- * @(#)lofs_vnops.c 1.2 (Berkeley) 6/18/92
- * ...and...
- * @(#)null_vnodeops.c 1.20 92/07/07 UCLA Ficus project
- */
-
-/*
- * Null Layer
- *
- * (See mount_null(8) for more information.)
- *
- * The null layer duplicates a portion of the file system
- * name space under a new name. In this respect, it is
- * similar to the loopback file system. It differs from
- * the loopback fs in two respects: it is implemented using
- * a stackable layers techniques, and it's "null-node"s stack above
- * all lower-layer vnodes, not just over directory vnodes.
- *
- * The null layer has two purposes. First, it serves as a demonstration
- * of layering by proving a layer which does nothing. (It actually
- * 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 be 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_null(8).
- * Mount_null(8) takes two arguments, the pathname
- * of the lower vfs (target-pn) and the pathname where the null
- * layer will appear in the namespace (alias-pn). After
- * the null layer is put into place, the contents
- * of target-pn subtree will be aliased under alias-pn.
- *
- *
- * OPERATION OF A NULL LAYER
- *
- * The null layer is the minimum file system layer,
- * simply bypassing all possible operations to the lower layer
- * for processing there. The majority of its activity centers
- * on the bypass routine, though which nearly all vnode operations
- * pass.
- *
- * The bypass routine accepts arbitrary vnode operations for
- * handling by the lower layer. It begins by examing vnode
- * operation arguments and replacing any null-nodes by their
- * lower-layer equivlants. It then invokes the operation
- * on the lower layer. Finally, it replaces the null-nodes
- * in the arguments and, if a vnode is return by the operation,
- * stacks a null-node on top of the returned vnode.
- *
- * Although bypass handles most operations, vop_getattr, vop_lock,
- * vop_unlock, vop_inactive, vop_reclaim, and vop_print are not
- * bypassed. Vop_getattr must change the fsid being returned.
- * Vop_lock and vop_unlock must handle any locking for the
- * current vnode as well as pass the lock request down.
- * 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.
- * Also, certain vnode operations change the locking state within
- * the operation (create, mknod, remove, link, rename, mkdir, rmdir,
- * and symlink). Ideally these operations should not change the
- * lock state, but should be changed to let the caller of the
- * function unlock them. Otherwise all intermediate vnode layers
- * (such as union, umapfs, etc) must catch these functions to do
- * the necessary locking at their layer.
- *
- *
- * INSTANTIATING VNODE STACKS
- *
- * Mounting associates the null layer with a lower layer,
- * 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 existance as a result of
- * an operation 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_null /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 return a vnode representing
- * the UFS "sys". Null_bypass then builds a null-node
- * aliasing the UFS "sys" and returns this to the caller.
- * Later operations 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 modifing the copy. Sed can be used to easily 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 appropriate in different situations. In both cases,
- * it is the responsibility of the aliasing layer to make
- * the operation arguments "correct" for the lower layer
- * by mapping an vnode arguments 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 operation
- * currently being hanldled 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 invoked 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 vnodes arguments must be manualy mapped.
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