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33 .Nd "memory management in the kernel IPC subsystem"
40 .Ss Mbuf allocation macros
41 .Fn MGET "struct mbuf *mbuf" "int how" "short type"
42 .Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
44 .Fn MCLGET "struct mbuf *mbuf" "int how"
46 .Fa "struct mbuf *mbuf"
49 .Fa "void (*free)(struct mbuf *)"
56 .Ss Mbuf utility macros
57 .Fn mtod "struct mbuf *mbuf" "type"
58 .Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
59 .Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
61 .Fn M_LEADINGSPACE "struct mbuf *mbuf"
63 .Fn M_TRAILINGSPACE "struct mbuf *mbuf"
64 .Fn M_MOVE_PKTHDR "struct mbuf *to" "struct mbuf *from"
65 .Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
66 .Fn MCHTYPE "struct mbuf *mbuf" "short type"
68 .Fn M_WRITABLE "struct mbuf *mbuf"
70 .Ss Mbuf allocation functions
72 .Fn m_get "int how" "short type"
74 .Fn m_get2 "int size" "int how" "short type" "int flags"
76 .Fn m_get3 "int size" "int how" "short type" "int flags"
78 .Fn m_getm "struct mbuf *orig" "int len" "int how" "short type"
80 .Fn m_getjcl "int how" "short type" "int flags" "int size"
82 .Fn m_getcl "int how" "short type" "int flags"
84 .Fn m_gethdr "int how" "short type"
86 .Fn m_free "struct mbuf *mbuf"
88 .Fn m_freem "struct mbuf *mbuf"
90 .Ss Mbuf utility functions
92 .Fn m_adj "struct mbuf *mbuf" "int len"
94 .Fn m_align "struct mbuf *mbuf" "int len"
96 .Fn m_append "struct mbuf *mbuf" "int len" "c_caddr_t cp"
98 .Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
100 .Fn m_copyup "struct mbuf *mbuf" "int len" "int dstoff"
102 .Fn m_pullup "struct mbuf *mbuf" "int len"
104 .Fn m_pulldown "struct mbuf *mbuf" "int offset" "int len" "int *offsetp"
106 .Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
108 .Fn m_copypacket "struct mbuf *mbuf" "int how"
110 .Fn m_dup "const struct mbuf *mbuf" "int how"
112 .Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
114 .Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
120 .Fa "struct ifnet *ifp"
121 .Fa "void (*copy)(char *from, caddr_t to, u_int len)"
124 .Fn m_cat "struct mbuf *m" "struct mbuf *n"
126 .Fn m_catpkt "struct mbuf *m" "struct mbuf *n"
128 .Fn m_fixhdr "struct mbuf *mbuf"
130 .Fn m_dup_pkthdr "struct mbuf *to" "const struct mbuf *from" "int how"
132 .Fn m_move_pkthdr "struct mbuf *to" "struct mbuf *from"
134 .Fn m_length "struct mbuf *mbuf" "struct mbuf **last"
136 .Fn m_split "struct mbuf *mbuf" "int len" "int how"
138 .Fn m_apply "struct mbuf *mbuf" "int off" "int len" "int (*f)(void *arg, void *data, u_int len)" "void *arg"
140 .Fn m_getptr "struct mbuf *mbuf" "int loc" "int *off"
142 .Fn m_defrag "struct mbuf *m0" "int how"
144 .Fn m_collapse "struct mbuf *m0" "int how" "int maxfrags"
146 .Fn m_unshare "struct mbuf *m0" "int how"
151 is a basic unit of memory management in the kernel IPC subsystem.
152 Network packets and socket buffers are stored in
154 A network packet may span multiple
159 which allows adding or trimming
160 network headers with little overhead.
