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DESCRIPTION

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;
        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.

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.

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.

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.