1Kmod: The Kernel Module Loader
2Kirk Petersen
3
4Kmod is a simple replacement for kerneld.  It consists of a
5request_module() replacement and a kernel thread called kmod.  When the
6kernel requests a module, the kmod wakes up and execve()s modprobe,
7passing it the name that was requested.
8
9If you have the /proc filesystem mounted, you can set the path of
10modprobe (where the kernel looks for it) by doing:
11
12	echo "/sbin/modprobe" > /proc/sys/kernel/modprobe
13
14To periodically unload unused modules, put something like the following
15in root's crontab entry:
16
17	0-59/5 * * * * /sbin/rmmod -a
18
19Kmod only loads modules.  Kerneld could do more (although
20nothing in the standard kernel used its other features).  If you
21require features such as request_route, we suggest that you take
22a similar approach.  A simple request_route function could be called,
23and a kroute kernel thread could be sent off to do the work.  But
24we should probably keep this to a minimum.
25
26Kerneld also had a mechanism for storing device driver settings.  This
27can easily be done with modprobe.  When a module is unloaded, modprobe
28could look at a per-driver-configurable location (/proc/sys/drivers/blah)
29for device driver settings and save them to a file.  When a module
30is loaded, simply cat that file back to that location in the proc
31filesystem.  Or perhaps a script could be a setting in /etc/modules.conf.
32There are many user-land methods that will work (I prefer using /proc,
33myself).
34
35If kerneld worked, why replace it?
36
37- kerneld used SysV IPC, which can now be made into a module.  Besides,
38  SysV IPC is ugly and should therefore be avoided (well, certainly for
39  kernel level stuff)
40
41- both kmod and kerneld end up doing the same thing (calling modprobe),
42  so why not skip the middle man?
43
44- removing kerneld related stuff from ipc/msg.c made it 40% smaller
45
46- kmod reports errors through the normal kernel mechanisms, which avoids
47  the chicken and egg problem of kerneld and modular Unix domain sockets
48
49
50Keith Owens <kaos@ocs.com.au> December 1999
51
52The combination of kmod and modprobe can loop, especially if modprobe uses a
53system call that requires a module.  If modules.dep does not exist and modprobe
54was started with the -s option (kmod does this), modprobe tries to syslog() a
55message.  syslog() needs Unix sockets, if Unix sockets are modular then kmod
56runs "modprobe -s net-pf-1".  This runs a second copy of modprobe which
57complains that modules.dep does not exist, tries to use syslog() and starts yet
58another copy of modprobe.  This is not the only possible kmod/modprobe loop,
59just the most common.
60
61To detect loops caused by "modprobe needs a service which is in a module", kmod
62limits the number of concurrent kmod issued modprobes.  See MAX_KMOD_CONCURRENT
63in kernel/kmod.c.  When this limit is exceeded, the kernel issues message "kmod:
64runaway modprobe loop assumed and stopped".
65
66Note for users building a heavily modularised system.  It is a good idea to
67create modules.dep after installing the modules and before booting a kernel for
68the first time.  "depmod -ae m.n.p" where m.n.p is the new kernel version.
69