162 While a developer should not bother with
164 internals without serious
165 reason in order to avoid incompatibilities with future changes, it
166 is useful to understand the general structure of an
171 consists of a variable-sized header and a small internal
176 is a constant defined in
181 .Bl -tag -width "m_nextpkt" -offset indent
184 A pointer to the next
190 A pointer to the next
195 A pointer to data attached to this
199 The length of the data.
202 The type of the data.
212 flag bits are defined as follows:
214 #define M_EXT 0x00000001 /* has associated external storage */
215 #define M_PKTHDR 0x00000002 /* start of record */
216 #define M_EOR 0x00000004 /* end of record */
217 #define M_RDONLY 0x00000008 /* associated data marked read-only */
218 #define M_BCAST 0x00000010 /* send/received as link-level broadcast */
219 #define M_MCAST 0x00000020 /* send/received as link-level multicast */
220 #define M_PROMISC 0x00000040 /* packet was not for us */
221 #define M_VLANTAG 0x00000080 /* ether_vtag is valid */
222 #define M_EXTPG 0x00000100 /* has array of unmapped pages and TLS */
223 #define M_NOFREE 0x00000200 /* do not free mbuf, embedded in cluster */
224 #define M_TSTMP 0x00000400 /* rcv_tstmp field is valid */
225 #define M_TSTMP_HPREC 0x00000800 /* rcv_tstmp is high-prec, typically
226 hw-stamped on port (useful for IEEE 1588
229 #define M_PROTO1 0x00001000 /* protocol-specific */
230 #define M_PROTO2 0x00002000 /* protocol-specific */
231 #define M_PROTO3 0x00004000 /* protocol-specific */
232 #define M_PROTO4 0x00008000 /* protocol-specific */
233 #define M_PROTO5 0x00010000 /* protocol-specific */
234 #define M_PROTO6 0x00020000 /* protocol-specific */
235 #define M_PROTO7 0x00040000 /* protocol-specific */
236 #define M_PROTO8 0x00080000 /* protocol-specific */
237 #define M_PROTO9 0x00100000 /* protocol-specific */
238 #define M_PROTO10 0x00200000 /* protocol-specific */
239 #define M_PROTO11 0x00400000 /* protocol-specific */
240 #define M_PROTO12 0x00800000 /* protocol-specific */
245 types are defined as follows:
247 #define MT_DATA 1 /* dynamic (data) allocation */
248 #define MT_HEADER MT_DATA /* packet header */
250 #define MT_VENDOR1 4 /* for vendor-internal use */
251 #define MT_VENDOR2 5 /* for vendor-internal use */
252 #define MT_VENDOR3 6 /* for vendor-internal use */
253 #define MT_VENDOR4 7 /* for vendor-internal use */
255 #define MT_SONAME 8 /* socket name */
257 #define MT_EXP1 9 /* for experimental use */
258 #define MT_EXP2 10 /* for experimental use */
259 #define MT_EXP3 11 /* for experimental use */
260 #define MT_EXP4 12 /* for experimental use */
262 #define MT_CONTROL 14 /* extra-data protocol message */
263 #define MT_EXTCONTROL 15 /* control message with externalized contents */
264 #define MT_OOBDATA 16 /* expedited data */
267 The available external buffer types are defined as follows:
269 #define EXT_CLUSTER 1 /* mbuf cluster */
270 #define EXT_SFBUF 2 /* sendfile(2)'s sf_bufs */
271 #define EXT_JUMBOP 3 /* jumbo cluster 4096 bytes */
272 #define EXT_JUMBO9 4 /* jumbo cluster 9216 bytes */
273 #define EXT_JUMBO16 5 /* jumbo cluster 16184 bytes */
274 #define EXT_PACKET 6 /* mbuf+cluster from packet zone */
275 #define EXT_MBUF 7 /* external mbuf reference */
276 #define EXT_RXRING 8 /* data in NIC receive ring */
277 #define EXT_PGS 9 /* array of unmapped pages */
279 #define EXT_VENDOR1 224 /* for vendor-internal use */
280 #define EXT_VENDOR2 225 /* for vendor-internal use */
281 #define EXT_VENDOR3 226 /* for vendor-internal use */
282 #define EXT_VENDOR4 227 /* for vendor-internal use */
284 #define EXT_EXP1 244 /* for experimental use */
285 #define EXT_EXP2 245 /* for experimental use */
286 #define EXT_EXP3 246 /* for experimental use */
287 #define EXT_EXP4 247 /* for experimental use */
289 #define EXT_NET_DRV 252 /* custom ext_buf provided by net driver(s) */
290 #define EXT_MOD_TYPE 253 /* custom module's ext_buf type */
291 #define EXT_DISPOSABLE 254 /* can throw this buffer away w/page flipping */
292 #define EXT_EXTREF 255 /* has externally maintained ref_cnt ptr */
298 .Vt struct pkthdr Va m_pkthdr
302 It contains a pointer to the interface
303 the packet has been received from
304 .Pq Vt struct ifnet Va *rcvif ,
305 and the total packet length
307 Optionally, it may also contain an attached list of packet tags
308 .Pq Vt "struct m_tag" .
