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26 .\" $FreeBSD$
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28 .Dd September 26, 2004
29 .Dt VM_MAP 9
30 .Os
31 .Sh NAME
32 .Nm vm_map
33 .Nd virtual address space portion of virtual memory subsystem
34 .Sh SYNOPSIS
35 .In sys/param.h
36 .In vm/vm.h
37 .In vm/vm_map.h
38 .Sh DESCRIPTION
39 The
40 .Nm
41 subsystem is used to manage virtual address spaces.
42 This section describes the main data structures used within the code.
43 .Pp
44 The
45 .Vt "struct vm_map"
46 is a generic representation of an address space.
47 This address space may belong to a user process or the kernel.
48 The kernel actually uses several maps, which are maintained as
49 subordinate maps, created using the
50 .Xr vm_map_submap 9
51 function.
52 .Bd -literal -offset indent
53 struct vm_map {
54 struct vm_map_entry header;
55 struct sx lock;
56 struct mtx system_mtx;
57 int nentries;
58 vm_size_t size;
59 u_int timestamp;
60 u_char needs_wakeup;
61 u_char system_map;
62 vm_flags_t flags;
63 vm_map_entry_t root;
64 pmap_t pmap;
65 #define min_offset header.start
66 #define max_offset header.end
67 };
68 .Ed
69 .Pp
70 The fields of
71 .Vt struct vm_map
72 are as follows:
73 .Bl -tag -width ".Va needs_wakeup"
74 .It Va header
75 Head node of a circular, doubly linked list of
76 .Vt struct vm_map_entry
77 objects.
78 Each object defines a particular region within this map's address space.
79 .It Va lock
80 Used to serialize access to the structure.
81 .It Va system_mtx
82 A mutex which is used if the map is a system map.
83 .It Va nentries
84 A count of the members in use within the circular map entry list.
85 .It Va size
86 Specifies the size of the virtual address space.
87 .It Va timestamp
88 Used to determine if the map has changed since its last access.
89 .It Va needs_wakeup
90 Indicates if a thread is waiting for an allocation within the map.
91 Used only by system maps.
92 .It Va system_map
93 Set to TRUE to indicate that map is a system map; otherwise, it belongs
94 to a user process.
95 .It Va flags
96 Map flags, described below.
97 .It Va root
98 Root node of a binary search tree used for fast lookup of map entries.
99 .It Va pmap
100 Pointer to the underlying physical map with which this virtual map
101 is associated.
102 .It Va min_offset
103 The minimum
104 .Vt vm_offset_t
105 in this map.
106 Programs should never use
107 .Va header.start
108 or
109 .Va header.end
110 directly, use
111 .Va min_offset
112 and
113 .Va max_offset
114 instead.
115 .It Va max_offset
116 The maximum
117 .Vt vm_offset_t
118 in this map.
119 .El
120 .Pp
121 There is one possible map flag:
122 .Bl -tag -width ".Dv MAP_PREFAULT_MADVISE"
123 .It Dv MAP_WIREFUTURE
124 Wire all future pages in this map.
125 .El
126 .Pp
127 The following flags can be passed to
128 .Xr vm_map_find 9
129 and
130 .Xr vm_map_insert 9
131 to specify the copy-on-write properties of regions within the map:
132 .Bl -tag -width ".Dv MAP_PREFAULT_MADVISE"
133 .It Dv MAP_COPY_ON_WRITE
134 The mapping is copy-on-write.
135 .It Dv MAP_NOFAULT
136 The mapping should not generate page faults.
137 .It Dv MAP_PREFAULT
138 The mapping should be prefaulted into physical memory.
139 .It Dv MAP_PREFAULT_PARTIAL
140 The mapping should be partially prefaulted into physical memory.
141 .It Dv MAP_DISABLE_SYNCER
142 Do not periodically flush dirty pages; only flush them when absolutely
143 necessary.
