V/Inode Cache
Active vnodes are reference- counted by the structures that hold pointers to them.
- process current directory
- file system mount points
Each specific file system maintains its own hash of vnodes (BSD).
- specific FS handles initialization
- free list is maintained by VFS
vget(vp): reclaim cached inactive vnode from VFS free list
vref(vp): increment reference count on an active vnode
vrele(vp): release reference count on a vnode
vgone(vp): vnode is no longer valid (file is removed)