remount chrooted read-only subdirectory (non-mountpoint) to read-write from within jail?

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http://unix.stackexchange.com – I'm trying to do the same of this question (multiple linux installation on same filesystem) but I get this result (root filesystem mounted as read-only) . I'm using debian/sid, have successfully modified the initrd /init script and patched the busybox switch_root to chroot into a subdirectory, the filesystem is ext4. The kernel boot but the filesystem is mounted read-only and any variant of mount -o rw,remount fail. I just guess (not investigated yet) the mount command is unable to resolve the new root / with the old /mountpoint. Mounting the filesystem as read-write before chrooting (as th (HowTos)