Support swap without NODATACOW

[DemiMarie]

Swap on btrfs currently requires NODATACOW, which implies NODATASUM. bcachefs will be able to do swap with the normal I/O path, including CoW and checksumming. This is an RFE for btrfs to have the same ability.

[kdave]

IIRC it should be possible to wire the reads/writes to direct io, but this was not available at the time the support was added initially.

[DemiMarie]

IIRC it should be possible to wire the reads/writes to direct io, but this was not available at the time the support was added initially.

It is, but this is only safe if btrfs does not need to allocate memory in the write path. Otherwise, a deadlock may result.