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 March 24, 2024 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 nocache 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.It Cm cache 100Force enable metadata caching. 101.El 102.El 103.Pp 104The 105.Dv vfs.nullfs.cache_vnodes 106sysctl specifies global default for mount-specific cache/nocache option. 107.Pp 108The null layer has two purposes. 109First, it serves as a demonstration of layering by providing a layer 110which does nothing. 111(It actually does everything the loopback file system does, 112which is slightly more than nothing.) 113Second, the null layer can serve as a prototype layer. 114Since it provides all necessary layer framework, 115new file system layers can be created very easily by starting 116with a null layer. 117.Pp 118The remainder of this man page examines the null layer as a basis 119for constructing new layers. 120.\" 121.\" 122.Sh INSTANTIATING NEW NULL LAYERS 123New null layers are created with 124.Nm . 125The 126.Nm 127utility takes two arguments, the pathname 128of the lower vfs (target-pn) and the pathname where the null 129layer will appear in the namespace (mount-point-pn). 130After 131the null layer is put into place, the contents 132of target-pn subtree will be aliased under mount-point-pn. 133.\" 134.\" 135.Sh OPERATION OF A NULL LAYER 136The null layer is the minimum file system layer, 137simply bypassing all possible operations to the lower layer 138for processing there. 139The majority of its activity centers 140on the bypass routine, through which nearly all vnode operations 141pass. 142.Pp 143The bypass routine accepts arbitrary vnode operations for 144handling by the lower layer. 145It begins by examining vnode 146operation arguments and replacing any null-nodes by their 147lower-layer equivalents. 148It then invokes the operation 149on the lower layer. 150Finally, it replaces the null-nodes 151in the arguments and, if a vnode is returned by the operation, 152stacks a null-node on top of the returned vnode. 153.Pp 154Although bypass handles most operations, 155.Em vop_getattr , 156.Em vop_inactive , 157.Em vop_reclaim , 158and 159.Em vop_print 160are not bypassed. 161.Em Vop_getattr 162must change the fsid being returned. 163.Em Vop_inactive 164and 165.Em vop_reclaim 166are not bypassed so that 167they can handle freeing null-layer specific data. 168.Em Vop_print 169is not bypassed to avoid excessive debugging 170information. 171.\" 172.\" 173.Sh INSTANTIATING VNODE STACKS 174Mounting associates the null layer with a lower layer, 175in effect stacking two VFSes. 176Vnode stacks are instead 177created on demand as files are accessed. 178.Pp 179The initial mount creates a single vnode stack for the 180root of the new null layer. 181All other vnode stacks 182are created as a result of vnode operations on 183this or other null vnode stacks. 184.Pp 185New vnode stacks come into existence as a result of 186an operation which returns a vnode. 187The bypass routine stacks a null-node above the new 188vnode before returning it to the caller. 189.Pp 190For example, imagine mounting a null layer with 191.Bd -literal -offset indent 192mount_nullfs /usr/include /dev/layer/null 193.Ed 194.Pp 195Changing directory to 196.Pa /dev/layer/null 197will assign 198the root null-node (which was created when the null layer was mounted). 199Now consider opening 200.Pa sys . 201A vop_lookup would be 202done on the root null-node. 203This operation would bypass through 204to the lower layer which would return a vnode representing 205the UFS 206.Pa sys . 207Null_bypass then builds a null-node 208aliasing the UFS 209.Pa sys 210and returns this to the caller. 211Later operations on the null-node 212.Pa sys 213will repeat this 214process when constructing other vnode stacks. 215.\" 216.\" 217.Sh CREATING OTHER FILE SYSTEM LAYERS 218One of the easiest ways to construct new file system layers is to make 219a copy of the null layer, rename all files and variables, and 220then begin modifying the copy. 221The 222.Xr sed 1 223utility can be used to easily rename 224all variables. 225.Pp 226The umap layer is an example of a layer descended from the 227null layer. 228.\" 229.\" 230.Sh INVOKING OPERATIONS ON LOWER LAYERS 231There are two techniques to invoke operations on a lower layer 232when the operation cannot be completely bypassed. 233Each method 234is appropriate in different situations. 235In both cases, 236it is the responsibility of the aliasing layer to make 237the operation arguments "correct" for the lower layer 238by mapping a vnode argument to the lower layer. 239.Pp 240The first approach is to call the aliasing layer's bypass routine. 241This method is most suitable when you wish to invoke the operation 242currently being handled on the lower layer. 243It has the advantage that 244the bypass routine already must do argument mapping. 245An example of this is 246.Em null_getattrs 247in the null layer. 248.Pp 249A second approach is to directly invoke vnode operations on 250the lower layer with the 251.Em VOP_OPERATIONNAME 252interface. 253The advantage of this method is that it is easy to invoke 254arbitrary operations on the lower layer. 255The disadvantage 256is that vnode arguments must be manually mapped. 257.\" 258.\" 259.Sh SEE ALSO 260.Xr nullfs 5 , 261.Xr mount 8 262.Pp 263UCLA Technical Report CSD-910056, 264.Em "Stackable Layers: an Architecture for File System Development" . 265.Sh HISTORY 266The 267.Nm mount_null 268utility first appeared in 269.Bx 4.4 . 270It was renamed to 271.Nm 272in 273.Fx 5.0 . 274