mmap() frequently needs to allocate large contiguous chunks
 357           of memory on which to store mappings, but it can only ask the system
 358           allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently
 359           more than it requires.  To deal with this, mmap() is able to trim off
 360           the excess and return it to the allocator.
 361
 362           If trimming is enabled, the excess is trimmed off and returned to the
 363           system allocator, which can cause extra fragmentation, particularly
 364           if there are a lot of transient processes.
 365
 366           If trimming is disabled, the excess is kept, but not used, which for
 367           long-term mappings means that the space is wasted.
 368
 369           Trimming can be dynamically controlled through a sysctl option
 370           (/proc/sys/vm/nr_trim_pages) which specifies the minimum number of
 371           excess pages there must be before trimming should occur, or zero if
 372           no trimming is to occur.
 373
 374           This option specifies the initial value of this option.  The default
 375           of 1 says that all excess pages should be trimmed.
 376
 377           See Documentation/nommu-mmap.txt for more information.
 378
 379 config TRANSPARENT_HUGEPAGE
 380         bool "Transparent Hugepage Support"
 381         depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
 382         select COMPACTION
 383         select RADIX_TREE_MULTIORDER
 384         help
 385           Transparent Hugepages allows the kernel to use huge pages and
 386           huge tlb transparently to the applications whenever possible.
 387           This feature can improve computing performance to certain
 388           applications by speeding up page faults during memory
 389           allocation, by reducing the number of tlb misses and by speeding
 390           up the pagetable walking.
 391
 392           If memory constrained on embedded, you may want to say N.
 393
 394 choice
 395         prompt "Transparent Hugepage Support sysfs defaults"
 396         depends on TRANSPARENT_HUGEPAGE
 397         default TRANSPARENT_HUGEPAGE_ALWAYS
 398         help
 399           Selects the sysfs defaults for Transparent Hugepage Support.
 400
 401         config TRANSPARENT_HUGEPAGE_ALWAYS
 402                 bool "always"
 403         help
 404           Enabling Transparent Hugepage always, can increase the
 405           memory footprint of applications without a guaranteed
 406           benefit but it will work automatically for all applications.
 407
 408         config TRANSPARENT_HUGEPAGE_MADVISE
 409                 bool "madvise"
 410         help
 411           Enabling Transparent Hugepage madvise, will only provide a
 412           performance improvement benefit to the applications using
 413           madvise(MADV_HUGEPAGE) but it won't risk to increase the
 414           memory footprint of applications without a guaranteed
 415           benefit.
 416 endchoice
 417
 418 config ARCH_WANTS_THP_SWAP
 419        def_bool n
 420
 421 config THP_SWAP
 422         def_bool y
 423         depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP
 424         help
 425           Swap transparent huge pages in one piece, without splitting.
 426           XXX: For now this only does clustered swap space allocation.
 427
 428           For selection by architectures with reasonable THP sizes.
 429
 430 config  TRANSPARENT_HUGE_PAGECACHE
 431         def_bool y
 432         depends on TRANSPARENT_HUGEPAGE
 433
 434 #
 435 # UP and nommu archs use km based percpu allocator
 436 #
 437 config NEED_PER_CPU_KM
 438         depends on !SMP
 439         bool
 440         default y
 441
 442 config CLEANCACHE
 443         bool "Enable cleancache driver to cache clean pages if tmem is present"
 444         default n
 445         help
 446           Cleancache can be thought of as a page-granularity victim cache
 447           for clean pages that the kernel's pageframe replacement algorithm
 448           (PFRA) would like to keep around, but can't since there isn't enough
 449           memory.  So when the PFRA "evicts" a page, it first attempts to use
 450           cleancache code to put the data contained in that page into
 451           "transcendent memory", memory that is not directly accessible or
 452           addressable by the kernel and is of unknown and possibly
 453           time-varying size.  And when a cleancache-enabled
 454           filesystem wishes to access a page in a file on disk, it first
 455           checks cleancache to see if it already contains it; if it does,
 456           the page is copied into the kernel and a disk access is avoided.
