Doubts About the Linux Root File System

Solution 1:

The mount command takes information about current mounts from /etc/mtab.

In the past, mtab was a normal file re-created after every boot and updated by the mount command – so it wouldn't have a rootfs / entry simply because the rootfs is never explicitly mounted; it just always exists. (Such a mtab sometimes also has duplicate entries, or entries for filesystems that aren't mounted anymore...)

Many current distros now symlink mtab to /proc/self/mounts, which (like everything else in /proc) is directly generated by the kernel. Because of this, it always shows everything that is currently mounted, including both rootfs / and /dev/blah /.

You can use cat /proc/self/mounts on all distros to compare the mounts. (There also is /proc/self/mountinfo, which uses an incompatible syntax but adds more detail.)