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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_getm "struct mbuf *orig" "int len" "int how" "short type"
78 .Fn m_getjcl "int how" "short type" "int flags" "int size"
80 .Fn m_getcl "int how" "short type" "int flags"
82 .Fn m_gethdr "int how" "short type"
84 .Fn m_free "struct mbuf *mbuf"
86 .Fn m_freem "struct mbuf *mbuf"
88 .Ss Mbuf utility functions
90 .Fn m_adj "struct mbuf *mbuf" "int len"
92 .Fn m_align "struct mbuf *mbuf" "int len"
94 .Fn m_append "struct mbuf *mbuf" "int len" "c_caddr_t cp"
96 .Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
98 .Fn m_copyup "struct mbuf *mbuf" "int len" "int dstoff"
100 .Fn m_pullup "struct mbuf *mbuf" "int len"
102 .Fn m_pulldown "struct mbuf *mbuf" "int offset" "int len" "int *offsetp"
104 .Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
106 .Fn m_copypacket "struct mbuf *mbuf" "int how"
108 .Fn m_dup "const struct mbuf *mbuf" "int how"
110 .Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
112 .Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
118 .Fa "struct ifnet *ifp"
119 .Fa "void (*copy)(char *from, caddr_t to, u_int len)"
122 .Fn m_cat "struct mbuf *m" "struct mbuf *n"
124 .Fn m_catpkt "struct mbuf *m" "struct mbuf *n"
126 .Fn m_fixhdr "struct mbuf *mbuf"
128 .Fn m_dup_pkthdr "struct mbuf *to" "const struct mbuf *from" "int how"
130 .Fn m_move_pkthdr "struct mbuf *to" "struct mbuf *from"
132 .Fn m_length "struct mbuf *mbuf" "struct mbuf **last"
134 .Fn m_split "struct mbuf *mbuf" "int len" "int how"
136 .Fn m_apply "struct mbuf *mbuf" "int off" "int len" "int (*f)(void *arg, void *data, u_int len)" "void *arg"
138 .Fn m_getptr "struct mbuf *mbuf" "int loc" "int *off"
140 .Fn m_defrag "struct mbuf *m0" "int how"
142 .Fn m_collapse "struct mbuf *m0" "int how" "int maxfrags"
144 .Fn m_unshare "struct mbuf *m0" "int how"
149 is a basic unit of memory management in the kernel IPC subsystem.
150 Network packets and socket buffers are stored in
152 A network packet may span multiple
157 which allows adding or trimming
158 network headers with little overhead.
160 While a developer should not bother with
162 internals without serious
163 reason in order to avoid incompatibilities with future changes, it
164 is useful to understand the general structure of an
169 consists of a variable-sized header and a small internal
174 is a constant defined in
179 .Bl -tag -width "m_nextpkt" -offset indent
182 A pointer to the next
188 A pointer to the next
193 A pointer to data attached to this
197 The length of the data.
200 The type of the data.
210 flag bits are defined as follows:
212 #define M_EXT 0x00000001 /* has associated external storage */
213 #define M_PKTHDR 0x00000002 /* start of record */
214 #define M_EOR 0x00000004 /* end of record */
215 #define M_RDONLY 0x00000008 /* associated data marked read-only */
216 #define M_NOMAP 0x00000100 /* mbuf data is unmapped */
217 #define M_NOFREE 0x00000200 /* do not free mbuf, embedded in cluster */
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_TSTMP 0x00000400 /* rcv_tstmp field is valid */
223 #define M_TSTMP_HPREC 0x00000800 /* rcv_tstmp is high-prec, typically
224 hw-stamped on port (useful for IEEE 1588
227 #define M_PROTO1 0x00001000 /* protocol-specific */
228 #define M_PROTO2 0x00002000 /* protocol-specific */
229 #define M_PROTO3 0x00004000 /* protocol-specific */
230 #define M_PROTO4 0x00008000 /* protocol-specific */
231 #define M_PROTO5 0x00010000 /* protocol-specific */
232 #define M_PROTO6 0x00020000 /* protocol-specific */
233 #define M_PROTO7 0x00040000 /* protocol-specific */
234 #define M_PROTO8 0x00080000 /* protocol-specific */
235 #define M_PROTO9 0x00100000 /* protocol-specific */
236 #define M_PROTO10 0x00200000 /* protocol-specific */
237 #define M_PROTO11 0x00400000 /* protocol-specific */
238 #define M_PROTO12 0x00800000 /* protocol-specific */
243 types are defined as follows:
245 #define MT_DATA 1 /* dynamic (data) allocation */
