1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
|
.\" $OpenBSD: mount_null.8,v 1.20 2003/06/02 20:06:15 millert Exp $
.\" $NetBSD: mount_null.8,v 1.4 1996/04/10 20:57:19 thorpej Exp $
.\"
.\" 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. 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 8.4 (Berkeley) 4/19/94
.\"
.Dd April 19, 1994
.Dt MOUNT_NULL 8
.Os
.Sh NAME
.Nm mount_null
.Nd demonstrate the use of a null file system layer
.Sh SYNOPSIS
.Nm mount_null
.Op Fl o Ar options
.Ar target
.Ar mount_point
.Sh DESCRIPTION
The
.Nm
command creates a
null layer, duplicating a sub-tree of the file system
namespace under another part of the global file system namespace.
It is implemented using a stackable layers technique, and its
.Do
null-nodes
.Dc
stack above
all lower-layer vnodes (not just above directory vnodes).
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl o Ar options
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
.Pp
The null layer has two purposes.
First, it serves as a demonstration of layering by proving a layer
which does 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 be starting
with a null layer.
.Pp
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 mount_null .
.Nm
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.
.Pp
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.
.Pp
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.
.Pp
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
null vnode stacks.
.Pp
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.
.Pp
For example, imagine mounting a null layer with
.Bd -literal -offset indent
# mount_null /usr/include /dev/layer/null
.Ed
.Pp
Changing directory to
.Pa /dev/layer/null
will assign
the root null-node (which was created when the null layer was mounted).
Now consider opening
.Pa sys .
A
.Em 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
.Pa sys .
Null_bypass then builds a null-node
aliasing the UFS
.Pa sys
and returns this to the caller.
Later operations on the null-node
.Pa 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.
.Pp
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 any vnode arguments to the lower layer.
.Pp
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.
.Pp
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 vnodes arguments must be manually mapped.
.\"
.\"
.Sh SEE ALSO
.Xr mount 2 ,
.Xr mount 8 ,
.Xr umount 8
.Pp
UCLA Technical Report CSD-910056,
.Em "Stackable Layers: an Architecture for File System Development" .
.Sh HISTORY
The
.Nm
utility first appeared in
.Bx 4.4 .
|