zswap: being able to recognize that part of SWAP space is allocated inside RAM
Zswap is a kernel feature that provides a compressed RAM cache for swap pages. Pages which would otherwise be swapped out to disk are instead compressed and stored into a memory pool in RAM. Once the pool is full or the RAM is exhausted, the least recently used (LRU) page is decompressed and written to disk, as if it had not been intercepted. After the page has been decompressed into the swap cache, the compressed version in the pool can be freed. 
As recently I've read I and others noticed that behavior with enabled zswap is weird, displaying incorrect informations.
This setup will be most likely commonly used by all Linux phones (Debian, Mobian etc.) and is being used even on desktops often.