vm_map.9 revision 131680 

 Copyright (c) 2003 Bruce M Simpson <bms@spc.org>
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 $FreeBSD: head/share/man/man9/vm_map.9 131680 2004-07-06 06:53:42Z ru $

.Dd July 19, 2003
.Dt VM_MAP 9
.Sh NAME
.Nm vm_map
.Nd virtual address space portion of virtual memory subsystem
.Sh SYNOPSIS
n sys/param.h n vm/vm.h n vm/vm_map.h .Sh DESCRIPTION
The
.Nm
subsystem is used to manage virtual address spaces.
This section describes the main data structures used within the code.

p
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
.Xr vm_map_submap 9
function.
d -literal -offset indent struct vm_map {
 struct vm_map_entry header;
 struct lock lock;
 struct mtx system_mtx;
 int nentries;
 vm_size_t size;
 u_char needs_wakeup;
 u_char system_map;
 u_char infork;
 vm_map_entry_t root;
 unsigned int timestamp;
 vm_map_entry_t first_free;
 pmap_t pmap;
};
.Ed

p
The fields of
.Vt struct vm_map
are as follows:
l -tag -width ".Va needs_wakeup" t Va header 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.
t Va lock Used to serialize access to the structure.
t Va system_mtx A mutex which is used if the map is a system map.
t Va nentries A count of the members in use within the circular map entry list.
t Va size Specifies the size of the virtual address space.
t Va infork Indicates if the map structure is currently undergoing fork processing.
t Va needs_wakeup Indicates if a thread is waiting for an allocation within the map.
Used only by system maps.
t Va system_map Set to TRUE to indicate that map is a system map; otherwise, it belongs
to a user process.
t Va root Root node of a binary search tree used for fast lookup of map entries.
t Va timestamp Used to determine if the map has changed since its last access.
t Va first_free Provides a hint to the first free space within the map.
t Va pmap Pointer to the underlying physical map with which this virtual map
is associated.
.El

p
The following flags can be passed to
.Xr vm_map_find 9
and
.Xr vm_map_insert 9
to specify the copy-on-write properties of regions within the map:
l -tag -width ".Dv MAP_PREFAULT_MADVISE" t Dv MAP_COPY_ON_WRITE The mapping is copy-on-write.
t Dv MAP_NOFAULT The mapping should not generate page faults.
t Dv MAP_PREFAULT The mapping should be prefaulted into physical memory.
t Dv MAP_PREFAULT_PARTIAL The mapping should be partially prefaulted into physical memory.
t Dv MAP_DISABLE_SYNCER Do not periodically flush dirty pages; only flush them when absolutely
necessary.
t Dv MAP_DISABLE_COREDUMP Do not include the mapping in a core dump.
t Dv MAP_PREFAULT_MADVISE Specify that the request from a user process calling
.Xr madvise 2 .
.El

p
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.
d -literal -offset indent 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;
 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;
};
.Ed

p
The fields of
.Vt struct vm_map_entry
are as follows:
l -tag -width ".Va avail_ssize" t Va prev Pointer to the previous node in a doubly-linked, circular list.
t Va next Pointer to the next node in a doubly-linked, circular list.
t Va left Pointer to the left node in a binary search tree.
t Va right Pointer to the right node in a binary search tree.
t Va start Lower address bound of this entry's region.
t Va end Upper address bound of this entry's region.
t Va avail_ssize If the entry is for a process stack, specifies how much the entry can grow.
t Va object Pointer to the
.Vt struct vm_map_object
with which this entry is associated.
t Va offset Offset within the
.Va object
which is mapped from
.Va start
onwards.
t Va eflags Flags applied to this entry, described below.
.El

p
The following five members are only valid for entries forming part of
a user process's address space:
l -tag -width ".Va max_protection" t Va protection Memory protection bits applied to this region.
These are identical to those defined for
.Xr vm_page_protect 9 .
t Va max_protection Mask for the memory protection bits which may be actually be applied to
this region.
These are identical to those defined for
.Xr vm_page_protect 9 .
t Va inheritance Contains flags which specify how this entry should be treated
during fork processing.
t Va wired_count Count of how many times this entry has been wired into physical memory.
t Va lastr Contains the address of the last read which caused a page fault.
.El

p
The following flags may be applied to each entry, by specifying them
as a mask within the
.Va eflags
member:
l -tag -width ".Dv MAP_ENTRY_BEHAV_SEQUENTIAL" t Dv MAP_ENTRY_NOSYNC The system should not flush the data associated with this map
periodically, but only when it needs to.
t Dv MAP_ENTRY_IS_SUB_MAP If set, then the
.Va object
member specifies a subordinate map.
t Dv MAP_ENTRY_COW Indicate that this is a copy-on-write region.
t Dv MAP_ENTRY_NEEDS_COPY Indicate that a copy-on-write region needs to be copied.
t Dv 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.
t Dv MAP_ENTRY_USER_WIRED Indicate that this region was wired on behalf of a user process.
t Dv MAP_ENTRY_BEHAV_NORMAL The system should use the default paging behaviour for this region.
t Dv MAP_ENTRY_BEHAV_SEQUENTIAL The system should depress the priority of pages immediately preceding
each page within this region when faulted in.
t Dv MAP_ENTRY_BEHAV_RANDOM Is a hint that pages within this region will be accessed randomly,
and that prefetching is likely not advantageous.
t Dv 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.
t Dv MAP_ENTRY_NEEDS_WAKEUP Indicate that there are kernel threads waiting for this region to become
available.
t Dv MAP_ENTRY_NOCOREDUMP The region should not be included in a core dump.
.El

p
The
.Va 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 :
l -tag -width ".Dv VM_INHERIT_DEFAULT" t Dv 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.
t Dv VM_INHERIT_COPY The object associated with the entry should be copied to the new map.
t Dv VM_INHERIT_NONE The entry should not be copied to the new map.
t Dv VM_INHERIT_DEFAULT Specifies the default behaviour,
.Dv VM_INHERIT_COPY .
.El

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

p
The fields of
.Vt union vm_map_object
are as follows:
d -literal -offset indent union vm_map_object {
 struct vm_object *vm_object;
 struct vm_map *sub_map;
};

p
.Ed
Normally, the
.Va 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 .
.Sh SEE ALSO
.Xr pmap 9 ,
.Xr vm_map_check_protection 9 ,
.Xr vm_map_clean 9 ,
.Xr vm_map_create 9 ,
.Xr vm_map_delete 9 ,
.Xr vm_map_find 9 ,
.Xr vm_map_findspace 9 ,
.Xr vm_map_inherit 9 ,
.Xr vm_map_init 9 ,
.Xr vm_map_insert 9 ,
.Xr vm_map_lock 9 ,
.Xr vm_map_lookup 9 ,
.Xr vm_map_madvise 9 ,
.Xr vm_map_max 9 ,
.Xr vm_map_min 9 ,
.Xr vm_map_pmap 9 ,
.Xr vm_map_protect 9 ,
.Xr vm_map_remove 9 ,
.Xr vm_map_simplify_entry 9 ,
.Xr vm_map_stack 9 ,
.Xr vm_map_submap 9 ,
.Xr vm_map_wire 9 ,
.Xr vm_page_protect 9
.Sh AUTHORS
This manual page was written by
.An Bruce M Simpson Aq bms@spc.org .