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Man Pages


Manual Reference Pages  -  MBUF (9)

NAME

mbuf - memory management in the kernel IPC subsystem

CONTENTS

Synopsis
     Mbuf allocation macros
     Mbuf utility macros
     Mbuf allocation functions
     Mbuf utility functions
Description
     Macros and Functions
Hardware-assisted Checksum Calculation
Stress Testing
Return Values
See Also
History
Authors

SYNOPSIS


.In sys/param.h
.In sys/systm.h
.In sys/mbuf.h

    Mbuf allocation macros

MGET struct mbuf *mbuf int how short type MGETHDR struct mbuf *mbuf int how short type MCLGET struct mbuf *mbuf int how
.Fo MEXTADD struct mbuf *mbuf caddr_t buf u_int size void (*free)(void *opt_arg1, void *opt_arg2) void *opt_arg1 void *opt_arg2 short flags int type
.Fc

    Mbuf utility macros

mtod struct mbuf *mbuf type M_ALIGN struct mbuf *mbuf u_int len MH_ALIGN struct mbuf *mbuf u_int len int M_LEADINGSPACE struct mbuf *mbuf int M_TRAILINGSPACE struct mbuf *mbuf M_MOVE_PKTHDR struct mbuf *to struct mbuf *from M_PREPEND struct mbuf *mbuf int len int how MCHTYPE struct mbuf *mbuf u_int type int M_WRITABLE struct mbuf *mbuf

    Mbuf allocation functions

struct mbuf * m_get int how int type struct mbuf * m_getm struct mbuf *orig int len int how int type struct mbuf * m_getcl int how short type int flags struct mbuf * m_getclr int how int type struct mbuf * m_gethdr int how int type struct mbuf * m_free struct mbuf *mbuf void m_freem struct mbuf *mbuf

    Mbuf utility functions

void m_adj struct mbuf *mbuf int len void m_align struct mbuf *mbuf int len int m_append struct mbuf *mbuf int len c_caddr_t cp struct mbuf * m_prepend struct mbuf *mbuf int len int how struct mbuf * m_copyup struct mbuf *mbuf int len int dstoff struct mbuf * m_pullup struct mbuf *mbuf int len struct mbuf * m_pulldown struct mbuf *mbuf int offset int len int *offsetp struct mbuf * m_copym struct mbuf *mbuf int offset int len int how struct mbuf * m_copypacket struct mbuf *mbuf int how struct mbuf * m_dup struct mbuf *mbuf int how void m_copydata const struct mbuf *mbuf int offset int len caddr_t buf void m_copyback struct mbuf *mbuf int offset int len caddr_t buf struct mbuf *
.Fo m_devget char *buf int len int offset struct ifnet *ifp void (*copy)(char *from, caddr_t to, u_int len)
.Fc void m_cat struct mbuf *m struct mbuf *n u_int m_fixhdr struct mbuf *mbuf void m_dup_pkthdr struct mbuf *to struct mbuf *from void m_move_pkthdr struct mbuf *to struct mbuf *from u_int m_length struct mbuf *mbuf struct mbuf **last struct mbuf * m_split struct mbuf *mbuf int len int how int m_apply struct mbuf *mbuf int off int len int (*f)(void *arg, void *data, u_int len) void *arg struct mbuf * m_getptr struct mbuf *mbuf int loc int *off struct mbuf * m_defrag struct mbuf *m0 int how struct mbuf * m_collapse struct mbuf *m0 int how int maxfrags struct mbuf * m_unshare struct mbuf *m0 int how

DESCRIPTION

An
.Vt mbuf is a basic unit of memory management in the kernel IPC subsystem. Network packets and socket buffers are stored in
.Vt mbufs . A network packet may span multiple
.Vt mbufs arranged into a
.Vt mbuf chain (linked list), which allows adding or trimming network headers with little overhead.

While a developer should not bother with
.Vt mbuf internals without serious reason in order to avoid incompatibilities with future changes, it is useful to understand the general structure of an
.Vt mbuf .