144 .It Dv MAP_DISABLE_COREDUMP
145 Do not include the mapping in a core dump.
146 .It Dv MAP_PREFAULT_MADVISE
147 Specify that the request is from a user process calling
148 .Xr madvise 2 .
149 .El
150 .Pp
151 The
152 .Vt struct vm_map_entry
153 is a generic representation of a region.
154 The region managed by each entry is associated with a
155 .Vt union vm_map_object ,
156 described below.
157 .Bd -literal -offset indent
158 struct vm_map_entry {
159 struct vm_map_entry *prev;
160 struct vm_map_entry *next;
161 struct vm_map_entry *left;
162 struct vm_map_entry *right;
163 vm_offset_t start;
164 vm_offset_t end;
165 vm_offset_t avail_ssize;
166 vm_size_t adj_free;
167 vm_size_t max_free;
168 union vm_map_object object;
169 vm_ooffset_t offset;
170 vm_eflags_t eflags;
171 /* Only in task maps: */
172 vm_prot_t protection;
173 vm_prot_t max_protection;
174 vm_inherit_t inheritance;
175 int wired_count;
176 vm_pindex_t lastr;
177 };
178 .Ed
179 .Pp
180 The fields of
181 .Vt struct vm_map_entry
182 are as follows:
183 .Bl -tag -width ".Va avail_ssize"
184 .It Va prev
185 Pointer to the previous node in a doubly-linked, circular list.
186 .It Va next
187 Pointer to the next node in a doubly-linked, circular list.
188 .It Va left
189 Pointer to the left node in a binary search tree.
190 .It Va right
191 Pointer to the right node in a binary search tree.
192 .It Va start
193 Lower address bound of this entry's region.
194 .It Va end
195 Upper address bound of this entry's region.
196 .It Va avail_ssize
197 If the entry is for a process stack, specifies how much the entry can grow.
198 .It Va adj_free
199 The amount of free, unmapped address space adjacent to and immediately
200 following this map entry.
201 .It Va max_free
202 The maximum amount of contiguous free space in this map entry's subtree.
203 .It Va object
204 Pointer to the
205 .Vt struct vm_map_object
206 with which this entry is associated.
207 .It Va offset
208 Offset within the
209 .Va object
210 which is mapped from
211 .Va start
212 onwards.
213 .It Va eflags
214 Flags applied to this entry, described below.
215 .El
216 .Pp
217 The following five members are only valid for entries forming part of
218 a user process's address space:
219 .Bl -tag -width ".Va max_protection"
220 .It Va protection
221 Memory protection bits applied to this region.
222 These are identical to those defined for
223 .Xr vm_page_protect 9 .
224 .It Va max_protection
225 Mask for the memory protection bits which may be actually be applied to
226 this region.
227 These are identical to those defined for
228 .Xr vm_page_protect 9 .
229 .It Va inheritance
230 Contains flags which specify how this entry should be treated
231 during fork processing.
232 .It Va wired_count
233 Count of how many times this entry has been wired into physical memory.
234 .It Va lastr
235 Contains the address of the last read which caused a page fault.
236 .El
237 .Pp
238 The following flags may be applied to each entry, by specifying them
239 as a mask within the
240 .Va eflags
241 member:
242 .Bl -tag -width ".Dv MAP_ENTRY_BEHAV_SEQUENTIAL"
243 .It Dv MAP_ENTRY_NOSYNC
244 The system should not flush the data associated with this map
245 periodically, but only when it needs to.
246 .It Dv MAP_ENTRY_IS_SUB_MAP
247 If set, then the
248 .Va object
249 member specifies a subordinate map.
