[$] The hard life of a virtual-filesystem developer

Filesystem development is not an easy task; the performance demands are
typically high, and the consequences for mistakes usually involve lost data
and irate users.  The implementation of a virtual (or "pseudo") filesystem
— a filesystem implemented within the kernel and lacking a normal backing
store — can also be challenging, but for different reasons.  A series of
conversations around the eventfs virtual filesystem has turned a spotlight
on the difficulty of creating a virtual filesystem for Linux.

https://lwn.net/Articles/960088/