Longer term ZFS might be changed to never set the raidz_expansion flag when the zpool is not raidz.

I think the least bad thing as a first step is to allow boot with the raidz_expansion flag set if the zpool is not zraid in the first place. That covers many users.

Why is this feature set in zpools that are not raidz, anyway? That seems easy to avoid.

Another can of worms is what I often do, which is prepare a new FreeBSD installation by connecting a new disk as a second disk to an existing install and prepare a root filesystem on the commandline. It will be very hard to prevent me from footshooting in that situation.

zfeature_common.c:759 says that this feature does not fall into the category of readonly-compatible features. That's the list that determines whether a lack of this feature in the running kernel would still allow a readonly mount.

In D52174#1191934, @imp wrote:

Linux has a policy of creating root partitions with a tiny subset of the capabilities in it. We should look into updating the installer to do that, but I'm pretty sure it wouldn't have big uptake.

But this change is needed, go ahead.

This looks good to me. I also ran it through my old SIGINT tests.

LGTM