MGET (struct mbuf *mbuf int how short type);
MGETHDR (struct mbuf *mbuf int how short type);
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_args)
Fa void *opt_args
Fa short flags
Fa int type
Fc Fn 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 *
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
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
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:
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 the sum of
MLEN
and
MHLEN
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.
Fn mtod mbuf type
Convert an
Fa mbuf
pointer to a data pointer.
The macro expands to the data pointer cast to the pointer of the specified
Fa type .
Note
It is advisable to ensure that there is enough contiguous data in
Fa mbuf .
See
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_TRYWAIT
or
M_DONTWAIT
It specifies whether the caller is willing to block if necessary.
If
Fa 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
(sysctl8
tunable)
number of ticks.
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 .
Programmers should be careful not to confuse the
Vt mbuf
allocation flag
M_DONTWAIT
with the
malloc(9)
allocation flag,
M_NOWAIT
They are not the same.
Fn MGETHDR mbuf how type
Allocate an
Vt mbuf
and initialize it to contain a packet header
and internal data.
See
MGET ();
for details.
Fn MCLGET mbuf how
Allocate and attach an
Vt mbuf cluster
to
Fa mbuf .
If the macro fails, the
M_EXT
flag will not be set in
Fa mbuf .
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
MGET ();
or
m_get (.);
Fn MH_ALIGN mbuf len
Serves the same purpose as
M_ALIGN ();
does, but only for
Fa mbuf
newly allocated with
MGETHDR ();
or
m_gethdr (,);
or initialized by
m_dup_pkthdr ();
or
m_move_pkthdr (.);
Fn m_align mbuf len
Services the same purpose as
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
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
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
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
MGET ();
for non-critical paths.
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
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
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_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
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
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
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
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 offp
is NULL, the region may be accessed using
mtod (mbuf type .);
If
Fa 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
Fa off
is not modified, therefore it is safe to hold pointers to data within
this region before calling
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
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
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
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 still valid after the function returned.
Note
It does not handle
M_PKTHDR
and friends.
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 ,
at offset
Fa off ,
for length
Fa len
bytes.
Typically used to avoid calls to
m_pullup ();
which would otherwise be unnecessary or undesirable.
Fa arg
is a convenience argument which is passed to the callback function
Fa f .
Each time
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
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,
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.
Fa 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.
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,
NULL
will be returned.
The original mbuf chain is always reclaimed and the reference
count of any shared mbuf clusters is decremented.
Fa 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
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.
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
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.
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
manual page was written by Yar Tikhiy.
The
uma(9)
Vt mbuf
allocator was written by Bosko Milekic.