312 Fields used in offloading checksum calculation to the hardware are kept in
316 .Sx HARDWARE-ASSISTED CHECKSUM CALCULATION
319 If small enough, data is stored in the internal data buffer of an
321 If the data is sufficiently large, another
325 or external storage may be associated with the
328 bytes of data can fit into an
336 If external storage is being associated with an
340 header is added at the cost of losing the internal data buffer.
341 It includes a pointer to external storage, the size of the storage,
342 a pointer to a function used for freeing the storage,
343 a pointer to an optional argument that can be passed to the function,
344 and a pointer to a reference counter.
347 using external storage has the
351 The system supplies a macro for allocating the desired external storage
355 The allocation and management of the reference counter is handled by the
358 The system also supplies a default type of external storage buffer called an
361 can be allocated and configured with the use of the
368 in size, where MCLBYTES is a machine-dependent constant.
369 The system defines an advisory macro
371 which is the smallest amount of data to put into an
376 It is typically preferable to store data into the data region of an
378 if size permits, as opposed to allocating a separate
380 to hold the same data.
382 .Ss Macros and Functions
383 There are numerous predefined macros and functions that provide the
384 developer with common utilities.
386 .Bl -ohang -offset indent
387 .It Fn mtod mbuf type
390 pointer to a data pointer.
391 The macro expands to the data pointer cast to the specified
394 It is advisable to ensure that there is enough contiguous data in
399 .It Fn MGET mbuf how type
402 and initialize it to contain internal data.
404 will point to the allocated
406 on success, or be set to
411 argument is to be set to
415 It specifies whether the caller is willing to block if necessary.
416 A number of other functions and macros related to
418 have the same argument because they may
419 at some point need to allocate new
421 .It Fn MGETHDR mbuf how type
424 and initialize it to contain a packet header
429 .It Fn MEXTADD mbuf buf size free opt_arg1 opt_arg2 flags type
430 Associate externally managed data with
432 Any internal data contained in the mbuf will be discarded, and the
439 arguments are the address and length, respectively, of the data.
442 argument points to a function which will be called to free the data
443 when the mbuf is freed; it is only used if
451 arguments will be saved in
460 argument specifies additional
462 flags; it is not necessary to specify
466 argument specifies the type of external data, which controls how it
467 will be disposed of when the
470 In most cases, the correct value is
472 .It Fn MCLGET mbuf how
473 Allocate and attach an
477 On success, a non-zero value returned; otherwise, 0.
478 Historically, consumers would check for success by testing the
480 flag on the mbuf, but this is now discouraged to avoid unnecessary awareness
481 of the implementation of external storage in protocol stacks and device
483 .It Fn M_ALIGN mbuf len
486 to place an object of the size
488 at the end of the internal data area of
493 is newly allocated with
497 .It Fn MH_ALIGN mbuf len
498 Serves the same purpose as
510 .It Fn m_align mbuf len
511 Services the same purpose as
513 but handles any type of mbuf.
