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.\" @(#)mount_null.8 8.6 (Berkeley) 5/1/95 34.\" $FreeBSD$ 35.\" 36.Dd June 11, 2023 37.Dt MOUNT_NULLFS 8 38.Os 39.Sh NAME 40.Nm mount_nullfs 41.Nd "mount a loopback file system sub-tree; demonstrate the use of a null file system layer" 42.Sh SYNOPSIS 43.Nm 44.Op Fl o Ar options 45.Ar target 46.Ar mount-point 47.Sh DESCRIPTION 48The 49.Nm 50utility creates a 51.Xr nullfs 5 52layer, duplicating a sub-tree of the file system 53name space under another part of the global file system namespace. 54This allows existing files and directories to be accessed 55using a different pathname. 56.Pp 57The primary differences between a virtual copy of the file system 58and a symbolic link are that the 59.Xr getcwd 3 60functions work correctly in the virtual copy, and that other file systems 61may be mounted on the virtual copy without affecting the original. 62A different device number for the virtual copy is returned by 63.Xr stat 2 , 64but in other respects it is indistinguishable from the original. 65.Pp 66The 67.Nm 68utility supports mounting both directories and single files. 69Both 70.Ar target 71and 72.Ar mount_point 73must be the same type. 74Mounting directories to files or files to 75directories is not supported. 76.Pp 77The 78.Nm 79file system differs from a traditional 80loopback file system in two respects: it is implemented using 81a stackable layers techniques, and its 82.Do null-node Dc Ns s 83stack above 84all lower-layer vnodes, not just over directory vnodes. 85.Pp 86The options are as follows: 87.Bl -tag -width indent 88.It Fl o 89Options are specified with a 90.Fl o 91flag followed by a comma separated string of options. 92See the 93.Xr mount 8 94man page for possible options and their meanings. 95Additionally the following option is supported: 96.Bl -tag -width indent 97.It Cm nocache 98Disable metadata caching in the null layer. 99Some lower-layer file systems may force this option. 100Depending on the access pattern, 101this may result in increased lock contention. 102.El 103.El 104.Pp 105The null layer has two purposes. 106First, it serves as a demonstration of layering by providing a layer 107which does nothing. 108(It actually does everything the loopback file system does, 109which is slightly more than nothing.) 110Second, the null layer can serve as a prototype layer. 111Since it provides all necessary layer framework, 112new file system layers can be created very easily by starting 113with a null layer. 114.Pp 115The remainder of this man page examines the null layer as a basis 116for constructing new layers. 117.\" 118.\" 119.Sh INSTANTIATING NEW NULL LAYERS 120New null layers are created with 121.Nm . 122The 123.Nm 124utility takes two arguments, the pathname 125of the lower vfs (target-pn) and the pathname where the null 126layer will appear in the namespace (mount-point-pn). 127After 128the null layer is put into place, the contents 129of target-pn subtree will be aliased under mount-point-pn. 130.\" 131.\" 132.Sh OPERATION OF A NULL LAYER 133The null layer is the minimum file system layer, 134simply bypassing all possible operations to the lower layer 135for processing there. 136The majority of its activity centers 137on the bypass routine, through which nearly all vnode operations 138pass. 139.Pp 140The bypass routine accepts arbitrary vnode operations for 141handling by the lower layer. 142It begins by examining vnode 143operation arguments and replacing any null-nodes by their 144lower-layer equivalents. 145It then invokes the operation 146on the lower layer. 147Finally, it replaces the null-nodes 148in the arguments and, if a vnode is returned by the operation, 149stacks a null-node on top of the returned vnode. 150.Pp 151Although bypass handles most operations, 152.Em vop_getattr , 153.Em vop_inactive , 154.Em vop_reclaim , 155and 156.Em vop_print 157are not bypassed. 158.Em Vop_getattr 159must change the fsid being returned. 160.Em Vop_inactive 161and 162.Em vop_reclaim 163are not bypassed so that 164they can handle freeing null-layer specific data. 165.Em Vop_print 166is not bypassed to avoid excessive debugging 167information. 168.\" 169.\" 170.Sh INSTANTIATING VNODE STACKS 171Mounting associates the null layer with a lower layer, 172in effect stacking two VFSes. 173Vnode stacks are instead 174created on demand as files are accessed. 175.Pp 176The initial mount creates a single vnode stack for the 177root of the new null layer. 178All other vnode stacks 179are created as a result of vnode operations on 180this or other null vnode stacks. 181.Pp 182New vnode stacks come into existence as a result of 183an operation which returns a vnode. 184The bypass routine stacks a null-node above the new 185vnode before returning it to the caller. 186.Pp 187For example, imagine mounting a null layer with 188.Bd -literal -offset indent 189mount_nullfs /usr/include /dev/layer/null 190.Ed 191.Pp 192Changing directory to 193.Pa /dev/layer/null 194will assign 195the root null-node (which was created when the null layer was mounted). 196Now consider opening 197.Pa sys . 198A vop_lookup would be 199done on the root null-node. 200This operation would bypass through 201to the lower layer which would return a vnode representing 202the UFS 203.Pa sys . 204Null_bypass then builds a null-node 205aliasing the UFS 206.Pa sys 207and returns this to the caller. 208Later operations on the null-node 209.Pa sys 210will repeat this 211process when constructing other vnode stacks. 212.\" 213.\" 214.Sh CREATING OTHER FILE SYSTEM LAYERS 215One of the easiest ways to construct new file system layers is to make 216a copy of the null layer, rename all files and variables, and 217then begin modifying the copy. 218The 219.Xr sed 1 220utility can be used to easily rename 221all variables. 222.Pp 223The umap layer is an example of a layer descended from the 224null layer. 225.\" 226.\" 227.Sh INVOKING OPERATIONS ON LOWER LAYERS 228There are two techniques to invoke operations on a lower layer 229when the operation cannot be completely bypassed. 230Each method 231is appropriate in different situations. 232In both cases, 233it is the responsibility of the aliasing layer to make 234the operation arguments "correct" for the lower layer 235by mapping a vnode argument to the lower layer. 236.Pp 237The first approach is to call the aliasing layer's bypass routine. 238This method is most suitable when you wish to invoke the operation 239currently being handled on the lower layer. 240It has the advantage that 241the bypass routine already must do argument mapping. 242An example of this is 243.Em null_getattrs 244in the null layer. 245.Pp 246A second approach is to directly invoke vnode operations on 247the lower layer with the 248.Em VOP_OPERATIONNAME 249interface. 250The advantage of this method is that it is easy to invoke 251arbitrary operations on the lower layer. 252The disadvantage 253is that vnode arguments must be manually mapped. 254.\" 255.\" 256.Sh SEE ALSO 257.Xr nullfs 5 , 258.Xr mount 8 259.Pp 260UCLA Technical Report CSD-910056, 261.Em "Stackable Layers: an Architecture for File System Development" . 262.Sh HISTORY 263The 264.Nm mount_null 265utility first appeared in 266.Bx 4.4 . 267It was renamed to 268.Nm 269in 270.Fx 5.0 . 271