[SOLVED] On my PC, I have multiple different installed operating systems with root on zfs.
These are arranged:
rpool
rpool/root
rpool/root/arch
rpool/root/kubuntu21
rpool/root/ubuntu

The leaves all have mountpoint=/ and canmount=noauto

The unbuntu process for building the initramfs is perfectly happy with this arrangement.

But the Arch process, which includes the zfs hook scripts called by mkinitcpio generates a bad mtab. Specifically, it include a line for each of these different
operating systems.
I expect to see just a line for the arch os - which corresponds to the dataset that is mounted (in my chroot jail) at the time I run mkinitcpio.

Of course, this causes the system not to boot, because it can't find the root.
Specifically, rpool/root (which is empty) ends up mounted at /new_root, and the boot process fails to find the init script and bails.

Can anybody help me? I've spent some time googling and reading the documentation, but nothing like this is mentioned.

There are a couple of bugs in the grub-mkconfig and zfs-linux-git packages.

The zfs mkinitcpio documentation is unclear about the meaning of the zfs= kernel parameter.
I took it to mean listing the pools to be mounted. This is wrong, it is the dataset containing
the root. So by quoting zfs=rpool, rather than zfs=rpool/root/arch, this explains why it mounted
the wrong dataset(s).

grub-mkconfig doesn't know about the zfs= parameter, only about root=zfs=. And then it gets it wrong,
missing out the pool name. This is a known bug (https://github.com/zfsonlinux/grub/issues/22).

There is another issue with the zfs init hook, which attempts to mount all pools (zfs import -aN),
and ignores the cachefile that it has copied into the initcpio. Add -c <cachefilename> is necessary
when there are devices attached with pools that should not be imported (e.g. if they have conflicting
mounts). Such conflicts are likely if copying entire pools between physical devices using zfs send & recv.