kern/162591: [nullfs] cross-filesystem nullfs does not work as expected

[Gleb Kurtsou]

The following reply was made to PR kern/162591; it has been noted by GNATS.

From: Gleb Kurtsou <gleb.kurtsou at gmail.com>
To: bug-followup at FreeBSD.org, mw at wzff.de
Cc:  
Subject: Re: kern/162591: [nullfs] cross-filesystem nullfs does not work as
 expected
Date: Mon, 21 Nov 2011 16:23:47 +0200

 That is expected behaviour, according to mount_nullfs(8):
 	The mount_nullfs utility creates a null layer, duplicating a
 	sub-tree of the file system name space under another part of the
 	global file system namespace.
 
 It mentions sub-tree of a single file system only. nullfs is technically
 very different from loop mounts in Linux or nullfs in DragonFly, it
 doesn't map part of file name space elsewhere, but duplicates file
 system sub-tree.
 
 Messing with v_mountedhere in nullfs would bring much more trouble than
 it can potentially solve: VFS_VGET(), full path lookups, etc.  We could
 add mount option to automatically nullfs-mount nested mounts, but it
 also feels overcomplicated: unmounting may be difficult, should we track
 new mounts, etc. Mounting nullfs over nullfs is a very bad idea imo (it
 it's not forbidden, I'd also suggest not to NFS export nullfs).
 
 I think writing a small script to do nested mounts/unmounts that suites
 your needs would be the best option here.