246 #define MT_HEADER MT_DATA /* packet header */
248 #define MT_VENDOR1 4 /* for vendor-internal use */
249 #define MT_VENDOR2 5 /* for vendor-internal use */
250 #define MT_VENDOR3 6 /* for vendor-internal use */
251 #define MT_VENDOR4 7 /* for vendor-internal use */
253 #define MT_SONAME 8 /* socket name */
255 #define MT_EXP1 9 /* for experimental use */
256 #define MT_EXP2 10 /* for experimental use */
257 #define MT_EXP3 11 /* for experimental use */
258 #define MT_EXP4 12 /* for experimental use */
260 #define MT_CONTROL 14 /* extra-data protocol message */
261 #define MT_EXTCONTROL 15 /* control message with externalized contents */
262 #define MT_OOBDATA 16 /* expedited data */
265 The available external buffer types are defined as follows:
267 #define EXT_CLUSTER 1 /* mbuf cluster */
268 #define EXT_SFBUF 2 /* sendfile(2)'s sf_bufs */
269 #define EXT_JUMBOP 3 /* jumbo cluster 4096 bytes */
270 #define EXT_JUMBO9 4 /* jumbo cluster 9216 bytes */
271 #define EXT_JUMBO16 5 /* jumbo cluster 16184 bytes */
272 #define EXT_PACKET 6 /* mbuf+cluster from packet zone */
273 #define EXT_MBUF 7 /* external mbuf reference */
274 #define EXT_RXRING 8 /* data in NIC receive ring */
275 #define EXT_PGS 9 /* array of unmapped pages */
277 #define EXT_VENDOR1 224 /* for vendor-internal use */
278 #define EXT_VENDOR2 225 /* for vendor-internal use */
279 #define EXT_VENDOR3 226 /* for vendor-internal use */
280 #define EXT_VENDOR4 227 /* for vendor-internal use */
282 #define EXT_EXP1 244 /* for experimental use */
283 #define EXT_EXP2 245 /* for experimental use */
284 #define EXT_EXP3 246 /* for experimental use */
285 #define EXT_EXP4 247 /* for experimental use */
287 #define EXT_NET_DRV 252 /* custom ext_buf provided by net driver(s) */
288 #define EXT_MOD_TYPE 253 /* custom module's ext_buf type */
289 #define EXT_DISPOSABLE 254 /* can throw this buffer away w/page flipping */
290 #define EXT_EXTREF 255 /* has externally maintained ref_cnt ptr */
296 .Vt struct pkthdr Va m_pkthdr
300 It contains a pointer to the interface
301 the packet has been received from
302 .Pq Vt struct ifnet Va *rcvif ,
303 and the total packet length
305 Optionally, it may also contain an attached list of packet tags
306 .Pq Vt "struct m_tag" .
310 Fields used in offloading checksum calculation to the hardware are kept in
314 .Sx HARDWARE-ASSISTED CHECKSUM CALCULATION
317 If small enough, data is stored in the internal data buffer of an
319 If the data is sufficiently large, another
323 or external storage may be associated with the
326 bytes of data can fit into an
334 If external storage is being associated with an
338 header is added at the cost of losing the internal data buffer.
339 It includes a pointer to external storage, the size of the storage,
340 a pointer to a function used for freeing the storage,
341 a pointer to an optional argument that can be passed to the function,
342 and a pointer to a reference counter.
345 using external storage has the
349 The system supplies a macro for allocating the desired external storage
353 The allocation and management of the reference counter is handled by the
356 The system also supplies a default type of external storage buffer called an
359 can be allocated and configured with the use of the
366 in size, where MCLBYTES is a machine-dependent constant.
367 The system defines an advisory macro
369 which is the smallest amount of data to put into an
374 It is typically preferable to store data into the data region of an
376 if size permits, as opposed to allocating a separate
378 to hold the same data.
380 .Ss Macros and Functions
381 There are numerous predefined macros and functions that provide the
382 developer with common utilities.
384 .Bl -ohang -offset indent
385 .It Fn mtod mbuf type
388 pointer to a data pointer.
389 The macro expands to the data pointer cast to the specified
392 It is advisable to ensure that there is enough contiguous data in
397 .It Fn MGET mbuf how type
400 and initialize it to contain internal data.
