VM_MAP(9)

NAME
vm_map - virtual address space portion of virtual memory subsystem

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

DESCRIPTION
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;
    	union {
    	        struct sx lock;
    		struct mtx system_mtx;
    	};
            int nentries;
            vm_size_t size;
            u_int timestamp;
            u_int flags;
            vm_map_entry_t root;
            pmap_t pmap;
            int busy;
    };

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 list.

size
    Specifies the size of the virtual address space.

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

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.

busy
    Map busy counter, prevents forks.

Possible map flags:

MAP_WIREFUTURE
    Wire all future pages in this map.

MAP_BUSY_WAKEUP
    There are waiters for the map busy status.

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

MAP_SYSTEM_MAP
    If set, indicates that the map is the system map; otherwise, it belongs to a user process.

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 memory.

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 means.

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 list.

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 associated.

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.

max_protection
    Mask for the memory protection bits which may be actually be applied to this region.

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 fault.

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 structure.

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 dump.

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.

SEE ALSO
pmap(9), vm_map_check_protection(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_stack(9), vm_map_submap(9), vm_map_sync(9), vm_map_wire(9)

AUTHORS
This manual page was written by Bruce M Simpson <bms@spc.org>.