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     vm_map — virtual address space portion of virtual memory subsystem


     #include <sys/param.h>
     #include <vm/vm.h>
     #include <vm/vm_map.h>


     The vm_map subsystem is used to manage virtual address spaces.  This
     section describes the main data structures used within the code.

     The struct vm_map is a generic representation of an address space.  This
     address space may belong to a user process or the kernel.  The kernel
     actually uses several maps, which are maintained as subordinate maps,
     created using the vm_map_submap(9) function.

           struct vm_map {
                   struct vm_map_entry header;
                   struct sx lock;
                   struct mtx system_mtx;
                   int nentries;
                   vm_size_t size;
                   u_int timestamp;
                   u_char needs_wakeup;
                   u_char system_map;
                   vm_flags_t flags;
                   vm_map_entry_t root;
                   pmap_t pmap;
           #define min_offset      header.start
           #define max_offset      header.end

     The fields of struct vm_map are as follows:

     header        Head node of a circular, doubly linked list of struct
                   vm_map_entry objects.  Each object defines a particular
                   region within this map's address space.

     lock          Used to serialize access to the structure.

     system_mtx    A mutex which is used if the map is a system map.

     nentries      A count of the members in use within the circular map entry

     size          Specifies the size of the virtual address space.

     timestamp     Used to determine if the map has changed since its last

     needs_wakeup  Indicates if a thread is waiting for an allocation within
                   the map.  Used only by system maps.

     system_map    Set to TRUE to indicate that map is a system map;
                   otherwise, it belongs to a user process.

     flags         Map flags, described below.

     root          Root node of a binary search tree used for fast lookup of
                   map entries.

     pmap          Pointer to the underlying physical map with which this
                   virtual map is associated.

     min_offset    The minimum vm_offset_t in this map.  Programs should never
                   use header.start or header.end directly, use min_offset and
                   max_offset instead.

     max_offset    The maximum vm_offset_t in this map.

     There is one possible map flag:

     MAP_WIREFUTURE        Wire all future pages in this map.

     The following flags can be passed to vm_map_find(9) and vm_map_insert(9)
     to specify the copy-on-write properties of regions within the map:

     MAP_COPY_ON_WRITE     The mapping is copy-on-write.

     MAP_NOFAULT           The mapping should not generate page faults.

     MAP_PREFAULT          The mapping should be prefaulted into physical

     MAP_PREFAULT_PARTIAL  The mapping should be partially prefaulted into
                           physical memory.

     MAP_DISABLE_SYNCER    Do not periodically flush dirty pages; only flush
                           them when absolutely necessary.

     MAP_DISABLE_COREDUMP  Do not include the mapping in a core dump.

     MAP_PREFAULT_MADVISE  Specify that the request is from a user process
                           calling madvise(2).

     MAP_ACC_CHARGED       Region is already charged to the requestor by some

     MAP_ACC_NO_CHARGE     Do not charge for allocated region.

     The struct vm_map_entry is a generic representation of a region.  The
     region managed by each entry is associated with a union vm_map_object,
     described below.

           struct vm_map_entry {
                   struct vm_map_entry *prev;
                   struct vm_map_entry *next;
                   struct vm_map_entry *left;
                   struct vm_map_entry *right;
                   vm_offset_t start;
                   vm_offset_t end;
                   vm_offset_t avail_ssize;
                   vm_size_t adj_free;
                   vm_size_t max_free;
                   union vm_map_object object;
                   vm_ooffset_t offset;
                   vm_eflags_t eflags;
                   /* Only in task maps: */
                   vm_prot_t protection;
                   vm_prot_t max_protection;
                   vm_inherit_t inheritance;
                   int wired_count;
                   vm_pindex_t lastr;

     The fields of struct vm_map_entry are as follows:

     prev         Pointer to the previous node in a doubly-linked, circular

     next         Pointer to the next node in a doubly-linked, circular list.

     left         Pointer to the left node in a binary search tree.

     right        Pointer to the right node in a binary search tree.

     start        Lower address bound of this entry's region.

     end          Upper address bound of this entry's region.

     avail_ssize  If the entry is for a process stack, specifies how much the
                  entry can grow.

     adj_free     The amount of free, unmapped address space adjacent to and
                  immediately following this map entry.

     max_free     The maximum amount of contiguous free space in this map
                  entry's subtree.

     object       Pointer to the struct vm_map_object with which this entry is

     offset       Offset within the object which is mapped from start onwards.

     eflags       Flags applied to this entry, described below.