402 will point to the allocated
404 on success, or be set to
409 argument is to be set to
413 It specifies whether the caller is willing to block if necessary.
414 A number of other functions and macros related to
416 have the same argument because they may
417 at some point need to allocate new
419 .It Fn MGETHDR mbuf how type
422 and initialize it to contain a packet header
427 .It Fn MEXTADD mbuf buf size free opt_arg1 opt_arg2 flags type
428 Associate externally managed data with
430 Any internal data contained in the mbuf will be discarded, and the
437 arguments are the address and length, respectively, of the data.
440 argument points to a function which will be called to free the data
441 when the mbuf is freed; it is only used if
449 arguments will be saved in
458 argument specifies additional
460 flags; it is not necessary to specify
464 argument specifies the type of external data, which controls how it
465 will be disposed of when the
468 In most cases, the correct value is
470 .It Fn MCLGET mbuf how
471 Allocate and attach an
475 On success, a non-zero value returned; otherwise, 0.
476 Historically, consumers would check for success by testing the
478 flag on the mbuf, but this is now discouraged to avoid unnecessary awareness
479 of the implementation of external storage in protocol stacks and device
481 .It Fn M_ALIGN mbuf len
484 to place an object of the size
486 at the end of the internal data area of
491 is newly allocated with
495 .It Fn MH_ALIGN mbuf len
496 Serves the same purpose as
508 .It Fn m_align mbuf len
509 Services the same purpose as
511 but handles any type of mbuf.
512 .It Fn M_LEADINGSPACE mbuf
513 Returns the number of bytes available before the beginning
516 .It Fn M_TRAILINGSPACE mbuf
517 Returns the number of bytes available after the end of data in
519 .It Fn M_PREPEND mbuf len how
520 This macro operates on an
522 It is an optimized wrapper for
524 that can make use of possible empty space before data
525 (e.g.\& left after trimming of a link-layer header).
533 .It Fn M_MOVE_PKTHDR to from
534 Using this macro is equivalent to calling
535 .Fn m_move_pkthdr to from .
536 .It Fn M_WRITABLE mbuf
537 This macro will evaluate true if
543 does not contain external storage or,
545 then if the reference count of the storage is not greater than 1.
550 This can be achieved during setup of the external storage,
557 macro, or can be directly set in individual
559 .It Fn MCHTYPE mbuf type
564 This is a relatively expensive operation and should be avoided.
568 .Bl -ohang -offset indent
569 .It Fn m_get how type
570 A function version of
572 for non-critical paths.
573 .It Fn m_get2 size how type flags
576 with enough space to hold specified amount of data.
577 If the size is is larger than
578 .Dv MJUMPAGESIZE , NULL
580 .It Fn m_getm orig len how type
587 if necessary and append the resulting allocated
593 .No non- Ns Dv NULL .
594 If the allocation fails at any point,
595 free whatever was allocated and return
600 .No non- Ns Dv NULL ,
601 it will not be freed.
602 It is possible to use
610 (for example, one which may be sitting in a pre-allocated ring)
611 or to simply perform an all-or-nothing
616 .It Fn m_gethdr how type
617 A function version of
619 for non-critical paths.
620 .It Fn m_getcl how type flags
626 If one of the allocations fails, the entire allocation fails.
627 This routine is the preferred way of fetching both the
631 together, as it avoids having to unlock/relock between allocations.
635 .It Fn m_getjcl how type flags size
640 of the cluster to be allocated must be one of
641 .Dv MCLBYTES , MJUMPAGESIZE , MJUM9BYTES ,
653 The functions below operate on
655 .Bl -ohang -offset indent
659 including any external storage.
661 .It Fn m_adj mbuf len
664 bytes from the head of an
668 is positive, from the tail otherwise.
670 .It Fn m_append mbuf len cp
677 Extend the mbuf chain if the new data does not fit in
680 .It Fn m_prepend mbuf len how
683 and prepend it to the
689 It does not allocate any
701 .It Fn m_copyup mbuf len dstoff
706 bytes of data into a new mbuf at
711 argument aligns the data and leaves room for a link layer header.
721 The function does not allocate
728 .It Fn m_pullup mbuf len
729 Arrange that the first
733 are contiguous and lay in the data area of
735 so they are accessible with
737 It is important to remember that this may involve
738 reallocating some mbufs and moving data so all pointers
739 referencing data within the old mbuf chain
740 must be recalculated or made invalid.
748 is freed in this case).