An
.Vt mbuf consists of a variable-sized header and a small internal buffer for data. The total size of an
.Vt mbuf , MSIZE, is a constant defined in
.In sys/param.h . The
.Vt mbuf header includes:
m_next (Vt struct mbuf *) A pointer to the next
.Vt mbuf in the
.Vt mbuf chain .
m_nextpkt
  (Vt struct mbuf *) A pointer to the next
.Vt mbuf chain in the queue.
m_data (Vt caddr_t) A pointer to data attached to this
.Vt mbuf .
m_len (Vt int) The length of the data.
m_type (Vt short) The type of the data.
m_flags (Vt int) The
.Vt mbuf flags.

The
.Vt mbuf flag bits are defined as follows:

/* mbuf flags */
#define M_EXT           0x0001  /* has associated external storage */
#define M_PKTHDR        0x0002  /* start of record */
#define M_EOR           0x0004  /* end of record */
#define M_RDONLY        0x0008  /* associated data marked read-only */
#define M_PROTO1        0x0010  /* protocol-specific */
#define M_PROTO2        0x0020  /* protocol-specific */
#define M_PROTO3        0x0040  /* protocol-specific */
#define M_PROTO4        0x0080  /* protocol-specific */
#define M_PROTO5        0x0100  /* protocol-specific */
#define M_PROTO6        0x4000  /* protocol-specific (avoid M_BCAST conflict) */
#define M_FREELIST      0x8000  /* mbuf is on the free list */

/* mbuf pkthdr flags (also stored in m_flags) */ #define M_BCAST         0x0200  /* send/received as link-level broadcast */ #define M_MCAST         0x0400  /* send/received as link-level multicast */ #define M_FRAG          0x0800  /* packet is fragment of larger packet */ #define M_FIRSTFRAG     0x1000  /* packet is first fragment */ #define M_LASTFRAG      0x2000  /* packet is last fragment */

The available
.Vt mbuf types are defined as follows:

/* mbuf types */
#define MT_DATA         1       /* dynamic (data) allocation */
#define MT_HEADER       MT_DATA /* packet header */
#define MT_SONAME       8       /* socket name */
#define MT_CONTROL      14      /* extra-data protocol message */
#define MT_OOBDATA      15      /* expedited data */

The available external buffer types are defined as follows:

/* external buffer types */
#define EXT_CLUSTER     1       /* mbuf cluster */
#define EXT_SFBUF       2       /* sendfile(2)’s sf_bufs */
#define EXT_JUMBOP      3       /* jumbo cluster 4096 bytes */
#define EXT_JUMBO9      4       /* jumbo cluster 9216 bytes */
#define EXT_JUMBO16     5       /* jumbo cluster 16184 bytes */
#define EXT_PACKET      6       /* mbuf+cluster from packet zone */
#define EXT_MBUF        7       /* external mbuf reference (M_IOVEC) */
#define EXT_NET_DRV     100     /* custom ext_buf provided by net driver(s) */
#define EXT_MOD_TYPE    200     /* custom module’s ext_buf type */
#define EXT_DISPOSABLE  300     /* can throw this buffer away w/page flipping */
#define EXT_EXTREF      400     /* has externally maintained ref_cnt ptr */

If the M_PKTHDR flag is set, a
.Vt struct pkthdr Va m_pkthdr is added to the
.Vt mbuf header. It contains a pointer to the interface the packet has been received from (Vt struct ifnet *rcvif), and the total packet length (Vt int len). Optionally, it may also contain an attached list of packet tags (Vt struct m_tag). See mbuf_tags(9) for details. Fields used in offloading checksum calculation to the hardware are kept in m_pkthdr as well. See HARDWARE-ASSISTED CHECKSUM CALCULATION for details.

If small enough, data is stored in the internal data buffer of an
.Vt mbuf . If the data is sufficiently large, another
.Vt mbuf may be added to the
.Vt mbuf chain , or external storage may be associated with the
.Vt mbuf . MHLEN bytes of data can fit into an
.Vt mbuf with the M_PKTHDR flag set, MLEN bytes can otherwise.

If external storage is being associated with an
.Vt mbuf , the m_ext header is added at the cost of losing the internal data buffer. It includes a pointer to external storage, the size of the storage, a pointer to a function used for freeing the storage, a pointer to an optional argument that can be passed to the function, and a pointer to a reference counter. An
.Vt mbuf using external storage has the M_EXT flag set.

The system supplies a macro for allocating the desired external storage buffer, MEXTADD.