250 .It Dv MAP_ENTRY_COW
251 Indicate that this is a copy-on-write region.
252 .It Dv MAP_ENTRY_NEEDS_COPY
253 Indicate that a copy-on-write region needs to be copied.
254 .It Dv MAP_ENTRY_NOFAULT
255 Specifies that accesses within this region should never cause a page fault.
256 If a page fault occurs within this region, the system will panic.
257 .It Dv MAP_ENTRY_USER_WIRED
258 Indicate that this region was wired on behalf of a user process.
259 .It Dv MAP_ENTRY_BEHAV_NORMAL
260 The system should use the default paging behaviour for this region.
261 .It Dv MAP_ENTRY_BEHAV_SEQUENTIAL
262 The system should depress the priority of pages immediately preceding
263 each page within this region when faulted in.
264 .It Dv MAP_ENTRY_BEHAV_RANDOM
265 Is a hint that pages within this region will be accessed randomly,
266 and that prefetching is likely not advantageous.
267 .It Dv MAP_ENTRY_IN_TRANSITION
268 Indicate that wiring or unwiring of an entry is in progress, and that
269 other kernel threads should not attempt to modify fields in the structure.
270 .It Dv MAP_ENTRY_NEEDS_WAKEUP
271 Indicate that there are kernel threads waiting for this region to become
272 available.
273 .It Dv MAP_ENTRY_NOCOREDUMP
274 The region should not be included in a core dump.
275 .El
276 .Pp
277 The
278 .Va inheritance
279 member has type
280 .Vt vm_inherit_t .
281 This governs the inheritance behaviour for a map entry during fork processing.
282 The following values are defined for
283 .Vt vm_inherit_t :
284 .Bl -tag -width ".Dv VM_INHERIT_DEFAULT"
285 .It Dv VM_INHERIT_SHARE
286 The object associated with the entry should be cloned and shared
287 with the new map.
288 A new
289 .Vt struct vm_object
290 will be created if necessary.
291 .It Dv VM_INHERIT_COPY
292 The object associated with the entry should be copied to the new map.
293 .It Dv VM_INHERIT_NONE
294 The entry should not be copied to the new map.
295 .It Dv VM_INHERIT_DEFAULT
296 Specifies the default behaviour,
297 .Dv VM_INHERIT_COPY .
298 .El
299 .Pp
300 The
301 .Vt union vm_map_object
302 is used to specify the structure which a
303 .Vt struct vm_map_entry
304 is associated with.
305 .Pp
306 The fields of
307 .Vt union vm_map_object
308 are as follows:
309 .Bd -literal -offset indent
310 union vm_map_object {
311 struct vm_object *vm_object;
312 struct vm_map *sub_map;
313 };
314 .Ed
315 .Pp
316 Normally, the
317 .Va sub_map
318 member is only used by system maps to indicate that a memory range
319 is managed by a subordinate system map.
320 Within a user process map, each
321 .Vt struct vm_map_entry
322 is backed by a
323 .Vt struct vm_object .
324 .Sh SEE ALSO
325 .Xr pmap 9 ,
326 .Xr vm_map_check_protection 9 ,
327 .Xr vm_map_clean 9 ,
328 .Xr vm_map_create 9 ,
329 .Xr vm_map_delete 9 ,
330 .Xr vm_map_entry_resize_free 9 ,
331 .Xr vm_map_find 9 ,
332 .Xr vm_map_findspace 9 ,
333 .Xr vm_map_inherit 9 ,
334 .Xr vm_map_init 9 ,
335 .Xr vm_map_insert 9 ,
336 .Xr vm_map_lock 9 ,
337 .Xr vm_map_lookup 9 ,
338 .Xr vm_map_madvise 9 ,
339 .Xr vm_map_max 9 ,
340 .Xr vm_map_min 9 ,
341 .Xr vm_map_pmap 9 ,
342 .Xr vm_map_protect 9 ,
343 .Xr vm_map_remove 9 ,
344 .Xr vm_map_simplify_entry 9 ,
345 .Xr vm_map_stack 9 ,
346 .Xr vm_map_submap 9 ,
347 .Xr vm_map_wire 9 ,
348 .Xr vm_page_protect 9
349 .Sh AUTHORS
350 This manual page was written by
351 .An Bruce M Simpson Aq bms@spc.org .