[Bug 221512] Contents of ZFS datasets invisible after being mounted inside a jail with nullfs (Even after a reboot!)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221512

            Bug ID: 221512
           Summary: Contents of ZFS datasets invisible after being mounted
                    inside a jail with nullfs (Even after a reboot!)
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: tsuroerusu at gmail.com

I am running a FreeBSD 11.1 system with ZFS and jails and I mount a file system
on my storage pool (/storage/cloud) into the jail (/jails/cloud/storage) via
nullfs, and that works fine for what the jail does. However I just noticed that
outside the jail, I can only see the mount points of the datasets/file system
children, but not the contents.

For example if I do: ls -la /storage/cloud/* (As root) then the children of
"cloud" just appear empty, but the data (files and folders) are present inside
the jail at the nullfs-mounted location. Even if I stop the jail, they still
are not present outside the jail.

I just tried disabling jails in /etc/rc.conf and rebooting the system and EVEN
THEN the files do not reappear outside the jail in their original location, yet
zfs list reports the space being used. But if I then (without rebooting) enable
the jail, the files show up in the nullfs-mounted location inside the jail, but
still remain invisible outside the jail!

As an experiment, I tried creating /mnt/test and /mnt/test2 and creates a
folder and a few files in test, and then nullfs-mounted it onto test2, and I
could then see the files in both locations.

The fact that my data stays hidden after a reboot is rather worrisome.

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