Swap should take place on the Instance Storage (ephemeral) disk and not an EBS device. Swapping will cause a lot of IO and will increase cost on EBS. EBS is also slower than the Instance Store and the Instance Store comes free with the EC2 Instance.
It will usually be mounted to /mnt but if not run
sudo mount /dev/xvda2 /mnt
To then create a swap file on this device do the following for a 4GB swapfile
sudo dd if=/dev/zero of=/mnt/swapfile bs=1M count=4096
Make sure no other user can view the swap file
sudo chown root:root /mnt/swapfile
sudo chmod 600 /mnt/swapfile
Make and Flag as swap
sudo mkswap /mnt/swapfile
sudo swapon /mnt/swapfile
Add/Make sure the following are in your /etc/fstab
/dev/xvda2 /mnt auto defaults,nobootwait,comment=cloudconfig 0 2
/mnt/swapfile swap swap defaults 0 0
and lastly enable swap
sudo swapon -a