 457           When a transcendent memory driver is available (such as zcache or
 458           Xen transcendent memory), a significant I/O reduction
 459           may be achieved.  When none is available, all cleancache calls
 460           are reduced to a single pointer-compare-against-NULL resulting
 461           in a negligible performance hit.
 462
 463           If unsure, say Y to enable cleancache
 464
 465 config FRONTSWAP
 466         bool "Enable frontswap to cache swap pages if tmem is present"
 467         depends on SWAP
 468         default n
 469         help
 470           Frontswap is so named because it can be thought of as the opposite
 471           of a "backing" store for a swap device.  The data is stored into
 472           "transcendent memory", memory that is not directly accessible or
 473           addressable by the kernel and is of unknown and possibly
 474           time-varying size.  When space in transcendent memory is available,
 475           a significant swap I/O reduction may be achieved.  When none is
 476           available, all frontswap calls are reduced to a single pointer-
 477           compare-against-NULL resulting in a negligible performance hit
 478           and swap data is stored as normal on the matching swap device.
 479
 480           If unsure, say Y to enable frontswap.
 481
 482 config CMA
 483         bool "Contiguous Memory Allocator"
 484         depends on HAVE_MEMBLOCK && MMU
 485         select MIGRATION
 486         select MEMORY_ISOLATION
 487         help
 488           This enables the Contiguous Memory Allocator which allows other
 489           subsystems to allocate big physically-contiguous blocks of memory.
 490           CMA reserves a region of memory and allows only movable pages to
 491           be allocated from it. This way, the kernel can use the memory for
 492           pagecache and when a subsystem requests for contiguous area, the
 493           allocated pages are migrated away to serve the contiguous request.
 494
 495           If unsure, say "n".
 496
 497 config CMA_DEBUG
 498         bool "CMA debug messages (DEVELOPMENT)"
 499         depends on DEBUG_KERNEL && CMA
 500         help
 501           Turns on debug messages in CMA.  This produces KERN_DEBUG
 502           messages for every CMA call as well as various messages while
 503           processing calls such as dma_alloc_from_contiguous().
 504           This option does not affect warning and error messages.
 505
 506 config CMA_DEBUGFS
 507         bool "CMA debugfs interface"
 508         depends on CMA && DEBUG_FS
 509         help
 510           Turns on the DebugFS interface for CMA.
 511
 512 config CMA_AREAS
 513         int "Maximum count of the CMA areas"
 514         depends on CMA
 515         default 7
 516         help
 517           CMA allows to create CMA areas for particular purpose, mainly,
 518           used as device private area. This parameter sets the maximum
 519           number of CMA area in the system.
 520
 521           If unsure, leave the default value "7".
 522
 523 config MEM_SOFT_DIRTY
 524         bool "Track memory changes"
 525         depends on CHECKPOINT_RESTORE && HAVE_ARCH_SOFT_DIRTY && PROC_FS
 526         select PROC_PAGE_MONITOR
 527         help
 528           This option enables memory changes tracking by introducing a
 529           soft-dirty bit on pte-s. This bit it set when someone writes
 530           into a page just as regular dirty bit, but unlike the latter
 531           it can be cleared by hands.
 532
 533           See Documentation/admin-guide/mm/soft-dirty.rst for more details.
 534
 535 config ZSWAP
 536         bool "Compressed cache for swap pages (EXPERIMENTAL)"
 537         depends on FRONTSWAP && CRYPTO=y
 538         select CRYPTO_LZO
 539         select ZPOOL
 540         default n
 541         help
 542           A lightweight compressed cache for swap pages.  It takes
 543           pages that are in the process of being swapped out and attempts to
 544           compress them into a dynamically allocated RAM-based memory pool.
 545           This can result in a significant I/O reduction on swap device and,
 546           in the case where decompressing from RAM is faster that swap device
 547           reads, can also improve workload performance.
 548
 549           This is marked experimental because it is a new feature (as of
 550           v3.11) that interacts heavily with memory reclaim.  While these
 551           interactions don't cause any known issues on simple memory setups,
 552           they have not be fully explored on the large set of potential
 553           configurations and workloads that exist.
