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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 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.
