MBUF(9) MidnightBSD Kernel Developer’s Manual MBUF(9)

NAME

mbuf — memory management in the kernel IPC subsystem

SYNOPSIS

#include <sys/param.h>
#include <sys/systm.h>
#include <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);

MEXTADD(struct mbuf *mbuf, caddr_t buf, u_int size, void (*free)(void *opt_args), void *opt_args, short flags, int type);

MEXTFREE(struct mbuf *mbuf);

MFREE(struct mbuf *mbuf, struct mbuf *successor);

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 *

m_devget(char *buf, int len, int offset, struct ifnet *ifp, void (*copy)(char *from, caddr_t to, u_int len));

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_unshare(struct mbuf *m0, int how);

DESCRIPTION

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

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

An mbuf consists of a variable-sized header and a small internal buffer for data. The total size of an mbuf, MSIZE, is a constant defined in <sys/param.h>. The mbuf header includes:

m_next

(struct mbuf *) A pointer to the next mbuf in the mbuf chain.

m_nextpkt

(struct mbuf *) A pointer to the next mbuf chain in the queue.

m_data

(caddr_t) A pointer to data attached to this mbuf.

m_len

(int) The length of the data.

m_type

(short) The type of the data.

m_flags

(int) The mbuf flags.

The 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 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 */

If the M_PKTHDR flag is set, a struct pkthdr m_pkthdr is added to the mbuf header. It contains a pointer to the interface the packet has been received from (struct ifnet *rcvif), and the total packet length (int len). Optionally, it may also contain an attached list of packet tags (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 mbuf. If the data is sufficiently large, another mbuf may be added to the mbuf chain, or external storage may be associated with the mbuf. MHLEN bytes of data can fit into an mbuf with the M_PKTHDR flag set, MLEN bytes can otherwise.

If external storage is being associated with an 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 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 mbuf cluster. Mbuf clusters can be allocated and configured with the use of the MCLGET macro. Each 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 mbuf cluster. It is equal to the sum of MLEN and MHLEN. It is typically preferable to store data into the data region of an mbuf, if size permits, as opposed to allocating a separate 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 mbuf and initialize it to contain internal data. mbuf will point to the allocated mbuf on success, or be set to NULL on failure. The how argument is to be set to M_TRYWAIT or M_DONTWAIT. It specifies whether the caller is willing to block if necessary. If how is set to M_TRYWAIT, a failed allocation will result in the caller being put to sleep for a designated kern.ipc.mbuf_wait (sysctl(8) tunable) number of ticks. A number of other functions and macros related to mbufs have the same argument because they may at some point need to allocate new mbufs.

Programmers should be careful not to confuse the mbuf allocation flag M_DONTWAIT with the malloc(9) allocation flag, M_NOWAIT. They are not the same.

MGETHDR(mbuf, how, type)

Allocate an mbuf and initialize it to contain a packet header and internal data. See MGET() for details.

MCLGET(mbuf, how)

Allocate and attach an 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 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 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 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 mbufs and mbuf clusters if necessary and append the resulting allocated mbuf chain to the 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 mbuf or mbuf chain (for example, one which may be sitting in a pre-allocated ring) or to simply perform an all-or-nothing mbuf and mbuf cluster allocation.

m_gethdr(how, type)

A function version of MGETHDR() for non-critical paths.

m_getcl(how, type, flags)

Fetch an mbuf with a 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 mbuf and mbuf cluster together, as it avoids having to unlock/relock between allocations. Returns NULL on failure.

m_getclr(how, type)

Allocate an mbuf and zero out the data region.

m_free(mbuf)

Frees mbuf. Returns m_next of the freed mbuf.