 554
 555 config ZPOOL
 556         tristate "Common API for compressed memory storage"
 557         default n
 558         help
 559           Compressed memory storage API.  This allows using either zbud or
 560           zsmalloc.
 561
 562 config ZBUD
 563         tristate "Low (Up to 2x) density storage for compressed pages"
 564         default n
 565         help
 566           A special purpose allocator for storing compressed pages.
 567           It is designed to store up to two compressed pages per physical
 568           page.  While this design limits storage density, it has simple and
 569           deterministic reclaim properties that make it preferable to a higher
 570           density approach when reclaim will be used.
 571
 572 config Z3FOLD
 573         tristate "Up to 3x density storage for compressed pages"
 574         depends on ZPOOL
 575         default n
 576         help
 577           A special purpose allocator for storing compressed pages.
 578           It is designed to store up to three compressed pages per physical
 579           page. It is a ZBUD derivative so the simplicity and determinism are
 580           still there.
 581
 582 config ZSMALLOC
 583         tristate "Memory allocator for compressed pages"
 584         depends on MMU
 585         default n
 586         help
 587           zsmalloc is a slab-based memory allocator designed to store
 588           compressed RAM pages.  zsmalloc uses virtual memory mapping
 589           in order to reduce fragmentation.  However, this results in a
 590           non-standard allocator interface where a handle, not a pointer, is
 591           returned by an alloc().  This handle must be mapped in order to
 592           access the allocated space.
 593
 594 config PGTABLE_MAPPING
 595         bool "Use page table mapping to access object in zsmalloc"
 596         depends on ZSMALLOC
 597         help
 598           By default, zsmalloc uses a copy-based object mapping method to
 599           access allocations that span two pages. However, if a particular
 600           architecture (ex, ARM) performs VM mapping faster than copying,
 601           then you should select this. This causes zsmalloc to use page table
 602           mapping rather than copying for object mapping.
 603
 604           You can check speed with zsmalloc benchmark:
 605           https://github.com/spartacus06/zsmapbench
 606
 607 config ZSMALLOC_STAT
 608         bool "Export zsmalloc statistics"
 609         depends on ZSMALLOC
 610         select DEBUG_FS
 611         help
 612           This option enables code in the zsmalloc to collect various
 613           statistics about whats happening in zsmalloc and exports that
 614           information to userspace via debugfs.
 615           If unsure, say N.
 616
 617 config GENERIC_EARLY_IOREMAP
 618         bool
 619
 620 config MAX_STACK_SIZE_MB
 621         int "Maximum user stack size for 32-bit processes (MB)"
 622         default 80
 623         range 8 2048
 624         depends on STACK_GROWSUP && (!64BIT || COMPAT)
 625         help
 626           This is the maximum stack size in Megabytes in the VM layout of 32-bit
 627           user processes when the stack grows upwards (currently only on parisc
 628           arch). The stack will be located at the highest memory address minus
 629           the given value, unless the RLIMIT_STACK hard limit is changed to a
 630           smaller value in which case that is used.
 631
 632           A sane initial value is 80 MB.
 633
 634 config DEFERRED_STRUCT_PAGE_INIT
 635         bool "Defer initialisation of struct pages to kthreads"
 636         default n
 637         depends on NO_BOOTMEM
 638         depends on !FLATMEM
 639         depends on !NEED_PER_CPU_KM
 640         help
 641           Ordinarily all struct pages are initialised during early boot in a
 642           single thread. On very large machines this can take a considerable
 643           amount of time. If this option is set, large machines will bring up
 644           a subset of memmap at boot and then initialise the rest in parallel
 645           by starting one-off "pgdatinitX" kernel thread for each node X. This
 646           has a potential performance impact on processes running early in the
 647           lifetime of the system until these kthreads finish the
 648           initialisation.
 649
 650 config IDLE_PAGE_TRACKING
 651         bool "Enable idle page tracking"
 652         depends on SYSFS && MMU
 653         select PAGE_EXTENSION if !64BIT
 654         help
 655           This feature allows to estimate the amount of user pages that have
 656           not been touched during a given period of time. This information can
 657           be useful to tune memory cgroup limits and/or for job placement
 658           within a compute cluster.
