FreeBSD/Linux Kernel Cross Reference
sys/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 (!SELECT_MEMORY_MODEL && ARCH_SPARSEMEM_ENABLE) || 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_ALLOC_MEM_MAP_TOGETHER
           def_bool y
           depends on SPARSEMEM && X86_64
   
   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.
   
   config HAVE_MEMBLOCK
           boolean
   
   config HAVE_MEMBLOCK_NODE_MAP
           boolean
   
   config ARCH_DISCARD_MEMBLOCK
           boolean
   
   config NO_BOOTMEM
           boolean
   
   config MEMORY_ISOLATION
           boolean
   
   config MOVABLE_NODE
           boolean "Enable to assign a node which has only movable memory"
           depends on HAVE_MEMBLOCK
           depends on NO_BOOTMEM
           depends on X86_64
           depends on NUMA
           default n
           help
             Allow a node to have only movable memory.  Pages used by the kernel,
             such as direct mapping pages cannot be migrated.  So the corresponding
             memory device cannot be hotplugged.  This option allows users to
             online all the memory of a node as movable memory so that the whole
             node can be hotplugged.  Users who don't use the memory hotplug
             feature are fine with this option on since they don't online memory
             as movable.
   
             Say Y here if you want to hotplug a whole node.
             Say N here if you want kernel to use memory on all nodes evenly.
   
   # eventually, we can have this option just 'select SPARSEMEM'
   config MEMORY_HOTPLUG
           bool "Allow for memory hot-add"
           select MEMORY_ISOLATION
           depends on SPARSEMEM || X86_64_ACPI_NUMA
           depends on HOTPLUG && ARCH_ENABLE_MEMORY_HOTPLUG
           depends on (IA64 || X86 || PPC_BOOK3S_64 || SUPERH || 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.
   # DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC spinlock_t also enlarge struct page.
   #
   config SPLIT_PTLOCK_CPUS
           int
           default "999999" if ARM && !CPU_CACHE_VIPT
           default "999999" if PARISC && !PA20
           default "999999" if DEBUG_SPINLOCK || DEBUG_LOCK_ALLOC
           default "4"
   
   #
   # support for memory balloon compaction
   config BALLOON_COMPACTION
           bool "Allow for balloon memory compaction/migration"
           def_bool y
           depends on COMPACTION && VIRTIO_BALLOON
           help
             Memory fragmentation introduced by ballooning might reduce
             significantly the number of 2MB contiguous memory blocks that can be
             used within a guest, thus imposing performance penalties associated
             with the reduced number of transparent huge pages that could be used
             by the guest workload. Allowing the compaction & migration for memory
             pages enlisted as being part of memory balloon devices avoids the
             scenario aforementioned and helps improving memory defragmentation.
   
   #
   # support for memory compaction
   config COMPACTION
           bool "Allow for memory compaction"
           def_bool y
           select MIGRATION
           depends on MMU
           help
             Allows the compaction of memory for the allocation of huge pages.
   
   #
   # support for page migration
   #
   config MIGRATION
           bool "Page migration"
           def_bool y
           depends on NUMA || ARCH_ENABLE_MEMORY_HOTREMOVE || COMPACTION || CMA
           help
             Allows the migration of the physical location of pages of processes
             while the virtual addresses are not changed. This is useful in
             two situations. The first is on NUMA systems to put pages nearer
             to the processors accessing. The second is when allocating huge
             pages as migration can relocate pages to satisfy a huge page
             allocation instead of reclaiming.
   
