One other interesting point about early root filesystems: In a latter day 2.4 kernel, there is a root filesystem even earlier than the initial ramdisk one (initrd). rootfs is built into the kernel, and always contains a handful of filesystem that you need to get up far enough to load and mount the initrd filesystem (the root directory and a few device special files, as I recall). And it never gets unmounted; it's always down there with the real root filesystem mounted over it.
I don't know 2.6 booting, but it sounds from the article like 2.6's rootfs is an extension of this.
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