750 It does not allocate any
754 must be less than or equal to
757 .It Fn m_pulldown mbuf offset len offsetp
766 are contiguous and lay in the data area of
768 so they are accessible with
771 must be smaller than, or equal to, the size of an
773 Return a pointer to an intermediate
775 in the chain containing the requested region;
776 the offset in the data region of the
778 to the data contained in the returned mbuf is stored in
782 is NULL, the region may be accessed using
786 is non-NULL, the region may be accessed using
787 .Fn mtod mbuf uint8_t
789 The region of the mbuf chain between its beginning and
791 is not modified, therefore it is safe to hold pointers to data within
792 this region before calling
795 .It Fn m_copym mbuf offset len how
800 bytes from the beginning, continuing for
807 copy to the end of the
810 The copy is read-only, because the
812 are not copied, only their reference counts are incremented.
814 .It Fn m_copypacket mbuf how
815 Copy an entire packet including header, which must be present.
816 This is an optimized version of the common case
817 .Fn m_copym mbuf 0 M_COPYALL how .
819 the copy is read-only, because the
821 are not copied, only their reference counts are incremented.
823 .It Fn m_dup mbuf how
826 into a completely new
828 including copying any
832 when you need a writable copy of an
835 .It Fn m_copydata mbuf offset len buf
840 bytes from the beginning, continuing for
842 bytes, into the indicated buffer
845 .It Fn m_copyback mbuf offset len buf
848 bytes from the buffer
850 back into the indicated
854 bytes from the beginning of the
860 It does not allocate any
872 will be allocated to fill the space.
874 .It Fn m_length mbuf last
875 Return the length of the
877 and optionally a pointer to the last
880 .It Fn m_dup_pkthdr to from how
881 Upon the function's completion, the
884 will contain an identical copy of
886 and the per-packet attributes found in the
896 must be empty on entry.
898 .It Fn m_move_pkthdr to from
901 and the per-packet attributes from the
914 must be empty on entry.
915 Upon the function's completion,
919 and the per-packet attributes cleared.
922 Set the packet-header length to the length of the
925 .It Fn m_devget buf len offset ifp copy
926 Copy data from a device local memory pointed to by
930 The copy is done using a specified copy routine
946 must be of the same type.
948 is not guaranteed to be valid after
952 does not update any packet header fields or free mbuf tags.
957 that operates on packets.
962 must contain packet headers.
964 is not guaranteed to be valid after
968 .It Fn m_split mbuf len how
971 in two pieces, returning the tail:
975 In case of failure, it returns
977 and attempts to restore the
979 to its original state.
981 .It Fn m_apply mbuf off len f arg
982 Apply a function to an
989 Typically used to avoid calls to
991 which would otherwise be unnecessary or undesirable.
993 is a convenience argument which is passed to the callback function
998 is called, it will be passed
1002 in the current mbuf, and the length
1004 of the data in this mbuf to which the function should be applied.
1006 The function should return zero to indicate success;
1007 otherwise, if an error is indicated, then
1009 will return the error and stop iterating through the
1012 .It Fn m_getptr mbuf loc off
1013 Return a pointer to the mbuf containing the data located at
1015 bytes from the beginning of the
1017 The corresponding offset into the mbuf will be stored in
1019 .It Fn m_defrag m0 how
1020 Defragment an mbuf chain, returning the shortest possible
1021 chain of mbufs and clusters.
1022 If allocation fails and this can not be completed,
1024 will be returned and the original chain will be unchanged.
1025 Upon success, the original chain will be freed and the new
1026 chain will be returned.
1032 depending on the caller's preference.
1034 This function is especially useful in network drivers, where
1035 certain long mbuf chains must be shortened before being added
1036 to TX descriptor lists.
1037 .It Fn m_collapse m0 how maxfrags
1038 Defragment an mbuf chain, returning a chain of at most
1041 If allocation fails or the chain cannot be collapsed as requested,
1043 will be returned, with the original chain possibly modified.
1051 .It Fn m_unshare m0 how
1052 Create a version of the specified mbuf chain whose
1053 contents can be safely modified without affecting other users.
1054 If allocation fails and this operation can not be completed,
1057 The original mbuf chain is always reclaimed and the reference
1058 count of any shared mbuf clusters is decremented.
1064 depending on the caller's preference.
1065 As a side-effect of this process the returned
1066 mbuf chain may be compacted.
1068 This function is especially useful in the transmit path of
1069 network code, when data must be encrypted or otherwise
1070 altered prior to transmission.
