What command can be used to force release everything in swap partition back to memory?

Question

What command can be used to force release everything in swap partition back to memory ?

Presume that I have enough memory.

Answer 1

From this Ask Ubuntu question:

You can also clear your swap by running swapoff -a and then swapon -a as root instead of rebooting to achieve the same effect.

Thus:

$ free -tm
...
Swap:         6439        196       6243
...
$ sudo swapoff -a
$ sudo swapon -a
$ free -tm
...
Swap:         6439          0       6439
...

As noted in a comment, if you don't have enough memory, swapoff will result in "out of memory" errors and on the kernel killing processes to recover RAM.