187 lines
8.1 KiB
Perl
187 lines
8.1 KiB
Perl
.\" $NetBSD: 5.t,v 1.2 1998/01/09 06:55:43 perry Exp $
|
|
.\"
|
|
.\" Copyright (c) 1983, 1986, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)5.t 8.1 (Berkeley) 6/8/93
|
|
.\"
|
|
.nr H2 1
|
|
.\".ds RH "Memory management
|
|
.br
|
|
.ne 2i
|
|
.NH
|
|
\s+2Memory management\s0
|
|
.PP
|
|
A single mechanism is used for data storage: memory buffers, or
|
|
\fImbuf\fP's. An mbuf is a structure of the form:
|
|
.DS
|
|
._f
|
|
struct mbuf {
|
|
struct mbuf *m_next; /* next buffer in chain */
|
|
u_long m_off; /* offset of data */
|
|
short m_len; /* amount of data in this mbuf */
|
|
short m_type; /* mbuf type (accounting) */
|
|
u_char m_dat[MLEN]; /* data storage */
|
|
struct mbuf *m_act; /* link in higher-level mbuf list */
|
|
};
|
|
.DE
|
|
The \fIm_next\fP field is used to chain mbufs together on linked
|
|
lists, while the \fIm_act\fP field allows lists of mbuf chains to be
|
|
accumulated. By convention, the mbufs common to a single object
|
|
(for example, a packet) are chained together with the \fIm_next\fP
|
|
field, while groups of objects are linked via the \fIm_act\fP
|
|
field (possibly when in a queue).
|
|
.PP
|
|
Each mbuf has a small data area for storing information, \fIm_dat\fP.
|
|
The \fIm_len\fP field indicates the amount of data, while the \fIm_off\fP
|
|
field is an offset to the beginning of the data from the base of the
|
|
mbuf. Thus, for example, the macro \fImtod\fP, which converts a pointer
|
|
to an mbuf to a pointer to the data stored in the mbuf, has the form
|
|
.DS
|
|
._d
|
|
#define mtod(\fIx\fP,\fIt\fP) ((\fIt\fP)((int)(\fIx\fP) + (\fIx\fP)->m_off))
|
|
.DE
|
|
(note the \fIt\fP parameter, a C type cast, which is used to cast
|
|
the resultant pointer for proper assignment).
|
|
.PP
|
|
In addition to storing data directly in the mbuf's data area, data
|
|
of page size may be also be stored in a separate area of memory.
|
|
The mbuf utility routines maintain
|
|
a pool of pages for this purpose and manipulate a private page map
|
|
for such pages.
|
|
An mbuf with an external data area may be recognized by the larger
|
|
offset to the data area;
|
|
this is formalized by the macro M_HASCL(\fIm\fP), which is true
|
|
if the mbuf whose address is \fIm\fP has an external page cluster.
|
|
An array of reference counts on pages is also maintained
|
|
so that copies of pages may be made without core to core
|
|
copying (copies are created simply by duplicating the reference to the data
|
|
and incrementing the associated reference counts for the pages).
|
|
Separate data pages are currently used only
|
|
when copying data from a user process into the kernel,
|
|
and when bringing data in at the hardware level. Routines which
|
|
manipulate mbufs are not normally aware whether data is stored directly in
|
|
the mbuf data array, or if it is kept in separate pages.
|
|
.PP
|
|
The following may be used to allocate and free mbufs:
|
|
.LP
|
|
m = m_get(wait, type);
|
|
.br
|
|
MGET(m, wait, type);
|
|
.IP
|
|
The subroutine \fIm_get\fP and the macro \fIMGET\fP
|
|
each allocate an mbuf, placing its address in \fIm\fP.
|
|
The argument \fIwait\fP is either M_WAIT or M_DONTWAIT according
|
|
to whether allocation should block or fail if no mbuf is available.
|
|
The \fItype\fP is one of the predefined mbuf types for use in accounting
|
|
of mbuf allocation.
|
|
.IP "MCLGET(m);"
|
|
This macro attempts to allocate an mbuf page cluster
|
|
to associate with the mbuf \fIm\fP.
|
|
If successful, the length of the mbuf is set to CLSIZE,
|
|
the size of the page cluster.
