Find $DEVICE UUID and swap file offset
Instructions
The most reliable address for a device is the UUID.
To find the UUID of your devices/partitions you may use a command such as blkid.
When setting resume=$DEVICE using a UUID the option should appear as resume=UUID=$UUID.
And when using a swap file the UUID of the device or partition containing the swap file should be used.
Find UUID
Here we will assume it is on the same root / device.
Execute:
findmnt / -o UUID
Find offset
For btrfs:
sudo btrfs inspect-internal map-swapfile -r /swapfile
For all other filesystems:
Execute:
sudo filefrag -v /swapfile | awk '$1=="0:" {print substr($4, 1, length($4)-2)}'