514 .It Fn M_LEADINGSPACE mbuf
515 Returns the number of bytes available before the beginning
518 .It Fn M_TRAILINGSPACE mbuf
519 Returns the number of bytes available after the end of data in
521 .It Fn M_PREPEND mbuf len how
522 This macro operates on an
524 It is an optimized wrapper for
526 that can make use of possible empty space before data
527 (e.g.\& left after trimming of a link-layer header).
535 .It Fn M_MOVE_PKTHDR to from
536 Using this macro is equivalent to calling
537 .Fn m_move_pkthdr to from .
538 .It Fn M_WRITABLE mbuf
539 This macro will evaluate true if
545 does not contain external storage or,
547 then if the reference count of the storage is not greater than 1.
552 This can be achieved during setup of the external storage,
559 macro, or can be directly set in individual
561 .It Fn MCHTYPE mbuf type
566 This is a relatively expensive operation and should be avoided.
570 .Bl -ohang -offset indent
571 .It Fn m_get how type
572 A function version of
574 for non-critical paths.
575 .It Fn m_get2 size how type flags
578 with enough space to hold specified amount of data.
579 If the size is is larger than
580 .Dv MJUMPAGESIZE , NULL
582 .It Fn m_get3 size how type flags
585 with enough space to hold specified amount of data.
586 If the size is is larger than
587 .Dv MJUM16BYTES, NULL
589 .It Fn m_getm orig len how type
596 if necessary and append the resulting allocated
602 .No non- Ns Dv NULL .
603 If the allocation fails at any point,
604 free whatever was allocated and return
609 .No non- Ns Dv NULL ,
610 it will not be freed.
611 It is possible to use
619 (for example, one which may be sitting in a pre-allocated ring)
620 or to simply perform an all-or-nothing
625 .It Fn m_gethdr how type
626 A function version of
628 for non-critical paths.
629 .It Fn m_getcl how type flags
635 If one of the allocations fails, the entire allocation fails.
636 This routine is the preferred way of fetching both the
640 together, as it avoids having to unlock/relock between allocations.
644 .It Fn m_getjcl how type flags size
649 of the cluster to be allocated must be one of
650 .Dv MCLBYTES , MJUMPAGESIZE , MJUM9BYTES ,
662 The functions below operate on
664 .Bl -ohang -offset indent
668 including any external storage.
670 .It Fn m_adj mbuf len
673 bytes from the head of an
677 is positive, from the tail otherwise.
679 .It Fn m_append mbuf len cp
686 Extend the mbuf chain if the new data does not fit in
689 .It Fn m_prepend mbuf len how
692 and prepend it to the
698 It does not allocate any
710 .It Fn m_copyup mbuf len dstoff
715 bytes of data into a new mbuf at
720 argument aligns the data and leaves room for a link layer header.
730 The function does not allocate
737 .It Fn m_pullup mbuf len
738 Arrange that the first
742 are contiguous and lay in the data area of
744 so they are accessible with
746 It is important to remember that this may involve
747 reallocating some mbufs and moving data so all pointers
748 referencing data within the old mbuf chain
749 must be recalculated or made invalid.
757 is freed in this case).
759 It does not allocate any
763 must be less than or equal to
766 .It Fn m_pulldown mbuf offset len offsetp
775 are contiguous and lay in the data area of
777 so they are accessible with
780 must be smaller than, or equal to, the size of an
782 Return a pointer to an intermediate
784 in the chain containing the requested region;
785 the offset in the data region of the
787 to the data contained in the returned mbuf is stored in
791 is NULL, the region may be accessed using
795 is non-NULL, the region may be accessed using
796 .Fn mtod mbuf uint8_t
798 The region of the mbuf chain between its beginning and
800 is not modified, therefore it is safe to hold pointers to data within
801 this region before calling
804 .It Fn m_copym mbuf offset len how
809 bytes from the beginning, continuing for
816 copy to the end of the
819 The copy is read-only, because the
821 are not copied, only their reference counts are incremented.
823 .It Fn m_copypacket mbuf how
824 Copy an entire packet including header, which must be present.
825 This is an optimized version of the common case
826 .Fn m_copym mbuf 0 M_COPYALL how .
