Based on in-depth analysis, I found the cause of the error：
“Apr 03 12:59:15 ubuntu systemd[1]: Failed to set up mount unit: Device
or resource busy”
call tree on systemd mount.c & unit.c:
mount_dispatch_io
-> mount_load_proc_self_mountinfo
-> mount_setup_unit
-> mount_setup_existing_unit
-> mount_add_extras
-> unit_set_default_slice:
-> unit_set_slice:
if (unit_active_state(u) != UNIT_INACTIVE)
return -EBUSY;
"unit_set_slice" return EBUSY always, because of nfsroot always active
state in netbooting,
"mount_dispatch_io" give up updating mount state when
"mount_load_proc_self_mountinfo" return the error.
finally, all systemd mount service failed and then goto emergency shell.
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https://bugs.launchpad.net/bugs/1755863
Title:
netbooting the bionic live CD over NFS goes straight to maintenance
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