1.\" 2.\" Copyright (c) 1992, 1993, 1994 3.\" The Regents of the University of California. All rights reserved. 4.\" 5.\" This code is derived from software donated to Berkeley by 6.\" John Heidemann of the UCLA Ficus project. 7.\" 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. Neither the name of the University nor the names of its contributors 18.\" may be used to endorse or promote products derived from this software 19.\" without specific prior written permission. 20.\" 21.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 22.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 23.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 24.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 25.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 26.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 27.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 28.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 29.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 30.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.Dd June 11, 2023 34.Dt MOUNT_NULLFS 8 35.Os 36.Sh NAME 37.Nm mount_nullfs 38.Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer" 39.Sh SYNOPSIS 40.Nm 41.Op Fl o Ar options 42.Ar target 43.Ar mount-point 44.Sh DESCRIPTION 45The 46.Nm 47utility creates a 48.Xr nullfs 5 49layer, duplicating a sub-tree of the file system 50name space under another part of the global file system namespace. 51This allows existing files and directories to be accessed 52using a different pathname. 53.Pp 54The primary differences between a virtual copy of the file system 55and a symbolic link are that the 56.Xr getcwd 3 57functions work correctly in the virtual copy, and that other file systems 58may be mounted on the virtual copy without affecting the original. 59A different device number for the virtual copy is returned by 60.Xr stat 2 , 61but in other respects it is indistinguishable from the original. 62.Pp 63The 64.Nm 65utility supports mounting both directories and single files. 66Both 67.Ar target 68and 69.Ar mount_point 70must be the same type. 71Mounting directories to files or files to 72directories is not supported. 73.Pp 74The 75.Nm 76file system differs from a traditional 77loopback file system in two respects: it is implemented using 78a stackable layers techniques, and its 79.Do null-node Dc Ns s 80stack above 81all lower-layer vnodes, not just over directory vnodes. 82.Pp 83The options are as follows: 84.Bl -tag -width indent 85.It Fl o 86Options are specified with a 87.Fl o 88flag followed by a comma separated string of options. 89See the 90.Xr mount 8 91man page for possible options and their meanings. 92Additionally the following option is supported: 93.Bl -tag -width indent 94.It Cm nocache 95Disable metadata caching in the null layer. 96Some lower-layer file systems may force this option. 97Depending on the access pattern, 98this may result in increased lock contention. 99.El 100.El 101.Pp 102The null layer has two purposes. 103First, it serves as a demonstration of layering by providing a layer 104which does nothing. 105(It actually does everything the loopback file system does, 106which is slightly more than nothing.) 107Second, the null layer can serve as a prototype layer. 108Since it provides all necessary layer framework, 109new file system layers can be created very easily by starting 110with a null layer. 111.Pp 112The remainder of this man page examines the null layer as a basis 113for constructing new layers. 114.\" 115.\" 116.Sh INSTANTIATING NEW NULL LAYERS 117New null layers are created with 118.Nm . 119The 120.Nm 121utility takes two arguments, the pathname 122of the lower vfs (target-pn) and the pathname where the null 123layer will appear in the namespace (mount-point-pn). 124After 125the null layer is put into place, the contents 126of target-pn subtree will be aliased under mount-point-pn. 127.\" 128.\" 129.Sh OPERATION OF A NULL LAYER 130The null layer is the minimum file system layer, 131simply bypassing all possible operations to the lower layer 132for processing there. 133The majority of its activity centers 134on the bypass routine, through which nearly all vnode operations 135pass. 136.Pp 137The bypass routine accepts arbitrary vnode operations for 138handling by the lower layer. 139It begins by examining vnode 140operation arguments and replacing any null-nodes by their 141lower-layer equivalents. 142It then invokes the operation 143on the lower layer. 144Finally, it replaces the null-nodes 145in the arguments and, if a vnode is returned by the operation, 146stacks a null-node on top of the returned vnode. 147.Pp 148Although bypass handles most operations, 149.Em vop_getattr , 150.Em vop_inactive , 151.Em vop_reclaim , 152and 153.Em vop_print 154are not bypassed. 155.Em Vop_getattr 156must change the fsid being returned. 157.Em Vop_inactive 158and 159.Em vop_reclaim 160are not bypassed so that 161they can handle freeing null-layer specific data. 162.Em Vop_print 163is not bypassed to avoid excessive debugging 164information. 165.\" 166.\" 167.Sh INSTANTIATING VNODE STACKS 168Mounting associates the null layer with a lower layer, 169in effect stacking two VFSes. 170Vnode stacks are instead 171created on demand as files are accessed. 172.Pp 173The initial mount creates a single vnode stack for the 174root of the new null layer. 175All other vnode stacks 176are created as a result of vnode operations on 177this or other null vnode stacks. 178.Pp 179New vnode stacks come into existence as a result of 180an operation which returns a vnode. 181The bypass routine stacks a null-node above the new 182vnode before returning it to the caller. 183.Pp 184For example, imagine mounting a null layer with 185.Bd -literal -offset indent 186mount_nullfs /usr/include /dev/layer/null 187.Ed 188.Pp 189Changing directory to 190.Pa /dev/layer/null 191will assign 192the root null-node (which was created when the null layer was mounted). 193Now consider opening 194.Pa sys . 195A vop_lookup would be 196done on the root null-node. 197This operation would bypass through 198to the lower layer which would return a vnode representing 199the UFS 200.Pa sys . 201Null_bypass then builds a null-node 202aliasing the UFS 203.Pa sys 204and returns this to the caller. 205Later operations on the null-node 206.Pa sys 207will repeat this 208process when constructing other vnode stacks. 209.\" 210.\" 211.Sh CREATING OTHER FILE SYSTEM LAYERS 212One of the easiest ways to construct new file system layers is to make 213a copy of the null layer, rename all files and variables, and 214then begin modifying the copy. 215The 216.Xr sed 1 217utility can be used to easily rename 218all variables. 219.Pp 220The umap layer is an example of a layer descended from the 221null layer. 222.\" 223.\" 224.Sh INVOKING OPERATIONS ON LOWER LAYERS 225There are two techniques to invoke operations on a lower layer 226when the operation cannot be completely bypassed. 227Each method 228is appropriate in different situations. 229In both cases, 230it is the responsibility of the aliasing layer to make 231the operation arguments "correct" for the lower layer 232by mapping a vnode argument to the lower layer. 233.Pp 234The first approach is to call the aliasing layer's bypass routine. 235This method is most suitable when you wish to invoke the operation 236currently being handled on the lower layer. 237It has the advantage that 238the bypass routine already must do argument mapping. 239An example of this is 240.Em null_getattrs 241in the null layer. 242.Pp 243A second approach is to directly invoke vnode operations on 244the lower layer with the 245.Em VOP_OPERATIONNAME 246interface. 247The advantage of this method is that it is easy to invoke 248arbitrary operations on the lower layer. 249The disadvantage 250is that vnode arguments must be manually mapped. 251.\" 252.\" 253.Sh SEE ALSO 254.Xr nullfs 5 , 255.Xr mount 8 256.Pp 257UCLA Technical Report CSD-910056, 258.Em "Stackable Layers: an Architecture for File System Development" . 259.Sh HISTORY 260The 261.Nm mount_null 262utility first appeared in 263.Bx 4.4 . 264It was renamed to 265.Nm 266in 267.Fx 5.0 . 268