fallocate -l 4G /swapfile
# if no fallocate on your system, use the following
# dd if=/dev/zero of=/swapfile bs=1024 count=1048576
chmod 600 /swapfile
# use "force" to use the entire "device"
mkswap -f /swapfile
# save, then modify fstab
cp -a /etc/fstab /etc/fstab.save
echo '/swapfile swap swap defaults 0 0' >> /etc/fstab
# turn on swap for everything in fstab
swapon -a
# display the result
swapon --show