I know this is an old thread but I had the same problem and when I found it I wished that someone had answered the question :-)
I think if the currently running kernel image does not have
Multi devices support (RAID) -> select device mapper support
turned on (can be a module) then this warning will be issued. Also, the module this option builds has to be loaded (if you build it as a module). You can check if it is loaded by running:
cat /proc/modules | grep dm_mod
If it is not in the list, but you have compiled it as a module for the running kernel, you should run
For me after doing so, the warning has disappeared.
PS: I am using Slackware 12, kernel 188.8.131.52. I am not sure if this works on other systems.
Last edited by bjdodo; 11-27-2007 at 04:46 AM.