One of the first changes merged for the upcoming 7.0 release was nullfs,
an empty filesystem that cannot actually contain any files.  One might
logically wonder why the kernel would need such a thing.  It turns out,
though, that there are places where a null filesystem can come in handy.
For 7.0, nullfs will be used to make life a bit easier for init
programs; future releases will likely use nullfs to increase the isolation
of kernel threads from the init process.

https://lwn.net/Articles/1062163/
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