mount_nullfs(8)

NAME

mount_nullfs - mount a loopback file system sub-tree; demon

strate the use
of a null file system layer

SYNOPSIS

mount_nullfs [-o options] target mount-point

DESCRIPTION

The mount_nullfs utility creates a null layer, duplicating a

sub-tree of
the file system name space under another part of the global

file system
namespace. This allows existing files and directories to be

accessed
using a different pathname.

The primary differences between a virtual copy of the file

system and a
symbolic link are that the getcwd(3) functions work correct

ly in the virtual copy, and that other file systems may be mounted on the

virtual copy
without affecting the original. A different device number

for the virtual copy is returned by stat(2), but in other respects it

is indistinguishable from the original.

The mount_nullfs file system differs from a traditional

loopback file
system in two respects: it is implemented using a stackable

layers techniques, and its ``null-node''s stack above all lower-layer

vnodes, not
just over directory vnodes.

The options are as follows:

-o Options are specified with a -o flag followed by a

comma sepa

rated string of options. See the mount(8) man page

for possible
options and their meanings.

The null layer has two purposes. First, it serves as a

demonstration of
layering by providing a layer which does nothing. (It actu

ally does
everything the loopback file system does, which is slightly

more than
nothing.) Second, the null layer can serve as a prototype

layer. Since
it provides all necessary layer framework, new file system

layers can be
created very easily by starting with a null layer.

The remainder of this man page examines the null layer as a

basis for
constructing new layers.

INSTANTIATING NEW NULL LAYERS

New null layers are created with mount_nullfs. The

mount_nullfs utility
takes two arguments, the pathname of the lower vfs (target

pn) and the
pathname where the null layer will appear in the namespace

(mount-pointpn). After the null layer is put into place, the contents

of target-pn
subtree will be aliased under mount-point-pn.

OPERATION OF A NULL LAYER

The null layer is the minimum file system layer, simply by

passing all
possible operations to the lower layer for processing there.

The majority of its activity centers on the bypass routine, through

which nearly
all vnode operations pass.

The bypass routine accepts arbitrary vnode operations for

handling by the
lower layer. It begins by examining vnode operation argu

ments and
replacing any null-nodes by their lower-layer equivalents.

It then
invokes the operation on the lower layer. Finally, it re

places the nullnodes in the arguments and, if a vnode is returned by the

operation,
stacks a null-node on top of the returned vnode.

Although bypass handles most operations, vop_getattr,

vop_inactive,
vop_reclaim, and vop_print are not bypassed. Vop_getattr

must change the
fsid being returned. Vop_inactive and vop_reclaim are not

bypassed so
that they can handle freeing null-layer specific data.

Vop_print is not
bypassed to avoid excessive debugging information.

INSTANTIATING VNODE STACKS

Mounting associates the null layer with a lower layer, in

effect stacking
two VFSes. Vnode stacks are instead created on demand as

files are
accessed.

The initial mount creates a single vnode stack for the root

of the new
null layer. All other vnode stacks are created as a result

of vnode
operations on this or other null vnode stacks.

New vnode stacks come into existence as a result of an oper

ation which
returns a vnode. The bypass routine stacks a null-node

above the new
vnode before returning it to the caller.

For example, imagine mounting a null layer with

mount_nullfs /usr/include /dev/layer/null

Changing directory to /dev/layer/null will assign the root

null-node
(which was created when the null layer was mounted). Now

consider opening sys. A vop_lookup would be done on the root null-node.

This operation would bypass through to the lower layer which would re

turn a vnode
representing the UFS sys. Null_bypass then builds a null

node aliasing
the UFS sys and returns this to the caller. Later opera

tions on the
null-node sys will repeat this process when constructing

other vnode
stacks.

CREATING OTHER FILE SYSTEM LAYERS

One of the easiest ways to construct new file system layers

is to make a
copy of the null layer, rename all files and variables, and

then begin
modifying the copy. The sed(1) utility can be used to easi

ly rename all
variables.

The umap layer is an example of a layer descended from the

null layer.

INVOKING OPERATIONS ON LOWER LAYERS

There are two techniques to invoke operations on a lower

layer when the
operation cannot be completely bypassed. Each method is ap

propriate in
different situations. In both cases, it is the responsibil

ity of the
aliasing layer to make the operation arguments "correct" for

the lower
layer by mapping a vnode argument to the lower layer.

The first approach is to call the aliasing layer's bypass

routine. This
method is most suitable when you wish to invoke the opera

tion currently
being handled on the lower layer. It has the advantage that

the bypass
routine already must do argument mapping. An example of

this is
null_getattrs in the null layer.

A second approach is to directly invoke vnode operations on

the lower
layer with the VOP_OPERATIONNAME interface. The advantage

of this method
is that it is easy to invoke arbitrary operations on the

lower layer.
The disadvantage is that vnode arguments must be manually

mapped.

SEE ALSO

mount(8)

UCLA Technical Report CSD-910056, Stackable Layers: an

Architecture for
File System Development.

HISTORY

The mount_nullfs utility first appeared in 4.4BSD.

BSD May 1, 1995

BSD