> монтирование их поверх + добавление unionfs).

man mount_unionfs
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BUGS
     THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
     AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.  USE AT YOUR OWN
     RISK.  BEWARE OF DOG.  SLIPPERY WHEN WET.  BATTERIES NOT INCLUD
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=201677
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Konstantin Belousov  freebsd_committer 2015-11-10 06:24:40 UTC
(In reply to Franco Fichtner from comment #1)
tmpfs change is completely wrong, there is no reason for tmpfs vnode lock to be recursive.  Unionfs is broken architecturally, it was so for the whole its existence, which is cause of this backtrace and many other issues you could get with the unionfs breaking all assumptions of VFS and filesystems.
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