> I’m looking for a very simple in-kernel filesystem. What's motivating the desire for to be in-kernel? Nearly, every file system in Plan 9 runs in user space. All the ones that have been mentioned do. The only in-kernel file system in the labs' distribution is devroot which is read-only and intended only to provide enough bits to get the system up and running. 9atom also includes a devtinyfs that you could take a look at.
BLS
