Copyright (c) 1992, 1993, 1994
The Regents of the University of California. All rights reserved.
This code is derived from software donated to Berkeley by
John Heidemann of the UCLA Ficus project.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. All advertising materials mentioning features or use of this software
must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
@(#)mount_null.8 8.6 (Berkeley) 5/1/95
$FreeBSD: head/sbin/mount_nullfs/mount_nullfs.8 102231 2002-08-21 18:11:48Z trhodes $
.Dd May 1, 1995 .Dt MOUNT_NULLFS 8 .Os .Sh NAME .Nm mount_nullfs .Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer" .Sh SYNOPSIS .Nm .Op Fl o Ar options .Ar target .Ar mount-point .Sh DESCRIPTION The .Nm utility creates a null layer, duplicating a sub-tree of the file system name space under another part of the global file system namespace. This allows existing files and directories to be accessed using a different pathname.
p The primary differences between a virtual copy of the file system and a symbolic link are that the .Xr getcwd 3 functions work correctly in the virtual copy, and that other file systems may be mounted on the virtual copy without affecting the original. A different device number for the virtual copy is returned by .Xr stat 2 , but in other respects it is indistinguishable from the original.
p The .Nm file system differs from a traditional loopback file system in two respects: it is implemented using a stackable layers techniques, and it's .Do null-node Dc Ns s stack above all lower-layer vnodes, not just over directory vnodes.
p The options are as follows: l -tag -width indent t Fl o Options are specified with a .Fl o flag followed by a comma separated string of options. See the .Xr mount 8 man page for possible options and their meanings. .El
p The null layer has two purposes. First, it serves as a demonstration of layering by providing a layer which does nothing. (It actually does everything the loopback file system does, which is slightly more than nothing.) Second, the null layer can serve as a prototype layer. Since it provides all necessary layer framework, new file system layers can be created very easily by starting with a null layer.
p
The remainder of this man page examines the null layer as a basis
for constructing new layers.
.Sh INSTANTIATING NEW NULL LAYERS
New null layers are created with
.Nm .
The
.Nm
utility takes two arguments, the pathname
of the lower vfs (target-pn) and the pathname where the null
layer will appear in the namespace (mount-point-pn). After
the null layer is put into place, the contents
of target-pn subtree will be aliased under mount-point-pn.
.Sh OPERATION OF A NULL LAYER
The null layer is the minimum file system layer,
simply bypassing all possible operations to the lower layer
for processing there. The majority of its activity centers
on the bypass routine, through which nearly all vnode operations
pass.
p The bypass routine accepts arbitrary vnode operations for handling by the lower layer. It begins by examining vnode operation arguments and replacing any null-nodes by their lower-layer equivalents. It then invokes the operation on the lower layer. Finally, it replaces the null-nodes in the arguments and, if a vnode is returned by the operation, stacks a null-node on top of the returned vnode.
p
Although bypass handles most operations,
.Em vop_getattr ,
.Em vop_inactive ,
.Em vop_reclaim ,
and
.Em vop_print
are not bypassed.
.Em Vop_getattr
must change the fsid being returned.
.Em Vop_inactive
and
.Em vop_reclaim
are not bypassed so that
they can handle freeing null-layer specific data.
.Em Vop_print
is not bypassed to avoid excessive debugging
information.
.Sh INSTANTIATING VNODE STACKS
Mounting associates the null layer with a lower layer,
in effect stacking two VFSes. Vnode stacks are instead
created on demand as files are accessed.
p The initial mount creates a single vnode stack for the root of the new null layer. All other vnode stacks are created as a result of vnode operations on this or other null vnode stacks.
p New vnode stacks come into existence as a result of an operation which returns a vnode. The bypass routine stacks a null-node above the new vnode before returning it to the caller.
p For example, imagine mounting a null layer with d -literal -offset indent mount_nullfs /usr/include /dev/layer/null .Ed Changing directory to
a /dev/layer/null will assign the root null-node (which was created when the null layer was mounted). Now consider opening
a sys . A vop_lookup would be done on the root null-node. This operation would bypass through to the lower layer which would return a vnode representing the UFS
a sys . Null_bypass then builds a null-node aliasing the UFS
a sys and returns this to the caller. Later operations on the null-node
a sys
will repeat this
process when constructing other vnode stacks.
.Sh CREATING OTHER FILE SYSTEM LAYERS
One of the easiest ways to construct new file system layers is to make
a copy of the null layer, rename all files and variables, and
then begin modifying the copy.
.Xr Sed 1
can be used to easily rename
all variables.
p
The umap layer is an example of a layer descended from the
null layer.
.Sh INVOKING OPERATIONS ON LOWER LAYERS
There are two techniques to invoke operations on a lower layer
when the operation cannot be completely bypassed. Each method
is appropriate in different situations. In both cases,
it is the responsibility of the aliasing layer to make
the operation arguments "correct" for the lower layer
by mapping a vnode argument to the lower layer.
p The first approach is to call the aliasing layer's bypass routine. This method is most suitable when you wish to invoke the operation currently being handled on the lower layer. It has the advantage that the bypass routine already must do argument mapping. An example of this is .Em null_getattrs in the null layer.
p
A second approach is to directly invoke vnode operations on
the lower layer with the
.Em VOP_OPERATIONNAME
interface.
The advantage of this method is that it is easy to invoke
arbitrary operations on the lower layer. The disadvantage
is that vnode arguments must be manually mapped.
.Sh SEE ALSO
.Xr mount 8
p UCLA Technical Report CSD-910056, .Em "Stackable Layers: an Architecture for File System Development" . .Sh BUGS THIS FILESYSTEM 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.
p This code also needs an owner in order to be less dangerous - serious hackers can apply by sending mail to .Aq hackers@FreeBSD.org and announcing their intent to take it over. .Sh HISTORY The .Nm utility first appeared in x 4.4 .