This is an example of the directory layout for the chroot/tmp directory of a SELinux UML instance.
This directory must have enough free space to back the amount of memory given to the guest, otherwise the guest will eventually segfault.
Ideally it should be mounted using tmpfs for better performance, ie: mount -o size=532M -t tmpfs none ./chroot/tmp
As a precaution, just give it slightly more than the amount used by the guest as any file created in /tmp (for example by one of the startup scripts) will consume some of that space.

-rw-r--r--   1 uml-1 nogroup    root_fs
-r-xr-xr-x   1 root  root
-rw-r--r--   1 uml-1 nogroup    swap_file