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NAMEmbuf - memory management in the kernel IPC subsystem
SYNOPSISIn sys/param.h In sys/systm.h In sys/mbuf.h
Mbuf allocation macrosFn MGET struct mbuf *mbuf int how short type Fn MGETHDR struct mbuf *mbuf int how short type Ft int Fn MCLGET struct mbuf *mbuf int how Fo MEXTADD Fa struct mbuf *mbuf Fa caddr_t buf Fa u_int size Fa void (*free)(void *opt_arg1, void *opt_arg2) Fa void *opt_arg1 Fa void *opt_arg2 Fa short flags Fa int type Fc
Mbuf utility macrosFn mtod struct mbuf *mbuf type Fn M_ALIGN struct mbuf *mbuf u_int len Fn MH_ALIGN struct mbuf *mbuf u_int len Ft int Fn M_LEADINGSPACE struct mbuf *mbuf Ft int Fn M_TRAILINGSPACE struct mbuf *mbuf Fn M_MOVE_PKTHDR struct mbuf *to struct mbuf *from Fn M_PREPEND struct mbuf *mbuf int len int how Fn MCHTYPE struct mbuf *mbuf short type Ft int Fn M_WRITABLE struct mbuf *mbuf
Mbuf allocation functionsFt struct mbuf * Fn m_get int how short type Ft struct mbuf * Fn m_get2 int size int how short type int flags Ft struct mbuf * Fn m_getm struct mbuf *orig int len int how short type Ft struct mbuf * Fn m_getjcl int how short type int flags int size Ft struct mbuf * Fn m_getcl int how short type int flags Ft struct mbuf * Fn m_getclr int how short type Ft struct mbuf * Fn m_gethdr int how short type Ft struct mbuf * Fn m_free struct mbuf *mbuf Ft void Fn m_freem struct mbuf *mbuf
Mbuf utility functionsFt void Fn m_adj struct mbuf *mbuf int len Ft void Fn m_align struct mbuf *mbuf int len Ft int Fn m_append struct mbuf *mbuf int len c_caddr_t cp Ft struct mbuf * Fn m_prepend struct mbuf *mbuf int len int how Ft struct mbuf * Fn m_copyup struct mbuf *mbuf int len int dstoff Ft struct mbuf * Fn m_pullup struct mbuf *mbuf int len Ft struct mbuf * Fn m_pulldown struct mbuf *mbuf int offset int len int *offsetp Ft struct mbuf * Fn m_copym struct mbuf *mbuf int offset int len int how Ft struct mbuf * Fn m_copypacket struct mbuf *mbuf int how Ft struct mbuf * Fn m_dup struct mbuf *mbuf int how Ft void Fn m_copydata const struct mbuf *mbuf int offset int len caddr_t buf Ft void Fn m_copyback struct mbuf *mbuf int offset int len caddr_t buf Ft struct mbuf * Fo m_devget Fa char *buf Fa int len Fa int offset Fa struct ifnet *ifp Fa void (*copy)(char *from, caddr_t to, u_int len) Fc Ft void Fn m_cat struct mbuf *m struct mbuf *n Ft void Fn m_catpkt struct mbuf *m struct mbuf *n Ft u_int Fn m_fixhdr struct mbuf *mbuf Ft void Fn m_dup_pkthdr struct mbuf *to struct mbuf *from Ft void Fn m_move_pkthdr struct mbuf *to struct mbuf *from Ft u_int Fn m_length struct mbuf *mbuf struct mbuf **last Ft struct mbuf * Fn m_split struct mbuf *mbuf int len int how Ft int Fn m_apply struct mbuf *mbuf int off int len int (*f)(void *arg, void *data, u_int len) void *arg Ft struct mbuf * Fn m_getptr struct mbuf *mbuf int loc int *off Ft struct mbuf * Fn m_defrag struct mbuf *m0 int how Ft struct mbuf * Fn m_collapse struct mbuf *m0 int how int maxfrags Ft struct mbuf * Fn m_unshare struct mbuf *m0 int how
DESCRIPTIONAn 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:
- (Vt struct mbuf ) A pointer to the next Vt mbuf in the Vt mbuf chain .
