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33 .\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95
41 .Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer"
51 null layer, duplicating a sub-tree of the file system
52 name space under another part of the global file system namespace.
53 This allows existing files and directories to be accessed
54 using a different pathname.
56 The primary differences between a virtual copy of the file system
57 and a symbolic link are that the
59 functions work correctly in the virtual copy, and that other file systems
60 may be mounted on the virtual copy without affecting the original.
61 A different device number for the virtual copy is returned by
63 but in other respects it is indistinguishable from the original.
67 file system differs from a traditional
68 loopback file system in two respects: it is implemented using
69 a stackable layers techniques, and its
72 all lower-layer vnodes, not just over directory vnodes.
74 The options are as follows:
75 .Bl -tag -width indent
77 Options are specified with a
79 flag followed by a comma separated string of options.
82 man page for possible options and their meanings.
85 The null layer has two purposes.
86 First, it serves as a demonstration of layering by providing a layer
88 (It actually does everything the loopback file system does,
89 which is slightly more than nothing.)
90 Second, the null layer can serve as a prototype layer.
91 Since it provides all necessary layer framework,
92 new file system layers can be created very easily by starting
95 The remainder of this man page examines the null layer as a basis
96 for constructing new layers.
99 .Sh INSTANTIATING NEW NULL LAYERS
100 New null layers are created with
104 utility takes two arguments, the pathname
105 of the lower vfs (target-pn) and the pathname where the null
106 layer will appear in the namespace (mount-point-pn).
108 the null layer is put into place, the contents
109 of target-pn subtree will be aliased under mount-point-pn.
112 .Sh OPERATION OF A NULL LAYER
113 The null layer is the minimum file system layer,
114 simply bypassing all possible operations to the lower layer
115 for processing there.
116 The majority of its activity centers
117 on the bypass routine, through which nearly all vnode operations
120 The bypass routine accepts arbitrary vnode operations for
121 handling by the lower layer.
122 It begins by examining vnode
123 operation arguments and replacing any null-nodes by their
124 lower-layer equivalents.
125 It then invokes the operation
127 Finally, it replaces the null-nodes
128 in the arguments and, if a vnode is returned by the operation,
129 stacks a null-node on top of the returned vnode.
131 Although bypass handles most operations,
139 must change the fsid being returned.
143 are not bypassed so that
144 they can handle freeing null-layer specific data.
146 is not bypassed to avoid excessive debugging
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.
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.
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.
167 For example, imagine mounting a null layer with
168 .Bd -literal -offset indent
169 mount_nullfs /usr/include /dev/layer/null
171 Changing directory to
174 the root null-node (which was created when the null layer was mounted).
177 A vop_lookup would be
178 done on the root null-node.
179 This operation would bypass through
180 to the lower layer which would return a vnode representing
183 Null_bypass then builds a null-node
186 and returns this to the caller.
187 Later operations on the null-node
190 process when constructing other vnode stacks.
193 .Sh CREATING OTHER FILE SYSTEM LAYERS
194 One of the easiest ways to construct new file system layers is to make
195 a copy of the null layer, rename all files and variables, and
196 then begin modifying the copy.
199 utility can be used to easily rename
202 The umap layer is an example of a layer descended from the
206 .Sh INVOKING OPERATIONS ON LOWER LAYERS
207 There are two techniques to invoke operations on a lower layer
208 when the operation cannot be completely bypassed.
210 is appropriate in different situations.
212 it is the responsibility of the aliasing layer to make
213 the operation arguments "correct" for the lower layer
214 by mapping a vnode argument to the lower layer.
216 The first approach is to call the aliasing layer's bypass routine.
217 This method is most suitable when you wish to invoke the operation
218 currently being handled on the lower layer.
219 It has the advantage that
220 the bypass routine already must do argument mapping.
221 An example of this is
225 A second approach is to directly invoke vnode operations on
226 the lower layer with the
227 .Em VOP_OPERATIONNAME
229 The advantage of this method is that it is easy to invoke
230 arbitrary operations on the lower layer.
232 is that vnode arguments must be manually mapped.
238 UCLA Technical Report CSD-910056,
239 .Em "Stackable Layers: an Architecture for File System Development" .
241 THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
242 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.
248 This code also needs an owner in order to be less dangerous - serious
249 hackers can apply by sending mail to
250 .Aq hackers@FreeBSD.org
252 their intent to take it over.
256 utility first appeared in