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Kmod: The Kernel Module Loader
Kirk Petersen

Kmod is a simple replacement for kerneld.  It consists of a 
request_module() replacement and a kernel thread called kmod.  When the
kernel requests a module, the kmod wakes up and execve()s modprobe,
passing it the name that was requested.

If you have the /proc filesystem mounted, you can set the path of
modprobe (where the kernel looks for it) by doing:

        echo "/sbin/modprobe" > /proc/sys/kernel/modprobe

To periodically unload unused modules, put something like the following
in root's crontab entry:

        0-59/5 * * * * /sbin/rmmod -a

Kmod only loads modules.  Kerneld could do more (although
nothing in the standard kernel used its other features).  If you
require features such as request_route, we suggest that you take
a similar approach.  A simple request_route function could be called,
and a kroute kernel thread could be sent off to do the work.  But
we should probably keep this to a minimum.

Kerneld also had a mechanism for storing device driver settings.  This
can easily be done with modprobe.  When a module is unloaded, modprobe
could look at a per-driver-configurable location (/proc/sys/drivers/blah)
for device driver settings and save them to a file.  When a module
is loaded, simply cat that file back to that location in the proc
filesystem.  Or perhaps a script could be a setting in /etc/modules.conf.
There are many user-land methods that will work (I prefer using /proc,
myself).

If kerneld worked, why replace it?

- kerneld used SysV IPC, which can now be made into a module.  Besides,
  SysV IPC is ugly and should therefore be avoided (well, certainly for
  kernel level stuff)

- both kmod and kerneld end up doing the same thing (calling modprobe),
  so why not skip the middle man?

- removing kerneld related stuff from ipc/msg.c made it 40% smaller

- kmod reports errors through the normal kernel mechanisms, which avoids
  the chicken and egg problem of kerneld and modular Unix domain sockets


Keith Owens <kaos@ocs.com.au> December 1999

The combination of kmod and modprobe can loop, especially if modprobe uses a
system call that requires a module.  If modules.dep does not exist and modprobe
was started with the -s option (kmod does this), modprobe tries to syslog() a
message.  syslog() needs Unix sockets, if Unix sockets are modular then kmod
runs "modprobe -s net-pf-1".  This runs a second copy of modprobe which
complains that modules.dep does not exist, tries to use syslog() and starts yet
another copy of modprobe.  This is not the only possible kmod/modprobe loop,
just the most common.

To detect loops caused by "modprobe needs a service which is in a module", kmod
limits the number of concurrent kmod issued modprobes.  See MAX_KMOD_CONCURRENT
in kernel/kmod.c.  When this limit is exceeded, the kernel issues message "kmod:
runaway modprobe loop assumed and stopped".

Note for users building a heavily modularised system.  It is a good idea to
create modules.dep after installing the modules and before booting a kernel for
the first time.  "depmod -ae m.n.p" where m.n.p is the new kernel version.