The allocation and management of the reference counter is handled by the subsystem.

The system also supplies a default type of external storage buffer called an
.Vt mbuf cluster .
.Vt Mbuf clusters can be allocated and configured with the use of the MCLGET macro. Each
.Vt mbuf cluster is MCLBYTES in size, where MCLBYTES is a machine-dependent constant. The system defines an advisory macro MINCLSIZE, which is the smallest amount of data to put into an
.Vt mbuf cluster . It is equal to MHLEN plus one. It is typically preferable to store data into the data region of an
.Vt mbuf , if size permits, as opposed to allocating a separate
.Vt mbuf cluster to hold the same data.

    Macros and Functions

There are numerous predefined macros and functions that provide the developer with common utilities.
mtod mbuf type Convert an mbuf pointer to a data pointer. The macro expands to the data pointer cast to the pointer of the specified type. Note: It is advisable to ensure that there is enough contiguous data in mbuf. See m_pullup for details.
MGET mbuf how type Allocate an
.Vt mbuf and initialize it to contain internal data. mbuf will point to the allocated
.Vt mbuf on success, or be set to NULL on failure. The how argument is to be set to M_WAITOK or M_NOWAIT. It specifies whether the caller is willing to block if necessary. A number of other functions and macros related to
.Vt mbufs have the same argument because they may at some point need to allocate new
.Vt mbufs .

Historical
.Vt mbuf allocator (See HISTORY section) used allocation flags M_WAIT and M_DONTWAIT. These constants are kept for compatibility and their use in new code is discouraged.

MGETHDR mbuf how type Allocate an
.Vt mbuf and initialize it to contain a packet header and internal data. See MGET for details.
MEXTADD mbuf buf size free opt_arg1 opt_arg2 flags type Associate externally managed data with mbuf. Any internal data contained in the mbuf will be discarded, and the M_EXT flag will be set. The buf and size arguments are the address and length, respectively, of the data. The free argument points to a function which will be called to free the data when the mbuf is freed; it is only used if type is EXT_EXTREF. The opt_arg1 and opt_arg2 arguments will be passed unmodified to free. The flags argument specifies additional
.Vt mbuf flags; it is not necessary to specify M_EXT. Finally, the type argument specifies the type of external data, which controls how it will be disposed of when the
.Vt mbuf is freed. In most cases, the correct value is EXT_EXTREF.
MCLGET mbuf how Allocate and attach an
.Vt mbuf cluster to mbuf. If the macro fails, the M_EXT flag will not be set in mbuf.
M_ALIGN mbuf len Set the pointer mbuf->m_data to place an object of the size len at the end of the internal data area of mbuf, long word aligned. Applicable only if mbuf is newly allocated with MGET or m_get.
MH_ALIGN mbuf len Serves the same purpose as M_ALIGN does, but only for mbuf newly allocated with MGETHDR or m_gethdr, or initialized by m_dup_pkthdr or m_move_pkthdr.
m_align mbuf len Services the same purpose as M_ALIGN but handles any type of mbuf.
M_LEADINGSPACE mbuf Returns the number of bytes available before the beginning of data in mbuf.
M_TRAILINGSPACE mbuf Returns the number of bytes available after the end of data in mbuf.
M_PREPEND mbuf len how This macro operates on an
.Vt mbuf chain . It is an optimized wrapper for m_prepend that can make use of possible empty space before data (e.g. left after trimming of a link-layer header). The new
.Vt mbuf chain pointer or NULL is in mbuf after the call.
M_MOVE_PKTHDR to from Using this macro is equivalent to calling m_move_pkthdr to from.
M_WRITABLE mbuf This macro will evaluate true if mbuf is not marked M_RDONLY and if either mbuf does not contain external storage or, if it does, then if the reference count of the storage is not greater than 1. The M_RDONLY flag can be set in mbuf->m_flags. This can be achieved during setup of the external storage, by passing the M_RDONLY bit as a flags argument to the MEXTADD macro, or can be directly set in individual
.Vt mbufs .
MCHTYPE mbuf type Change the type of mbuf to type. This is a relatively expensive operation and should be avoided.

