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1 .\" $NetBSD: mount_null.8,v 1.20 2004/06/10 14:13:36 uebayasi Exp $
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33 .\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95
34 .\"
35 .\"
36 .Dd May 1, 1995
37 .Dt MOUNT_NULL 8
38 .Os
39 .Sh NAME
40 .Nm mount_null
41 .Nd mount a loopback filesystem sub-tree;
42 demonstrate the use of a null file system layer
43 .Sh SYNOPSIS
44 .Nm
45 .Op Fl o Ar options
46 .Ar target
47 .Ar mount-point
48 .Sh DESCRIPTION
49 The
50 .Nm
51 command creates a
52 null layer, duplicating a sub-tree of the file system
53 name space under another part of the global file system namespace.
54 This allows existing files and directories to be accessed
55 using a different pathname.
56 .Pp
57 The primary differences between a virtual copy of the filesystem
58 and a symbolic link are that
59 .Xr getcwd 3
60 functions correctly in the virtual copy, and that other filesystems
61 may be mounted on the virtual copy without affecting the original.
62 A different device number for the virtual copy is returned by
63 .Xr stat 2 ,
64 but in other respects it is indistinguishable from the original.
65 .Pp
66 The
67 .Nm
68 filesystem differs from a traditional
69 loopback file system in two respects: it is implemented using
70 a stackable layers technique, and its
71 .Do
72 null-nodes
73 .Dc
74 stack above
75 all lower-layer vnodes (not just above directory vnodes).
76 .Pp
77 Both
78 .Ar target
79 and
80 .Ar mount-point
81 are converted to absolute paths before use.
82 .Pp
83 The options are as follows:
84 .Bl -tag -width indent
85 .It Fl o
86 Options are specified with a
87 .Fl o
88 flag followed by a comma separated string of options.
89 See the
90 .Xr mount 8
91 man page for possible options and their meanings.
92 .El
93 .Pp
94 The null layer has two purposes.
95 First, it serves as a demonstration of layering by providing a layer
96 which does nothing.
97 Second, the null layer can serve as a prototype layer.
98 Since it provides all necessary layer framework,
99 new file system layers can be created very easily by starting
100 with a null layer.
101 .Pp
102 The remainder of this man page examines the null layer as a basis
103 for constructing new layers.
104 .\"
105 .\"
106 .Sh INSTANTIATING NEW NULL LAYERS
107 New null layers are created with
108 .Nm .
109 .Nm
110 takes two arguments, the pathname
111 of the lower vfs (target-pn) and the pathname where the null
112 layer will appear in the namespace (mount-point-pn).
113 After the null layer is put into place, the contents
114 of target-pn subtree will be aliased under mount-point-pn.
115 .\"
116 .\"
117 .Sh OPERATION OF A NULL LAYER
118 The null layer is the minimum file system layer,
119 simply passing all possible operations to the lower layer
120 for processing there.
121 The majority of its activity centers on the bypass routine,
122 through which nearly all vnode operations pass.
123 .Pp
124 The bypass routine accepts arbitrary vnode operations for
125 handling by the lower layer.
126 It begins by examining vnode operation arguments and replacing
127 any null-nodes by their lower-layer equivalents.
128 It then invokes the operation on the lower layer.
129 Finally, it replaces the null-nodes
130 in the arguments and, if a vnode is returned by the operation,
131 stacks a null-node on top of the returned vnode.
132 .Pp
133 Although bypass handles most operations,
134 .Em vop_getattr ,
135 .Em vop_inactive ,
136 .Em vop_reclaim ,
137 and
138 .Em vop_print
139 are not bypassed.
140 .Em vop_getattr
141 must change the fsid being returned.
142 .Em vop_inactive
143 and vop_reclaim are not bypassed so that
144 they can handle freeing null-layer specific data.
145 .Em vop_print
146 is not bypassed to avoid excessive debugging
147 information.
148 .\"
149 .\"
150 .Sh INSTANTIATING VNODE STACKS
151 Mounting associates the null layer with a lower layer,
152 in effect stacking two VFSes.
153 Vnode stacks are instead
154 created on demand as files are accessed.
155 .Pp
156 The initial mount creates a single vnode stack for the
157 root of the new null layer.
158 All other vnode stacks
159 are created as a result of vnode operations on
160 this or other null vnode stacks.
161 .Pp
162 New vnode stacks come into existence as a result of
163 an operation which returns a vnode.
164 The bypass routine stacks a null-node above the new
165 vnode before returning it to the caller.
166 .Pp
167 For example, imagine mounting a null layer with
168 .Bd -literal -offset indent
169 mount_null /usr/include /dev/layer/null
170 .Ed
171 Changing directory to
172 .Pa /dev/layer/null
173 will assign
174 the root null-node (which was created when the null layer was mounted).
175 Now consider opening
176 .Pa sys .
177 A vop_lookup would be done on the root null-node.
178 This operation would bypass through to the lower layer
179 which would return a vnode representing the UFS
180 .Pa sys .
181 null_bypass then builds a null-node aliasing the UFS
182 .Pa sys
183 and returns this to the caller.
184 Later operations on the null-node
185 .Pa sys
186 will repeat this process when constructing other vnode stacks.
187 .\"
188 .\"
189 .Sh CREATING OTHER FILE SYSTEM LAYERS
190 One of the easiest ways to construct new file system layers is to make
191 a copy of the null layer, rename all files and variables, and
192 then begin modifying the copy.
193 .Xr sed 1
194 can be used to easily rename all variables.
195 .Pp
196 The umap layer is an example of a layer descended from the
197 null layer.
198 .\"
199 .\"
200 .Sh INVOKING OPERATIONS ON LOWER LAYERS
201 There are two techniques to invoke operations on a lower layer
202 when the operation cannot be completely bypassed.
203 Each method is appropriate in different situations.
204 In both cases, it is the responsibility of the aliasing layer to make
205 the operation arguments "correct" for the lower layer
206 by mapping any vnode arguments to the lower layer.
207 .Pp
208 The first approach is to call the aliasing layer's bypass routine.
209 This method is most suitable when you wish to invoke the operation
210 currently being handled on the lower layer.
211 It has the advantage that the bypass routine already must do argument mapping.
212 An example of this is
213 .Em null_getattrs
214 in the null layer.
215 .Pp
216 A second approach is to directly invoke vnode operations on
217 the lower layer with the
218 .Em VOP_OPERATIONNAME
219 interface.
220 The advantage of this method is that it is easy to invoke
221 arbitrary operations on the lower layer.
222 The disadvantage is that vnode arguments must be manually mapped.
223 .\"
224 .\"
225 .Sh SEE ALSO
226 .Xr mount 8
227 .Pp
228 UCLA Technical Report CSD-910056,
229 .Em "Stackable Layers: an Architecture for File System Development" .
230 .Sh HISTORY
231 The
232 .Nm
233 utility first appeared in
234 .Bx 4.4 .