- (Vt struct mbuf ) A pointer to the next Vt mbuf chain in the queue.
- (Vt caddr_t ) A pointer to data attached to this Vt mbuf .
- (Vt int ) The length of the data.
- (Vt short ) The type of the data.
- (Vt int ) The Vt mbuf flags.
The Vt mbuf flag bits are defined as follows:
/* mbuf flags */ #define M_EXT 0x00000001 /* has associated external storage */ #define M_PKTHDR 0x00000002 /* start of record */ #define M_EOR 0x00000004 /* end of record */ #define M_RDONLY 0x00000008 /* associated data marked read-only */ #define M_PROTO1 0x00001000 /* protocol-specific */ #define M_PROTO2 0x00002000 /* protocol-specific */ #define M_PROTO3 0x00004000 /* protocol-specific */ #define M_PROTO4 0x00008000 /* protocol-specific */ #define M_PROTO5 0x00010000 /* protocol-specific */ #define M_PROTO6 0x00020000 /* protocol-specific */ #define M_PROTO7 0x00040000 /* protocol-specific */ #define M_PROTO8 0x00080000 /* protocol-specific */ #define M_PROTO9 0x00100000 /* protocol-specific */ #define M_PROTO10 0x00200000 /* protocol-specific */ #define M_PROTO11 0x00400000 /* protocol-specific */ #define M_PROTO12 0x00800000 /* protocol-specific */ /* mbuf pkthdr flags (also stored in m_flags) */ #define M_BCAST 0x00000010 /* send/received as link-level broadcast */ #define M_MCAST 0x00000020 /* send/received as link-level multicast */
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 252 /* custom ext_buf provided by net driver(s) */ #define EXT_MOD_TYPE 253 /* custom module's ext_buf type */ #define EXT_DISPOSABLE 254 /* can throw this buffer away w/page flipping */ #define EXT_EXTREF 255 /* 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_tags9 for details. Fields used in offloading checksum calculation to the hardware are kept in m_pkthdr as well. See Sx 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 FunctionsThere are numerous predefined macros and functions that provide the developer with common utilities.
- Fn mtod mbuf type
- Convert an Fa mbuf pointer to a data pointer. The macro expands to the data pointer cast to the specified Fa type . Note It is advisable to ensure that there is enough contiguous data in Fa mbuf . See Fn m_pullup for details.
- Fn MGET mbuf how type
- Allocate an Vt mbuf and initialize it to contain internal data. Fa mbuf will point to the allocated Vt mbuf on success, or be set to NULL on failure. The Fa 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 .
- Fn MGETHDR mbuf how type
- Allocate an Vt mbuf and initialize it to contain a packet header and internal data. See Fn MGET for details.
- Fn MEXTADD mbuf buf size free opt_arg1 opt_arg2 flags type
- Associate externally managed data with Fa mbuf . Any internal data contained in the mbuf will be discarded, and the M_EXT flag will be set. The Fa buf and Fa size arguments are the address and length, respectively, of the data. The Fa free argument points to a function which will be called to free the data when the mbuf is freed; it is only used if Fa type is EXT_EXTREF The Fa opt_arg1 and Fa opt_arg2 arguments will be passed unmodified to Fa free . The Fa flags argument specifies additional Vt mbuf flags; it is not necessary to specify M_EXT Finally, the Fa 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
- Fn MCLGET mbuf how
- Allocate and attach an Vt mbuf cluster to Fa mbuf . On success, a non-zero value returned; otherwise, 0. Historically, consumers would check for success by testing the M_EXT flag on the mbuf, but this is now discouraged to avoid unnecessary awareness of the implementation of external storage in protocol stacks and device drivers.
- Fn M_ALIGN mbuf len
- Set the pointer Fa mbuf->m_data to place an object of the size Fa len at the end of the internal data area of Fa mbuf , long word aligned. Applicable only if Fa mbuf is newly allocated with Fn MGET or Fn m_get .