The functions are:
m_get how type A function version of MGET for non-critical paths.
m_getm orig len how type Allocate len bytes worth of
.Vt mbufs and
.Vt mbuf clusters if necessary and append the resulting allocated
.Vt mbuf chain to the
.Vt mbuf chain orig, if it is non- NULL. If the allocation fails at any point, free whatever was allocated and return NULL. If orig is non- NULL, it will not be freed. It is possible to use m_getm to either append len bytes to an existing
.Vt mbuf or
.Vt mbuf chain (for example, one which may be sitting in a pre-allocated ring) or to simply perform an all-or-nothing
.Vt mbuf and
.Vt mbuf cluster allocation.
m_gethdr how type A function version of MGETHDR for non-critical paths.
m_getcl how type flags Fetch an
.Vt mbuf with a
.Vt mbuf cluster attached to it. If one of the allocations fails, the entire allocation fails. This routine is the preferred way of fetching both the
.Vt mbuf and
.Vt mbuf cluster together, as it avoids having to unlock/relock between allocations. Returns NULL on failure.
m_getclr how type Allocate an
.Vt mbuf and zero out the data region.
m_free mbuf Frees
.Vt mbuf . Returns m_next of the freed
.Vt mbuf .

The functions below operate on
.Vt mbuf chains .
m_freem mbuf Free an entire
.Vt mbuf chain , including any external storage.
m_adj mbuf len Trim len bytes from the head of an
.Vt mbuf chain if len is positive, from the tail otherwise.
m_append mbuf len cp Append
.Vt len bytes of data
.Vt cp to the
.Vt mbuf chain . Extend the mbuf chain if the new data does not fit in existing space.
m_prepend mbuf len how Allocate a new
.Vt mbuf and prepend it to the
.Vt mbuf chain , handle M_PKTHDR properly. Note: It does not allocate any
.Vt mbuf clusters , so len must be less than MLEN or MHLEN, depending on the M_PKTHDR flag setting.
m_copyup mbuf len dstoff Similar to m_pullup but copies len bytes of data into a new mbuf at dstoff bytes into the mbuf. The dstoff argument aligns the data and leaves room for a link layer header. Returns the new
.Vt mbuf chain on success, and frees the
.Vt mbuf chain and returns NULL on failure. Note: The function does not allocate
.Vt mbuf clusters , so len + dstoff must be less than MHLEN.
m_pullup mbuf len Arrange that the first len bytes of an
.Vt mbuf chain are contiguous and lay in the data area of mbuf, so they are accessible with mtod mbuf type. It is important to remember that this may involve reallocating some mbufs and moving data so all pointers referencing data within the old mbuf chain must be recalculated or made invalid. Return the new
.Vt mbuf chain on success, NULL on failure (the
.Vt mbuf chain is freed in this case). Note: It does not allocate any
.Vt mbuf clusters , so len must be less than or equal to MHLEN.
m_pulldown mbuf offset len offsetp Arrange that len bytes between offset and offset + len in the
.Vt mbuf chain are contiguous and lay in the data area of mbuf, so they are accessible with mtod mbuf type. len must be smaller than, or equal to, the size of an
.Vt mbuf cluster . Return a pointer to an intermediate
.Vt mbuf in the chain containing the requested region; the offset in the data region of the
.Vt mbuf chain to the data contained in the returned mbuf is stored in *offsetp. If offp is NULL, the region may be accessed using mtod mbuf type. If offp is non-NULL, the region may be accessed using mtod mbuf uint8_t + *offsetp. The region of the mbuf chain between its beginning and off is not modified, therefore it is safe to hold pointers to data within this region before calling m_pulldown.
m_copym mbuf offset len how Make a copy of an
.Vt mbuf chain starting offset bytes from the beginning, continuing for len bytes. If len is M_COPYALL, copy to the end of the
.Vt mbuf chain . Note: The copy is read-only, because the
.Vt mbuf clusters are not copied, only their reference counts are incremented.
m_copypacket mbuf how Copy an entire packet including header, which must be present. This is an optimized version of the common case m_copym mbuf 0 M_COPYALL how. Note: the copy is read-only, because the
.Vt mbuf clusters are not copied, only their reference counts are incremented.
m_dup mbuf how Copy a packet header
.Vt mbuf chain into a completely new
.Vt mbuf chain , including copying any
.Vt mbuf clusters . Use this instead of m_copypacket when you need a writable copy of an
.Vt mbuf chain .
m_copydata mbuf offset len buf Copy data from an
.Vt mbuf chain starting off bytes from the beginning, continuing for len bytes, into the indicated buffer buf.
m_copyback mbuf offset len buf Copy len bytes from the buffer buf back into the indicated
.Vt mbuf chain , starting at offset bytes from the beginning of the
.Vt mbuf chain , extending the
.Vt mbuf chain if necessary. Note: It does not allocate any
.Vt mbuf clusters , just adds
.Vt mbufs to the
.Vt mbuf chain . It is safe to set offset beyond the current
.Vt mbuf chain end: zeroed
.Vt mbufs will be allocated to fill the space.
m_length mbuf last Return the length of the
.Vt mbuf chain , and optionally a pointer to the last
.Vt mbuf .
m_dup_pkthdr to from how Upon the function’s completion, the
.Vt mbuf to will contain an identical copy of from->m_pkthdr and the per-packet attributes found in the
.Vt mbuf chain from. The
.Vt mbuf from must have the flag M_PKTHDR initially set, and to must be empty on entry.
m_move_pkthdr to from Move m_pkthdr and the per-packet attributes from the
.Vt mbuf chain from to the
.Vt mbuf to. The
.Vt mbuf from must have the flag M_PKTHDR initially set, and to must be empty on entry. Upon the function’s completion, from will have the flag M_PKTHDR and the per-packet attributes cleared.
m_fixhdr mbuf Set the packet-header length to the length of the
.Vt mbuf chain .
m_devget buf len offset ifp copy Copy data from a device local memory pointed to by buf to an
.Vt mbuf chain . The copy is done using a specified copy routine copy, or bcopy if copy is NULL.
m_cat m n Concatenate n to m. Both
.Vt mbuf chains must be of the same type. N is still valid after the function returned. Note: It does not handle M_PKTHDR and friends.
m_split mbuf len how Partition an
.Vt mbuf chain in two pieces, returning the tail: all but the first len bytes. In case of failure, it returns NULL and attempts to restore the
.Vt mbuf chain to its original state.
m_apply mbuf off len f arg Apply a function to an
.Vt mbuf chain , at offset off, for length len bytes. Typically used to avoid calls to m_pullup which would otherwise be unnecessary or undesirable. arg is a convenience argument which is passed to the callback function f.

