[Bug 241117] zfs_unlinked_drain takes "forever" for over-quota ZFS filesystems

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241117

            Bug ID: 241117
           Summary: zfs_unlinked_drain takes "forever" for over-quota ZFS
                    filesystems
           Product: Base System
           Version: 11.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: pen at lysator.liu.se

When rebooting a server "zfs mount -a" can take "forever" (or atleast a silly
amount of time) if there is a ZFS filesystems that are at or over 'refquota'
when the kernel tries to execute "zfs_unlinked_drain" (and that filesystem has
unlinked stuff to drain).

This effective allows a user to cause a denial-of-service if the server happens
to reboot due to a bug...

Workaround: 

Make sure "zfs mount" is done in the background in the /etc/rc.d/zfs startup
script - so that you can get a login prompt, and then give the filesystem more
quota so progress can continue.

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