I have seen that a workaround exist to be able to launch several virtual machine with COW files.
The VDSOs provided by the host kernel are confusing UML. Until this is fixed in UML, a workaround to to disable CONFIG_COMPAT_VDSO on the host.
Someone could explain me how to disable the variable CONFIG_COMPAT_VDSO ?
Note : I have download the following kernel http://user-mode-linux.sourceforge.net/linux-2.6.22-rc2.bz2