 659
 660           See Documentation/admin-guide/mm/idle_page_tracking.rst for
 661           more details.
 662
 663 # arch_add_memory() comprehends device memory
 664 config ARCH_HAS_ZONE_DEVICE
 665         bool
 666
 667 config ZONE_DEVICE
 668         bool "Device memory (pmem, HMM, etc...) hotplug support"
 669         depends on MEMORY_HOTPLUG
 670         depends on MEMORY_HOTREMOVE
 671         depends on SPARSEMEM_VMEMMAP
 672         depends on ARCH_HAS_ZONE_DEVICE
 673         select RADIX_TREE_MULTIORDER
 674
 675         help
 676           Device memory hotplug support allows for establishing pmem,
 677           or other device driver discovered memory regions, in the
 678           memmap. This allows pfn_to_page() lookups of otherwise
 679           "device-physical" addresses which is needed for using a DAX
 680           mapping in an O_DIRECT operation, among other things.
 681
 682           If FS_DAX is enabled, then say Y.
 683
 684 config ARCH_HAS_HMM
 685         bool
 686         default y
 687         depends on (X86_64 || PPC64)
 688         depends on ZONE_DEVICE
 689         depends on MMU && 64BIT
 690         depends on MEMORY_HOTPLUG
 691         depends on MEMORY_HOTREMOVE
 692         depends on SPARSEMEM_VMEMMAP
 693
 694 config MIGRATE_VMA_HELPER
 695         bool
 696
 697 config DEV_PAGEMAP_OPS
 698         bool
 699
 700 config HMM
 701         bool
 702         select MIGRATE_VMA_HELPER
 703
 704 config HMM_MIRROR
 705         bool "HMM mirror CPU page table into a device page table"
 706         depends on ARCH_HAS_HMM
 707         select MMU_NOTIFIER
 708         select HMM
 709         help
 710           Select HMM_MIRROR if you want to mirror range of the CPU page table of a
 711           process into a device page table. Here, mirror means "keep synchronized".
 712           Prerequisites: the device must provide the ability to write-protect its
 713           page tables (at PAGE_SIZE granularity), and must be able to recover from
 714           the resulting potential page faults.
 715
 716 config DEVICE_PRIVATE
 717         bool "Unaddressable device memory (GPU memory, ...)"
 718         depends on ARCH_HAS_HMM
 719         select HMM
 720         select DEV_PAGEMAP_OPS
 721
 722         help
 723           Allows creation of struct pages to represent unaddressable device
 724           memory; i.e., memory that is only accessible from the device (or
 725           group of devices). You likely also want to select HMM_MIRROR.
 726
 727 config DEVICE_PUBLIC
 728         bool "Addressable device memory (like GPU memory)"
 729         depends on ARCH_HAS_HMM
 730         select HMM
 731         select DEV_PAGEMAP_OPS
 732
 733         help
 734           Allows creation of struct pages to represent addressable device
 735           memory; i.e., memory that is accessible from both the device and
 736           the CPU
 737
 738 config FRAME_VECTOR
 739         bool
 740
 741 config ARCH_USES_HIGH_VMA_FLAGS
 742         bool
 743 config ARCH_HAS_PKEYS
 744         bool
 745
 746 config PERCPU_STATS
 747         bool "Collect percpu memory statistics"
 748         default n
 749         help
 750           This feature collects and exposes statistics via debugfs. The
 751           information includes global and per chunk statistics, which can
 752           be used to help understand percpu memory usage.
 753
 754 config GUP_BENCHMARK
 755         bool "Enable infrastructure for get_user_pages_fast() benchmarking"
 756         default n
 757         help
 758           Provides /sys/kernel/debug/gup_benchmark that helps with testing
 759           performance of get_user_pages_fast().
 760
 761           See tools/testing/selftests/vm/gup_benchmark.c
 762
 763 config ARCH_HAS_PTE_SPECIAL
 764         bool