The functions below operate on mbuf chains.

m_freem(mbuf)

Free an entire mbuf chain, including any external storage.

m_adj(mbuf, len)

Trim len bytes from the head of an mbuf chain if len is positive, from the tail otherwise.

m_append(mbuf, len, cp)

Append len bytes of data cp to the mbuf chain. Extend the mbuf chain if the new data does not fit in existing space.

m_prepend(mbuf, len, how)

Allocate a new mbuf and prepend it to the mbuf chain, handle M_PKTHDR properly. Note: It does not allocate any 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 mbuf chain on success, and frees the mbuf chain and returns NULL on failure. Note: The function does not allocate mbuf clusters, so len + dstoff must be less than MHLEN.

m_pullup(mbuf, len)

Arrange that the first len bytes of an 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 mbuf chain on success, NULL on failure (the mbuf chain is freed in this case). Note: It does not allocate any mbuf clusters, so len must be less than MHLEN.

m_pulldown(mbuf, offset, len, offsetp)

Arrange that len bytes between offset and offset + len in the 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 mbuf cluster. Return a pointer to an intermediate mbuf in the chain containing the requested region; the offset in the data region of the 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 mbuf chain starting offset bytes from the beginning, continuing for len bytes. If len is M_COPYALL, copy to the end of the mbuf chain. Note: The copy is read-only, because the 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 mbuf clusters are not copied, only their reference counts are incremented.

m_dup(mbuf, how)

Copy a packet header mbuf chain into a completely new mbuf chain, including copying any mbuf clusters. Use this instead of m_copypacket() when you need a writable copy of an mbuf chain.

m_copydata(mbuf, offset, len, buf)

Copy data from an 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 mbuf chain, starting at offset bytes from the beginning of the mbuf chain, extending the mbuf chain if necessary. Note: It does not allocate any mbuf clusters, just adds mbufs to the mbuf chain. It is safe to set offset beyond the current mbuf chain end: zeroed mbufs will be allocated to fill the space.

m_length(mbuf, last)

Return the length of the mbuf chain, and optionally a pointer to the last mbuf.

m_dup_pkthdr(to, from, how)

Upon the function’s completion, the mbuf to will contain an identical copy of from->m_pkthdr and the per-packet attributes found in the mbuf chain from. The 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 mbuf chain from to the mbuf to. The 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 mbuf chain.

m_devget(buf, len, offset, ifp, copy)

Copy data from a device local memory pointed to by buf to an 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 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 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 mbuf chain to its original state.

m_apply(mbuf, off, len, f, arg)

Apply a function to an 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 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 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_TRYWAIT or M_DONTWAIT, 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_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_TRYWAIT or M_DONTWAIT, 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 mbuf of a packet contains two fields used for that purpose, int csum_flags and int 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 mbuf in the 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.

For outbound packets which have been fragmented by the host CPU, the following will also be true, regardless of the checksum flag settings:

all fragments will have the flag M_FRAG set in their m_flags field;

the first and the last fragments in the chain will have M_FIRSTFRAG or M_LASTFRAG set in their m_flags, correspondingly;

the first fragment in the chain will have the total number of fragments contained in its csum_data field.

The last rule for fragmented packets takes precedence over the one for a TCP or UDP checksum. Nevertheless, offloading a TCP or UDP checksum is possible for a fragmented packet if the flag CSUM_IP_FRAGS is set in the field ifnet.if_data.ifi_hwassist associated with the network interface. However, in this case the interface is expected to figure out the location of the checksum field within the sequence of fragments by itself because csum_data contains a fragment count instead of a checksum offset value.

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.

For inbound packets which are IP fragments, all csum_data fields will be summed during reassembly to obtain the final checksum value passed to an upper layer in the csum_data field of the reassembled packet. The csum_flags fields of all fragments will be consolidated using logical AND to obtain the final value for csum_flags. Thus, in order to successfully offload checksum computation for fragmented data, all fragments should have the same value of csum_flags.

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 mbufs.

net.inet.ip.mbuf_frag_size

Causes ip_output() to fragment outgoing mbuf chains into fragments of the specified size. Setting this variable to 1 is an excellent way to test the long 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

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 FreeBSD use of mbufs is almost entirely limited to packet storage, with uma(9) zones being used directly to store other network-related memory.

Historically, the 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 FreeBSD 5.3, the mbuf allocator is a wrapper around uma(9), allowing caching of mbufs, clusters, and 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) mbuf allocator was written by Bosko Milekic.

MidnightBSD 0.3 February 26, 2007 MidnightBSD 0.3