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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. After a configurable period of
time, kmod will have delete_module() remove any unused modules.
Kmod is configurable through two entries in /proc/sys/kernel. You can
set the path of modprobe (where the kernel looks for it) by doing:
echo "/sbin/modprobe" > /proc/sys/kernel/modprobe
To tell kmod when to unload unused modules, do something like:
echo "120" > /proc/sys/kernel/kmod_unload_delay
Kmod only loads and unloads 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
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