- Fn MH_ALIGN mbuf len
- Serves the same purpose as Fn M_ALIGN does, but only for Fa mbuf newly allocated with Fn MGETHDR or Fn m_gethdr , or initialized by Fn m_dup_pkthdr or Fn m_move_pkthdr .
- Fn m_align mbuf len
- Services the same purpose as Fn M_ALIGN but handles any type of mbuf.
- Fn M_LEADINGSPACE mbuf
- Returns the number of bytes available before the beginning of data in Fa mbuf .
- Fn M_TRAILINGSPACE mbuf
- Returns the number of bytes available after the end of data in Fa mbuf .
- Fn M_PREPEND mbuf len how
- This macro operates on an Vt mbuf chain . It is an optimized wrapper for Fn 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 Fa mbuf after the call.
- Fn M_MOVE_PKTHDR to from
- Using this macro is equivalent to calling Fn m_move_pkthdr to from .
- Fn M_WRITABLE mbuf
- This macro will evaluate true if Fa mbuf is not marked M_RDONLY and if either Fa 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 Fa mbuf->m_flags . This can be achieved during setup of the external storage, by passing the M_RDONLY bit as a Fa flags argument to the Fn MEXTADD macro, or can be directly set in individual Vt mbufs .
- Fn MCHTYPE mbuf type
- Change the type of Fa mbuf to Fa type . This is a relatively expensive operation and should be avoided.
The functions are:
- Fn m_get how type
- A function version of Fn MGET for non-critical paths.
- Fn m_get2 size how type flags
- Allocate an Vt mbuf with enough space to hold specified amount of data.
- Fn m_getm orig len how type
- Allocate Fa 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 Fa orig , if it is non- NULL If the allocation fails at any point, free whatever was allocated and return NULL If Fa orig is non- NULL it will not be freed. It is possible to use Fn m_getm to either append Fa 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.
- Fn m_gethdr how type
- A function version of Fn MGETHDR for non-critical paths.
- Fn 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.
- Fn m_getjcl how type flags size
- This is like Fn m_getcl but it the size of the cluster allocated will be large enough for Fa size bytes.
- Fn m_getclr how type
- Allocate an Vt mbuf and zero out the data region.
- Fn m_free mbuf
- Frees Vt mbuf . Returns m_next of the freed Vt mbuf .
The functions below operate on Vt mbuf chains .
- Fn m_freem mbuf
- Free an entire Vt mbuf chain , including any external storage.
- Fn m_adj mbuf len
- Trim Fa len bytes from the head of an Vt mbuf chain if Fa len is positive, from the tail otherwise.
- Fn 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.
- Fn 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 Fa len must be less than MLEN or MHLEN depending on the M_PKTHDR flag setting.
- Fn m_copyup mbuf len dstoff
- Similar to Fn m_pullup but copies Fa len bytes of data into a new mbuf at Fa dstoff bytes into the mbuf. The Fa 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 Fa len + dstoff must be less than MHLEN
- Fn m_pullup mbuf len
- Arrange that the first Fa len bytes of an Vt mbuf chain are contiguous and lay in the data area of Fa mbuf , so they are accessible with Fn 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 Fa len must be less than or equal to MHLEN
- Fn m_pulldown mbuf offset len offsetp
- Arrange that Fa len bytes between Fa offset and Fa offset + len in the Vt mbuf chain are contiguous and lay in the data area of Fa mbuf , so they are accessible with Fn mtod mbuf type . Fa 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 Fa *offsetp . If Fa offsetp is NULL, the region may be accessed using Fn mtod mbuf type . If Fa offsetp is non-NULL, the region may be accessed using Fn mtod mbuf uint8_t + *offsetp. The region of the mbuf chain between its beginning and Fa offset is not modified, therefore it is safe to hold pointers to data within this region before calling Fn m_pulldown .
- Fn m_copym mbuf offset len how
- Make a copy of an Vt mbuf chain starting Fa offset bytes from the beginning, continuing for Fa len bytes. If Fa 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.
- Fn m_copypacket mbuf how
- Copy an entire packet including header, which must be present. This is an optimized version of the common case Fn 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.