Each time f is called, it will be passed arg, a pointer to the data in the current mbuf, and the length len of the data in this mbuf to which the function should be applied.

The function should return zero to indicate success; otherwise, if an error is indicated, then m_apply will return the error and stop iterating through the
.Vt mbuf chain .

m_getptr mbuf loc off Return a pointer to the mbuf containing the data located at loc bytes from the beginning of the
.Vt mbuf chain . The corresponding offset into the mbuf will be stored in *off.
m_defrag m0 how Defragment an mbuf chain, returning the shortest possible chain of mbufs and clusters. If allocation fails and this can not be completed, NULL will be returned and the original chain will be unchanged. Upon success, the original chain will be freed and the new chain will be returned. how should be either M_WAITOK or M_NOWAIT, depending on the caller’s preference.

This function is especially useful in network drivers, where certain long mbuf chains must be shortened before being added to TX descriptor lists.

m_collapse m0 how maxfrags Defragment an mbuf chain, returning a chain of at most maxfrags mbufs and clusters. If allocation fails or the chain cannot be collapsed as requested, NULL will be returned, with the original chain possibly modified. As with m_defrag, how should be one of M_WAITOK or M_NOWAIT.
m_unshare m0 how Create a version of the specified mbuf chain whose contents can be safely modified without affecting other users. If allocation fails and this operation can not be completed, NULL will be returned. The original mbuf chain is always reclaimed and the reference count of any shared mbuf clusters is decremented. how should be either M_WAITOK or M_NOWAIT, depending on the caller’s preference. As a side-effect of this process the returned mbuf chain may be compacted.

This function is especially useful in the transmit path of network code, when data must be encrypted or otherwise altered prior to transmission.