|
|
.LP
|
|
n = m_free(m);
|
|
.br
|
|
MFREE(m,n);
|
|
.IP
|
|
The routine \fIm_free\fP and the macro \fIMFREE\fP
|
|
each free a single mbuf, \fIm\fP, and any associated external storage area,
|
|
placing a pointer to its successor in the chain it heads, if any, in \fIn\fP.
|
|
.IP "m_freem(m);"
|
|
This routine frees an mbuf chain headed by \fIm\fP.
|
|
.PP
|
|
The following utility routines are available for manipulating mbuf
|
|
chains:
|
|
.IP "m = m_copy(m0, off, len);"
|
|
.br
|
|
The \fIm_copy\fP routine create a copy of all, or part, of a
|
|
list of the mbufs in \fIm0\fP. \fILen\fP bytes of data, starting
|
|
\fIoff\fP bytes from the front of the chain, are copied.
|
|
Where possible, reference counts on pages are used instead
|
|
of core to core copies. The original mbuf chain must have at
|
|
least \fIoff\fP + \fIlen\fP bytes of data. If \fIlen\fP is
|
|
specified as M_COPYALL, all the data present, offset
|
|
as before, is copied.
|
|
.IP "m_cat(m, n);"
|
|
.br
|
|
The mbuf chain, \fIn\fP, is appended to the end of \fIm\fP.
|
|
Where possible, compaction is performed.
|
|
.IP "m_adj(m, diff);"
|
|
.br
|
|
The mbuf chain, \fIm\fP is adjusted in size by \fIdiff\fP
|
|
bytes. If \fIdiff\fP is non-negative, \fIdiff\fP bytes
|
|
are shaved off the front of the mbuf chain. If \fIdiff\fP
|
|
is negative, the alteration is performed from back to front.
|
|
No space is reclaimed in this operation; alterations are
|
|
accomplished by changing the \fIm_len\fP and \fIm_off\fP
|
|
fields of mbufs.
|
|
.IP "m = m_pullup(m0, size);"
|
|
.br
|
|
After a successful call to \fIm_pullup\fP, the mbuf at
|
|
the head of the returned list, \fIm\fP, is guaranteed
|
|
to have at least \fIsize\fP
|
|
bytes of data in contiguous memory within the data area of the mbuf
|
|
(allowing access via a pointer, obtained using the \fImtod\fP macro,
|
|
and allowing the mbuf to be located from a pointer to the data area
|
|
using \fIdtom\fP, defined below).
|
|
If the original data was less than \fIsize\fP bytes long,
|
|
\fIlen\fP was greater than the size of an mbuf data
|
|
area (112 bytes), or required resources were unavailable,
|
|
\fIm\fP is 0 and the original mbuf chain is deallocated.
|
|
.IP
|
|
This routine is particularly useful when verifying packet
|
|
header lengths on reception. For example, if a packet is
|
|
received and only 8 of the necessary 16 bytes required
|
|
for a valid packet header are present at the head of the list
|
|
of mbufs representing the packet, the remaining 8 bytes
|
|
may be ``pulled up'' with a single \fIm_pullup\fP call.
|
|
If the call fails the invalid packet will have been discarded.
|
|
.PP
|
|
By insuring that mbufs always reside on 128 byte boundaries,
|
|
it is always possible to locate the mbuf associated with a data
|
|
area by masking off the low bits of the virtual address.
|
|
This allows modules to store data structures in mbufs and
|
|
pass them around without concern for locating the original
|
|
mbuf when it comes time to free the structure.
|
|
Note that this works only with objects stored in the internal data
|
|
buffer of the mbuf.
|
|
The \fIdtom\fP macro is used to convert a pointer into an mbuf's
|
|
data area to a pointer to the mbuf,
|
|
.DS
|
|
#define dtom(x) ((struct mbuf *)((int)x & ~(MSIZE-1)))
|
|
.DE
|
|
.PP
|
|
Mbufs are used for dynamically allocated data structures such as
|
|
sockets as well as memory allocated for packets and headers. Statistics are
|
|
maintained on mbuf usage and can be viewed by users using the
|
|
\fInetstat\fP\|(1) program.
|