Here's a script which works in ubuntu bionic. i've not enough rep to comment, but, at least in ubuntu bionic, the initramfs looks in /etc/fstab for /usr immediately after mounting the root, so adding an init-bottom script is "too late". instead see the tweak suggested here:
#!/bin/bash -f #copyleft 2018 greg mott #set a subdirectory as root (so multiple installs don't need partitions) #these work in ubuntu bionic, could be different elsewhere #1st choice: tweak initramfs-tools/scripts/local # pro: no risk that some initramfs script might see the partition root # con: only works if the (subdirectory's) initramfs/initrd is tweaked and rebuilt #2nd choice: specify this script as init= on the kernel commandline # pro: no need to tweak the initramfs # con: this script runs in a shell in the partition root #the tweak for bionic:/usr/share/initramfs-tools/scripts/local is replace: # mount ${roflag} ${FSTYPE:+-t ${FSTYPE} }${ROOTFLAGS} ${ROOT} ${rootmnt} # mountroot_status="$?" #with: # (set -x # mkdir part #mountpoint for partition root # part=$rootmnt a=" $(cat<proc/cmdline) " b=${a#* subroot=};[ "$a" = "$b" ]&&echo subroot not specifyed||subroot=${b%% *} part=part #extract subroot from kernel commandline # mount ${roflag} ${FSTYPE:+-t ${FSTYPE} }${ROOTFLAGS} ${ROOT} $part&&[ $subroot ]&&mount --bind part/$subroot $rootmnt&&umount part #mount partition, mount subroot, unmount partition root # ) # mountroot_status="$?" # #sleep 90 #if needed to debug, also remove splash, and maybe quiet, from kernel cmdline #then run "update-initramfs -u" for the current kernel, future kernel installs will do that automatically #for either choice copy /etc/grub.d/40_custom to /etc/grub.d/07_custom and add one or more menuentries that specify subroot: #menuentry "subroot foo" { # echo "subroot foo" # sub=foo # uuid=22e7c84a-a416-43e9-ae9d-ee0119fc3894 #use your partition's uuid # search --no-floppy --fs-uuid --set=root $uuid # linux /$sub/vmlinuz ro root=UUID=$uuid subroot=$sub # echo "initrd /$sub/initrd.img" # initrd /$sub/initrd.img #works in recent releases where the /initrd.img softlink is relative #} #to use this script, in addition to subroot= on the kernel commandline also specify: # init=pat pathname (in partition root) to this script (chmod 744) subroot(){ a=" $(cat<proc/cmdline) " b=${a#* subroot=} [ "$a" = "$b" ]&&{ echo no subroot on kernel cmdline;return 1;}||subroot=${b%% *} #extract subroot from kernel commandline while read -r m r m do for m in $M x #build list of what's already mounted do [[ $r = $m* ]]&&break #exclude subtrees (ie dev/**) done||[[ $r = / ]]||M=$M\ $r #exclude / done<proc/mounts (set -x;mount --bind $subroot mnt)||return #mount subroot for m in $M do (set -x;mount -n --move $m mnt$m)||return #move listed mounts to subroot done set -x cd mnt&& pivot_root . mnt&& #subroot becomes root umount -l mnt #unmount partition root } subroot&&exec chroot . init "$@"||exec bash #land in a shell if anything fails