- Fn 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 Fn m_copypacket when you need a writable copy of an Vt mbuf chain .
- Fn m_copydata mbuf offset len buf
- Copy data from an Vt mbuf chain starting Fa off bytes from the beginning, continuing for Fa len bytes, into the indicated buffer Fa buf .
- Fn m_copyback mbuf offset len buf
- Copy Fa len bytes from the buffer Fa buf back into the indicated Vt mbuf chain , starting at Fa 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 Fa offset beyond the current Vt mbuf chain end: zeroed Vt mbufs will be allocated to fill the space.
- Fn m_length mbuf last
- Return the length of the Vt mbuf chain , and optionally a pointer to the last Vt mbuf .
- Fn m_dup_pkthdr to from how
- Upon the function's completion, the Vt mbuf Fa to will contain an identical copy of Fa from->m_pkthdr and the per-packet attributes found in the Vt mbuf chain Fa from . The Vt mbuf Fa from must have the flag M_PKTHDR initially set, and Fa to must be empty on entry.
- Fn m_move_pkthdr to from
- Move m_pkthdr and the per-packet attributes from the Vt mbuf chain Fa from to the Vt mbuf Fa to . The Vt mbuf Fa from must have the flag M_PKTHDR initially set, and Fa to must be empty on entry. Upon the function's completion, Fa from will have the flag M_PKTHDR and the per-packet attributes cleared.
- Fn m_fixhdr mbuf
- Set the packet-header length to the length of the Vt mbuf chain .
- Fn m_devget buf len offset ifp copy
- Copy data from a device local memory pointed to by Fa buf to an Vt mbuf chain . The copy is done using a specified copy routine Fa copy , or Fn bcopy if Fa copy is NULL
- Fn m_cat m n
- Concatenate Fa n to Fa m . Both Vt mbuf chains must be of the same type. Fa n is not guaranteed to be valid after Fn m_cat returns. Fn m_cat does not update any packet header fields or free mbuf tags.
- Fn m_catpkt m n
- A variant of Fn m_cat that operates on packets. Both Fa m and Fa n must contain packet headers. Fa n is not guaranteed to be valid after Fn m_catpkt returns.
- Fn m_split mbuf len how
- Partition an Vt mbuf chain in two pieces, returning the tail: all but the first Fa len bytes. In case of failure, it returns NULL and attempts to restore the Vt mbuf chain to its original state.
- Fn m_apply mbuf off len f arg
Apply a function to an
Vt mbuf chain ,
Fa off ,
Typically used to avoid calls to
which would otherwise be unnecessary or undesirable.
is a convenience argument which is passed to the callback function
Fa f .
Each time Fn f is called, it will be passed Fa arg , a pointer to the Fa data in the current mbuf, and the length Fa 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 Fn m_apply will return the error and stop iterating through the Vt mbuf chain .
- Fn m_getptr mbuf loc off
- Return a pointer to the mbuf containing the data located at Fa loc bytes from the beginning of the Vt mbuf chain . The corresponding offset into the mbuf will be stored in Fa *off .
- Fn 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,
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.
should be either
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.
- Fn m_collapse m0 how maxfrags
- Defragment an mbuf chain, returning a chain of at most Fa 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 Fn m_defrag , Fa how should be one of M_WAITOK or M_NOWAIT
- Fn 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,
will be returned.
The original mbuf chain is always reclaimed and the reference
count of any shared mbuf clusters is decremented.
should be either
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 CALCULATIONThis 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:
- 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.
- The TCP checksum is to be computed. (See below.)
- 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:
- The IP header checksum has been computed.
- The IP header has a valid checksum. This flag can appear only in combination with CSUM_IP_CHECKED
- The checksum of the data portion of the IP packet has been computed and stored in the field csum_data in network byte order.
- 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 TESTINGWhen 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 .
- Causes Fn 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.
- Causes the function Fn m_defrag to randomly fail, returning NULL Any piece of code which uses Fn m_defrag should be tested with this feature.
RETURN VALUESSee above.
SEE ALSOifnet(9), mbuf_tags9
HISTORYVt 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.