828 the copy is read-only, because the
830 are not copied, only their reference counts are incremented.
832 .It Fn m_dup mbuf how
835 into a completely new
837 including copying any
841 when you need a writable copy of an
844 .It Fn m_copydata mbuf offset len buf
849 bytes from the beginning, continuing for
851 bytes, into the indicated buffer
854 .It Fn m_copyback mbuf offset len buf
857 bytes from the buffer
859 back into the indicated
863 bytes from the beginning of the
869 It does not allocate any
881 will be allocated to fill the space.
883 .It Fn m_length mbuf last
884 Return the length of the
886 and optionally a pointer to the last
889 .It Fn m_dup_pkthdr to from how
890 Upon the function's completion, the
893 will contain an identical copy of
895 and the per-packet attributes found in the
905 must be empty on entry.
907 .It Fn m_move_pkthdr to from
910 and the per-packet attributes from the
923 must be empty on entry.
924 Upon the function's completion,
928 and the per-packet attributes cleared.
931 Set the packet-header length to the length of the
934 .It Fn m_devget buf len offset ifp copy
935 Copy data from a device local memory pointed to by
939 The copy is done using a specified copy routine
955 must be of the same type.
957 is not guaranteed to be valid after
961 does not update any packet header fields or free mbuf tags.
966 that operates on packets.
971 must contain packet headers.
973 is not guaranteed to be valid after
977 .It Fn m_split mbuf len how
980 in two pieces, returning the tail:
984 In case of failure, it returns
986 and attempts to restore the
988 to its original state.
990 .It Fn m_apply mbuf off len f arg
991 Apply a function to an
998 Typically used to avoid calls to
1000 which would otherwise be unnecessary or undesirable.
1002 is a convenience argument which is passed to the callback function
1007 is called, it will be passed
1011 in the current mbuf, and the length
1013 of the data in this mbuf to which the function should be applied.
1015 The function should return zero to indicate success;
1016 otherwise, if an error is indicated, then
1018 will return the error and stop iterating through the
1021 .It Fn m_getptr mbuf loc off
1022 Return a pointer to the mbuf containing the data located at
1024 bytes from the beginning of the
1026 The corresponding offset into the mbuf will be stored in
1028 .It Fn m_defrag m0 how
1029 Defragment an mbuf chain, returning the shortest possible
1030 chain of mbufs and clusters.
1031 If allocation fails and this can not be completed,
1033 will be returned and the original chain will be unchanged.
1034 Upon success, the original chain will be freed and the new
1035 chain will be returned.
1041 depending on the caller's preference.
1043 This function is especially useful in network drivers, where
1044 certain long mbuf chains must be shortened before being added
1045 to TX descriptor lists.
1046 .It Fn m_collapse m0 how maxfrags
1047 Defragment an mbuf chain, returning a chain of at most
1050 If allocation fails or the chain cannot be collapsed as requested,
1052 will be returned, with the original chain possibly modified.
1060 .It Fn m_unshare m0 how
1061 Create a version of the specified mbuf chain whose
1062 contents can be safely modified without affecting other users.
1063 If allocation fails and this operation can not be completed,
1066 The original mbuf chain is always reclaimed and the reference
1067 count of any shared mbuf clusters is decremented.
1073 depending on the caller's preference.
1074 As a side-effect of this process the returned
1075 mbuf chain may be compacted.
1077 This function is especially useful in the transmit path of
1078 network code, when data must be encrypted or otherwise
1079 altered prior to transmission.
1081 .Sh HARDWARE-ASSISTED CHECKSUM CALCULATION
1082 This section currently applies to TCP/IP only.
1083 In order to save the host CPU resources, computing checksums is
1084 offloaded to the network interface hardware if possible.
1087 member of the leading
1089 of a packet contains two fields used for that purpose,
1090 .Vt int Va csum_flags
1092 .Vt int Va csum_data .
1093 The meaning of those fields depends on the direction a packet flows in,
1094 and on whether the packet is fragmented.
1100 will denote the corresponding field of the
1102 member of the leading
1106 containing the packet.
