The
subsystem is used to manage virtual address spaces.
This section describes the main data structures used within the code.
The
Vt 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_submap9
function.
Head node of a circular, doubly linked list of
Vt 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.
min_offset
The minimum
Vt 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
Vt 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_find9
and
vm_map_insert9
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).
The
Vt struct vm_map_entry
is a generic representation of a region.
The region managed by each entry is associated with a
Vt union vm_map_object ,
described below.
The fields of
Vt 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
Vt 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.
These are identical to those defined for
vm_page_protect9.
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_protect9.
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
Vt vm_inherit_t .
This governs the inheritance behaviour for a map entry during fork processing.
The following values are defined for
Vt vm_inherit_t :
VM_INHERIT_SHARE
The object associated with the entry should be cloned and shared
with the new map.
A new
Vt 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
Vt union vm_map_object
is used to specify the structure which a
Vt struct vm_map_entry
is associated with.
The fields of
Vt 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
Vt struct vm_map_entry
is backed by a
Vt struct vm_object .