   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 AVR32
           default "1"
   
   config VIRT_TO_BUS
           def_bool y
           depends on !ARCH_NO_VIRT_TO_BUS
   
   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 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.
             See Documentation/vm/ksm.txt for more information: KSM is inactive
             until a program has madvised that an area is MADV_MERGEABLE, and
             root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).
   
   config DEFAULT_MMAP_MIN_ADDR
           int "Low address space to protect from user allocation"
           depends on MMU
           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 ARCH_SUPPORTS_MEMORY_FAILURE
           bool
   
   config MEMORY_FAILURE
           depends on MMU
           depends on ARCH_SUPPORTS_MEMORY_FAILURE
           bool "Enable recovery from hardware memory errors"
           select MEMORY_ISOLATION
           help
             Enables code to recover from some memory failures on systems
             with MCA recovery. This allows a system to continue running
             even when some of its memory has uncorrected errors. This requires
             special hardware support and typically ECC memory.
   
   config HWPOISON_INJECT
           tristate "HWPoison pages injector"
           depends on MEMORY_FAILURE && DEBUG_KERNEL && PROC_FS
           select PROC_PAGE_MONITOR
   
   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.
   
   config TRANSPARENT_HUGEPAGE
           bool "Transparent Hugepage Support"
           depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
           select COMPACTION
           help
             Transparent Hugepages allows the kernel to use huge pages and
             huge tlb transparently to the applications whenever possible.
             This feature can improve computing performance to certain
             applications by speeding up page faults during memory
             allocation, by reducing the number of tlb misses and by speeding
             up the pagetable walking.
   
             If memory constrained on embedded, you may want to say N.
   
   choice
           prompt "Transparent Hugepage Support sysfs defaults"
           depends on TRANSPARENT_HUGEPAGE
           default TRANSPARENT_HUGEPAGE_ALWAYS
           help
             Selects the sysfs defaults for Transparent Hugepage Support.
   
           config TRANSPARENT_HUGEPAGE_ALWAYS
                   bool "always"
           help
             Enabling Transparent Hugepage always, can increase the
             memory footprint of applications without a guaranteed
             benefit but it will work automatically for all applications.
   
           config TRANSPARENT_HUGEPAGE_MADVISE
                   bool "madvise"
           help
             Enabling Transparent Hugepage madvise, will only provide a
             performance improvement benefit to the applications using
             madvise(MADV_HUGEPAGE) but it won't risk to increase the
             memory footprint of applications without a guaranteed
             benefit.
   endchoice
   
   config CROSS_MEMORY_ATTACH
           bool "Cross Memory Support"
           depends on MMU
           default y
           help
             Enabling this option adds the system calls process_vm_readv and
             process_vm_writev which allow a process with the correct privileges
             to directly read from or write to to another process's address space.
             See the man page for more details.
   
   #
   # UP and nommu archs use km based percpu allocator
   #
   config NEED_PER_CPU_KM
           depends on !SMP
           bool
           default y
   
   config CLEANCACHE
           bool "Enable cleancache driver to cache clean pages if tmem is present"
           default n
           help
             Cleancache can be thought of as a page-granularity victim cache
             for clean pages that the kernel's pageframe replacement algorithm
             (PFRA) would like to keep around, but can't since there isn't enough
             memory.  So when the PFRA "evicts" a page, it first attempts to use
             cleancache code to put the data contained in that page into
             "transcendent memory", memory that is not directly accessible or
             addressable by the kernel and is of unknown and possibly
             time-varying size.  And when a cleancache-enabled
             filesystem wishes to access a page in a file on disk, it first
             checks cleancache to see if it already contains it; if it does,
             the page is copied into the kernel and a disk access is avoided.
             When a transcendent memory driver is available (such as zcache or
             Xen transcendent memory), a significant I/O reduction
             may be achieved.  When none is available, all cleancache calls
             are reduced to a single pointer-compare-against-NULL resulting
             in a negligible performance hit.
   
             If unsure, say Y to enable cleancache
   
   config FRONTSWAP
           bool "Enable frontswap to cache swap pages if tmem is present"
           depends on SWAP
           default n
           help
             Frontswap is so named because it can be thought of as the opposite
             of a "backing" store for a swap device.  The data is stored into
             "transcendent memory", memory that is not directly accessible or
             addressable by the kernel and is of unknown and possibly
             time-varying size.  When space in transcendent memory is available,
             a significant swap I/O reduction may be achieved.  When none is
             available, all frontswap calls are reduced to a single pointer-
             compare-against-NULL resulting in a negligible performance hit
             and swap data is stored as normal on the matching swap device.
   
             If unsure, say Y to enable frontswap.

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