1108 On output, checksum offloading is attempted after the outgoing
1109 interface has been determined for a packet.
1110 The interface-specific field
1111 .Va ifnet.if_data.ifi_hwassist
1114 is consulted for the capabilities of the interface to assist in
1115 computing checksums.
1118 field of the packet header is set to indicate which actions the interface
1119 is supposed to perform on it.
1120 The actions unsupported by the network interface are done in the
1121 software prior to passing the packet down to the interface driver;
1122 such actions will never be requested through
1125 The flags demanding a particular action from an interface are as follows:
1126 .Bl -tag -width ".Dv CSUM_TCP" -offset indent
1128 The IP header checksum is to be computed and stored in the
1129 corresponding field of the packet.
1130 The hardware is expected to know the format of an IP header
1131 to determine the offset of the IP checksum field.
1133 The TCP checksum is to be computed.
1136 The UDP checksum is to be computed.
1140 Should a TCP or UDP checksum be offloaded to the hardware,
1143 will contain the byte offset of the checksum field relative to the
1144 end of the IP header.
1145 In this case, the checksum field will be initially
1146 set by the TCP/IP module to the checksum of the pseudo header
1147 defined by the TCP and UDP specifications.
1149 On input, an interface indicates the actions it has performed
1150 on a packet by setting one or more of the following flags in
1152 associated with the packet:
1153 .Bl -tag -width ".Dv CSUM_IP_CHECKED" -offset indent
1154 .It Dv CSUM_IP_CHECKED
1155 The IP header checksum has been computed.
1156 .It Dv CSUM_IP_VALID
1157 The IP header has a valid checksum.
1158 This flag can appear only in combination with
1159 .Dv CSUM_IP_CHECKED .
1160 .It Dv CSUM_DATA_VALID
1161 The checksum of the data portion of the IP packet has been computed
1162 and stored in the field
1164 in network byte order.
1165 .It Dv CSUM_PSEUDO_HDR
1166 Can be set only along with
1168 to indicate that the IP data checksum found in
1170 allows for the pseudo header defined by the TCP and UDP specifications.
1171 Otherwise the checksum of the pseudo header must be calculated by
1172 the host CPU and added to
1174 to obtain the final checksum to be used for TCP or UDP validation purposes.
1177 If a particular network interface just indicates success or
1178 failure of TCP or UDP checksum validation without returning
1179 the exact value of the checksum to the host CPU, its driver can mark
1189 hexadecimal to indicate a valid checksum.
1190 It is a peculiarity of the algorithm used that the Internet checksum
1191 calculated over any valid packet will be
1193 as long as the original checksum field is included.
1195 When running a kernel compiled with the option
1196 .Dv MBUF_STRESS_TEST ,
1199 -controlled options may be used to create
1200 various failure/extreme cases for testing of network drivers
1201 and other parts of the kernel that rely on
1203 .Bl -tag -width ident
1204 .It Va net.inet.ip.mbuf_frag_size
1207 to fragment outgoing
1209 into fragments of the specified size.
1210 Setting this variable to 1 is an excellent way to
1213 handling ability of network drivers.
1214 .It Va kern.ipc.m_defragrandomfailures
1217 to randomly fail, returning
1219 Any piece of code which uses
1221 should be tested with this feature.
1234 .%T Networking Implementation Notes
1235 .%B 4.4BSD System Manager's Manual (SMM)
1238 .\" Please correct me if I'm wrong
1240 appeared in an early version of
1242 Besides being used for network packets, they were used
1243 to store various dynamic structures, such as routing table
1244 entries, interface addresses, protocol control blocks, etc.
1249 is almost entirely limited to packet storage, with
1251 zones being used directly to store other network-related memory.
1255 allocator has been a special-purpose memory allocator able to run in
1256 interrupt contexts and allocating from a special kernel address space map.
1261 allocator is a wrapper around
1267 + cluster pairs in per-CPU caches, as well as bringing other benefits of
1272 manual page was written by
1277 allocator was written by