This question already has an answer here:
Please take care that you have no unsaved work if you test this.
I ran this command as an experiment, but how can I undo it without rebooting?
# mount --rbind / /mnt
What I have tried
# umount /mnt umount: /mnt: target is busy.
# strace -f -e umount,umount2 umount -R /mnt umount2("/mnt/var/lib/nfs/rpc_pipefs", 0) = 0 umount2("/mnt/sys/kernel/security", 0) = 0 umount2("/mnt/sys/fs/cgroup/unified", 0) = -1 EBUSY (Device or resource busy) umount: /mnt/sys/fs/cgroup/unified: target is busy. +++ exited with 32 +++
# umount -l /mnt # findmnt findmnt: can't read /proc/mounts: No such file or directory
(this is the catastrophe I warned you about).