on an read-only root

[chengjianwen]

My pi boot by nfs from a piserver, and I want to use overlayroot to set root writable. I succeed on the os from piserver (kernel version: 4.19.97-v7+), but failed on the latest raspberry pi os (kernel version: 5.4.51-v7l+).
I set debug option on cmdline.txt to see the initramfs debug, and found the error occurred on init-bottom-overlay script from line 64:
mount -t overlay overlay -olowerdir=/overlay/lower,upperdir=/overlay/upper,workdir=/overlay/work ${rootmnt}
mount: mount overlay on /root failed: No such device.
cp: can't stat '/root/etc/fstab': No such file or directory.
/script/init-bottom/init-bottom-overlay: line 71: can't create /root/etc/fstab: nonexistent directory.
/script/init-bottom/init-bottom-overlay: line 72: can't create /root/etc/fstab: nonexistent directory.

[chesty]

It drops you into a busybox shell? The only thing I can suggest is to look around and find where the read-only nfs root is mounted, if it's mounted, maybe it's not mounted yet?

[chengjianwen]

It is mounted on ${rootmnt} already.
It seems the script want to move the mount point to another directory (/overlay/lower), but failed.

[chesty]

oh, because it's read-only. You'll need to comment out lines 70, 71 and 72. They are trying to write to ${rootmnt}

edit: no, that's not it. I guess you'll have to look into why moving the mount point doesn't work, or maybe you can modify the script so it doesn't need to move it.

[chengjianwen]

I think the problem was line 64:
mount -t overlay overlay -olowerdir=/overlay/lower,upperdir=/overlay/upper,workdir=/overlay/work ${rootmnt}
error msg:
mount: mounting overlay on /root failed: no such device.

[chengjianwen]

mount -o move worked, and mount -t overlay overlay ..... failed.

[chesty]

If you want help you need to give move information. xxx failed isn't enough. what is the exact command you ran and what is the exact error message.