HARDWARE-ASSISTED CHECKSUM CALCULATION

This section currently applies to TCP/IP only. In order to save the host CPU resources, computing checksums is offloaded to the network interface hardware if possible. The m_pkthdr member of the leading
.Vt mbuf of a packet contains two fields used for that purpose,
.Vt int Va csum_flags and
.Vt int Va csum_data . The meaning of those fields depends on the direction a packet flows in, and on whether the packet is fragmented. Henceforth, csum_flags or csum_data of a packet will denote the corresponding field of the m_pkthdr member of the leading
.Vt mbuf in the
.Vt mbuf chain containing the packet.

On output, checksum offloading is attempted after the outgoing interface has been determined for a packet. The interface-specific field ifnet.if_data.ifi_hwassist (see ifnet(9)) is consulted for the capabilities of the interface to assist in computing checksums. The csum_flags field of the packet header is set to indicate which actions the interface is supposed to perform on it. The actions unsupported by the network interface are done in the software prior to passing the packet down to the interface driver; such actions will never be requested through csum_flags.

The flags demanding a particular action from an interface are as follows:
CSUM_IP The IP header checksum is to be computed and stored in the corresponding field of the packet. The hardware is expected to know the format of an IP header to determine the offset of the IP checksum field.
CSUM_TCP
  The TCP checksum is to be computed. (See below.)
CSUM_UDP
  The UDP checksum is to be computed. (See below.)

Should a TCP or UDP checksum be offloaded to the hardware, the field csum_data will contain the byte offset of the checksum field relative to the end of the IP header. In this case, the checksum field will be initially set by the TCP/IP module to the checksum of the pseudo header defined by the TCP and UDP specifications.

On input, an interface indicates the actions it has performed on a packet by setting one or more of the following flags in csum_flags associated with the packet:
CSUM_IP_CHECKED
  The IP header checksum has been computed.
CSUM_IP_VALID The IP header has a valid checksum. This flag can appear only in combination with CSUM_IP_CHECKED.
CSUM_DATA_VALID
  The checksum of the data portion of the IP packet has been computed and stored in the field csum_data in network byte order.
CSUM_PSEUDO_HDR
  Can be set only along with CSUM_DATA_VALID to indicate that the IP data checksum found in csum_data allows for the pseudo header defined by the TCP and UDP specifications. Otherwise the checksum of the pseudo header must be calculated by the host CPU and added to csum_data to obtain the final checksum to be used for TCP or UDP validation purposes.

If a particular network interface just indicates success or failure of TCP or UDP checksum validation without returning the exact value of the checksum to the host CPU, its driver can mark CSUM_DATA_VALID and CSUM_PSEUDO_HDR in csum_flags, and set csum_data to 0xFFFF hexadecimal to indicate a valid checksum. It is a peculiarity of the algorithm used that the Internet checksum calculated over any valid packet will be 0xFFFF as long as the original checksum field is included.

STRESS TESTING

When running a kernel compiled with the option MBUF_STRESS_TEST, the following sysctl 8 -controlled options may be used to create various failure/extreme cases for testing of network drivers and other parts of the kernel that rely on
.Vt mbufs .
net.inet.ip.mbuf_frag_size
  Causes ip_output to fragment outgoing
.Vt mbuf chains into fragments of the specified size. Setting this variable to 1 is an excellent way to test the long
.Vt mbuf chain handling ability of network drivers.
kern.ipc.m_defragrandomfailures
  Causes the function m_defrag to randomly fail, returning NULL. Any piece of code which uses m_defrag should be tested with this feature.

RETURN VALUES

See above.

SEE ALSO

ifnet(9), mbuf_tags(9)

HISTORY


.Vt Mbufs appeared in an early version of BSD . Besides being used for network packets, they were used to store various dynamic structures, such as routing table entries, interface addresses, protocol control blocks, etc. In more recent
.Fx use of
.Vt mbufs is almost entirely limited to packet storage, with uma(9) zones being used directly to store other network-related memory.

Historically, the
.Vt mbuf allocator has been a special-purpose memory allocator able to run in interrupt contexts and allocating from a special kernel address space map. As of
.Fx 5.3 , the
.Vt mbuf allocator is a wrapper around uma(9), allowing caching of
.Vt mbufs , clusters, and
.Vt mbuf + cluster pairs in per-CPU caches, as well as bringing other benefits of slab allocation.

AUTHORS

The original mbuf manual page was written by Yar Tikhiy. The uma(9)
.Vt mbuf allocator was written by Bosko Milekic.
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