1072 .Sh HARDWARE-ASSISTED CHECKSUM CALCULATION
1073 This section currently applies to TCP/IP only.
1074 In order to save the host CPU resources, computing checksums is
1075 offloaded to the network interface hardware if possible.
1078 member of the leading
1080 of a packet contains two fields used for that purpose,
1081 .Vt int Va csum_flags
1083 .Vt int Va csum_data .
1084 The meaning of those fields depends on the direction a packet flows in,
1085 and on whether the packet is fragmented.
1091 will denote the corresponding field of the
1093 member of the leading
1097 containing the packet.
1099 On output, checksum offloading is attempted after the outgoing
1100 interface has been determined for a packet.
1101 The interface-specific field
1102 .Va ifnet.if_data.ifi_hwassist
1105 is consulted for the capabilities of the interface to assist in
1106 computing checksums.
1109 field of the packet header is set to indicate which actions the interface
1110 is supposed to perform on it.
1111 The actions unsupported by the network interface are done in the
1112 software prior to passing the packet down to the interface driver;
1113 such actions will never be requested through
1116 The flags demanding a particular action from an interface are as follows:
1117 .Bl -tag -width ".Dv CSUM_TCP" -offset indent
1119 The IP header checksum is to be computed and stored in the
1120 corresponding field of the packet.
1121 The hardware is expected to know the format of an IP header
1122 to determine the offset of the IP checksum field.
1124 The TCP checksum is to be computed.
1127 The UDP checksum is to be computed.
1131 Should a TCP or UDP checksum be offloaded to the hardware,
1134 will contain the byte offset of the checksum field relative to the
1135 end of the IP header.
1136 In this case, the checksum field will be initially
1137 set by the TCP/IP module to the checksum of the pseudo header
1138 defined by the TCP and UDP specifications.
1140 On input, an interface indicates the actions it has performed
1141 on a packet by setting one or more of the following flags in
1143 associated with the packet:
1144 .Bl -tag -width ".Dv CSUM_IP_CHECKED" -offset indent
1145 .It Dv CSUM_IP_CHECKED
1146 The IP header checksum has been computed.
1147 .It Dv CSUM_IP_VALID
1148 The IP header has a valid checksum.
1149 This flag can appear only in combination with
1150 .Dv CSUM_IP_CHECKED .
1151 .It Dv CSUM_DATA_VALID
1152 The checksum of the data portion of the IP packet has been computed
1153 and stored in the field
1155 in network byte order.
1156 .It Dv CSUM_PSEUDO_HDR
1157 Can be set only along with
1159 to indicate that the IP data checksum found in
1161 allows for the pseudo header defined by the TCP and UDP specifications.
1162 Otherwise the checksum of the pseudo header must be calculated by
1163 the host CPU and added to
1165 to obtain the final checksum to be used for TCP or UDP validation purposes.
1168 If a particular network interface just indicates success or
1169 failure of TCP or UDP checksum validation without returning
1170 the exact value of the checksum to the host CPU, its driver can mark
1180 hexadecimal to indicate a valid checksum.
1181 It is a peculiarity of the algorithm used that the Internet checksum
1182 calculated over any valid packet will be
1184 as long as the original checksum field is included.
1186 When running a kernel compiled with the option
1187 .Dv MBUF_STRESS_TEST ,
1190 -controlled options may be used to create
1191 various failure/extreme cases for testing of network drivers
1192 and other parts of the kernel that rely on
1194 .Bl -tag -width ident
1195 .It Va net.inet.ip.mbuf_frag_size
1198 to fragment outgoing
1200 into fragments of the specified size.
1201 Setting this variable to 1 is an excellent way to
1204 handling ability of network drivers.
1205 .It Va kern.ipc.m_defragrandomfailures
1208 to randomly fail, returning
1210 Any piece of code which uses
1212 should be tested with this feature.
1220 .\" Please correct me if I'm wrong
1222 appeared in an early version of
1224 Besides being used for network packets, they were used
1225 to store various dynamic structures, such as routing table
1226 entries, interface addresses, protocol control blocks, etc.
1231 is almost entirely limited to packet storage, with
1233 zones being used directly to store other network-related memory.
1237 allocator has been a special-purpose memory allocator able to run in
1238 interrupt contexts and allocating from a special kernel address space map.
1243 allocator is a wrapper around
1249 + cluster pairs in per-CPU caches, as well as bringing other benefits of
1254 manual page was written by
1259 allocator was written by
projects / FreeBSD / FreeBSD.git / blob