mount cmd 後見到

rootfs on / type rootfs

rootfs is a special instance of ramfs (or tmpfs, if "CONFIG_TMPFS" is enabled)

when the kernel at boot mounts initramfs as rootfs

gzipped "cpio" format archive, which is extracted into rootfs when the kernel boots up.

After extracting, the kernel checks to see if rootfs contains a file "init", and if so it executes it as PID 1.

this init process is responsible for bringing the system the rest of the way up,

including locating and mounting the real root device