First of all, thanks this r news for me. But I don't think is a good idea to use the swap file in btrfs.

It is supported since kernel 5.0

There are some limitations of the implementation in BTRFS and Linux swap subsystem:

filesystem - must be only single device

filesystem - must have only single data profile

subvolume - cannot be snapshotted if it contains any active swapfiles

swapfile - must be preallocated (i.e. no holes)

swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)

With active swapfiles, the following whole-filesystem operations will skip swapfile extents or may fail:

balance - block groups with extents of any active swapfiles are skipped and reported, the rest will be processed normally

resize grow - unaffected

resize shrink - works as long as the extents of any active swapfiles are outside of the shrunk range

device add - if the new devices do not interfere with any already active swapfiles this operation will work, though no new swapfile can be activated afterwards

device delete - if the device has been added as above, it can be also deleted

device replace - ditto