I know that this is an old issue but I can't find the solution no matter how much I've Googled.
I've build a 2.6.27 64 bit kernel and build modules with make modules ARCH=um.
When I run the linux binary with mem=256M I can "insmod" the module loop.ko without any problem.
However if I use mem=512M and try to "insmod" loop.ko I'm getting the message;
# cd /lib/modules
# insmod loop.ko
overflow in relocation type 11 val 848351e2
`loop' likely not compiled with -mcmodel=kernel
insmod: cannot insert 'loop.ko': Invalid module format (-1): Exec format error
My host is;
$ uname -a
Linux lnx6004 2.6.9-42.ELsmp #1 SMP Wed Jul 12 23:32:02 EDT 2006 x86_64 x86_64 x86_64 GNU/Linux