Its features are steadily getting added to the Linux kernel, at least in some form: - You have FUSE, which has some overlap with 9P. You lose the network trans

Its features are steadily getting added to the Linux kernel, at least in some form: - You have FUSE, which has some overlap with 9P. You lose the network transparency, but still can build specialized filesystems entirely in userspace with no special permissions, rather than requiring a kernel driver. - The clone(2) system call, which has a lot of options equivalent to those of Plan 9's rfork(2) allowing explicit control of what gets shared between the parent and child processes. - In particular, the filesystem namespace (read: mount table) can be one of the unshared items, though my understanding is that this still requires root permissions.