     The following five members are only valid for entries forming part of a
     user process's address space:

     protection      Memory protection bits applied to this region.  These are
                     identical to those defined for vm_page_protect(9).

     max_protection  Mask for the memory protection bits which may be actually
                     be applied to this region.  These are identical to those
                     defined for vm_page_protect(9).

     inheritance     Contains flags which specify how this entry should be
                     treated during fork processing.

     wired_count     Count of how many times this entry has been wired into
                     physical memory.

     lastr           Contains the address of the last read which caused a page

     The following flags may be applied to each entry, by specifying them as a
     mask within the eflags member:

     MAP_ENTRY_NOSYNC            The system should not flush the data
                                 associated with this map periodically, but
                                 only when it needs to.

     MAP_ENTRY_IS_SUB_MAP        If set, then the object member specifies a
                                 subordinate map.

     MAP_ENTRY_COW               Indicate that this is a copy-on-write region.

     MAP_ENTRY_NEEDS_COPY        Indicate that a copy-on-write region needs to
                                 be copied.

     MAP_ENTRY_NOFAULT           Specifies that accesses within this region
                                 should never cause a page fault.  If a page
                                 fault occurs within this region, the system
                                 will panic.

     MAP_ENTRY_USER_WIRED        Indicate that this region was wired on behalf
                                 of a user process.

     MAP_ENTRY_BEHAV_NORMAL      The system should use the default paging
                                 behaviour for this region.

     MAP_ENTRY_BEHAV_SEQUENTIAL  The system should depress the priority of
                                 pages immediately preceding each page within
                                 this region when faulted in.

     MAP_ENTRY_BEHAV_RANDOM      Is a hint that pages within this region will
                                 be accessed randomly, and that prefetching is
                                 likely not advantageous.

     MAP_ENTRY_IN_TRANSITION     Indicate that wiring or unwiring of an entry
                                 is in progress, and that other kernel threads
                                 should not attempt to modify fields in the

     MAP_ENTRY_NEEDS_WAKEUP      Indicate that there are kernel threads
                                 waiting for this region to become available.

     MAP_ENTRY_NOCOREDUMP        The region should not be included in a core

     The inheritance member has type vm_inherit_t.  This governs the
     inheritance behaviour for a map entry during fork processing.  The
     following values are defined for vm_inherit_t:

     VM_INHERIT_SHARE    The object associated with the entry should be cloned
                         and shared with the new map.  A new struct vm_object
                         will be created if necessary.

     VM_INHERIT_COPY     The object associated with the entry should be copied
                         to the new map.

     VM_INHERIT_NONE     The entry should not be copied to the new map.

     VM_INHERIT_DEFAULT  Specifies the default behaviour, VM_INHERIT_COPY.

     The union vm_map_object is used to specify the structure which a struct
     vm_map_entry is associated with.

     The fields of union vm_map_object are as follows:

           union vm_map_object {
                   struct vm_object *vm_object;
                   struct vm_map *sub_map;

     Normally, the sub_map member is only used by system maps to indicate that
     a memory range is managed by a subordinate system map.  Within a user
     process map, each struct vm_map_entry is backed by a struct vm_object.


     pmap(9), vm_map_check_protection(9), vm_map_clean(9), vm_map_create(9),
     vm_map_delete(9), vm_map_entry_resize_free(9), vm_map_find(9),
     vm_map_findspace(9), vm_map_inherit(9), vm_map_init(9), vm_map_insert(9),
     vm_map_lock(9), vm_map_lookup(9), vm_map_madvise(9), vm_map_max(9),
     vm_map_min(9), vm_map_pmap(9), vm_map_protect(9), vm_map_remove(9),
     vm_map_simplify_entry(9), vm_map_stack(9), vm_map_submap(9),
     vm_map_wire(9), vm_page_protect(9)


     This manual page was written by Bruce M Simpson ⟨⟩.