VNODE(9) OpenBSD Kernel Manual VNODE(9)NAMEvnode - an overview of vnodes
DESCRIPTION
A vnode is an object in kernel memory that speaks the UNIX file interface
(open, read, write, close, readdir, etc.). Vnodes can represent files,
directories, FIFOs, domain sockets, block devices, character devices.
Each vnode has a set of methods which start with the string ``VOP_''.
These methods include VOP_OPEN(), VOP_READ(), VOP_WRITE(), VOP_RENAME(),
VOP_CLOSE(), and VOP_MKDIR(). Many of these methods correspond closely
to the equivalent file system call - open(2), read(2), write(2),
rename(2), etc. Each file system (FFS, NFS, etc.) provides
implementations for these methods.
The Virtual File System library (see vfs(9)) maintains a pool of vnodes.
File systems cannot allocate their own vnodes; they must use the
functions provided by the VFS to create and manage vnodes.
The definition of a vnode is as follows:
struct vnode {
struct uvm_vnode v_uvm; /* uvm(9) data */
int (**v_op)(void *); /* vnode operations vector */
enum vtype v_type; /* vnode type */
u_int v_flag; /* vnode flags (see below) */
u_int v_usecount; /* reference count of users */
u_int v_writecount; /* reference count of writers */
/* Flags that can be read/written in interrupts */
u_int v_bioflag; /* flags used by intr handlers */
u_int v_holdcnt; /* buffer references */
u_int v_id; /* capability identifier */
struct mount *v_mount; /* ptr to vfs we are in */
TAILQ_ENTRY(vnode) v_freelist; /* vnode freelist */
LIST_ENTRY(vnode) v_mntvnodes; /* vnodes for mount point */
struct buflists v_cleanblkhd; /* clean blocklist head */
struct buflists v_dirtyblkhd; /* dirty blocklist head */
u_int v_numoutput; /* num of writes in progress */
LIST_ENTRY(vnode) v_synclist; /* vnode with dirty buffers */
union {
struct mount *vu_mountedhere;/* ptr to mounted vfs (VDIR) */
struct socket *vu_socket; /* UNIX IPC (VSOCK) */
struct specinfo *vu_specinfo; /* device (VCHR, VBLK) */
struct fifoinfo *vu_fifoinfo; /* fifo (VFIFO) */
} v_un;
enum vtagtype v_tag; /* type of underlying data */
void *v_data; /* private data for fs */
struct {
struct simplelock vsi_lock; /* lock to protect below */
struct selinfo vsi_selinfo; /* identity of poller(s) */
} v_selectinfo;
};
#define v_mountedhere v_un.vu_mountedhere
#define v_socket v_un.vu_socket
#define v_specinfo v_un.vu_specinfo
#define v_fifoinfo v_un.vu_fifoinfo
Vnode life cycle
When a client of the VFS requests a new vnode, the vnode allocation code
can reuse an old vnode object that is no longer in use. Whether a vnode
is in use is tracked by the vnode reference count (v_usecount). By
convention, each open file handle holds a reference as do VM objects
backed by files. A vnode with a reference count of 1 or more will not be
deallocated or reused to point to a different file. So, if you want to
ensure that your vnode doesn't become a different file under you, you
better be sure you have a reference to it. A vnode that points to a
valid file and has a reference count of 1 or more is called active.
When a vnode's reference count drops to zero, it becomes inactive, that
is, a candidate for reuse. An inactive vnode still refers to a valid
file and one can try to reactivate it using vget(9) (this is used a lot
by caches).
Before the VFS can reuse an inactive vnode to refer to another file, it
must clean all information pertaining to the old file. A cleaned out
vnode is called a reclaimed vnode.
To support forceable unmounts and the revoke(2) system call, the VFS may
reclaim a vnode with a positive reference count. The reclaimed vnode is
given to the dead file system, which returns errors for most operations.
The reclaimed vnode will not be reused for another file until its
reference count hits zero.
Vnode pool
The getnewvnode(9) call allocates a vnode from the pool, possibly reusing
an inactive vnode, and returns it to the caller. The vnode returned has
a reference count (v_usecount) of 1.
The vref(9) call increments the reference count on the vnode. It may
only be on a vnode with reference count of 1 or greater. The vrele(9)
and vput(9) calls decrement the reference count. In addition, the
vput(9) call also releases the vnode lock.
The vget(9) call, when used on an inactive vnode, will make the vnode
active by bumping the reference count to one. When called on an active
vnode, vget() increases the reference count by one. However, if the
vnode is being reclaimed concurrently, then vget() will fail and return
an error.
The vgone(9) and vgonel(9) calls orchestrate the reclamation of a vnode.
They can be called on both active and inactive vnodes.
When transitioning a vnode to the reclaimed state, the VFS will call
VOP_RECLAIM(9) method. File systems use this method to free any file-
system-specific data they attached to the vnode.
Vnode locks
The vnode actually has two different types of locks: the vnode lock and
the vnode reclamation lock (VXLOCK).
The vnode lock
The vnode lock and its consistent use accomplishes the following:
o It keeps a locked vnode from changing across certain pairs of VOP_
calls, thus preserving cached data. For example, it keeps the
directory from changing between a VOP_LOOKUP(9) call and a
VOP_CREATE(9). The VOP_LOOKUP() call makes sure the name doesn't
already exist in the directory and finds free room in the directory
for the new entry. The VOP_CREATE() call can then go ahead and
create the file without checking if it already exists or looking for
free space.
o Some file systems rely on it to ensure that only one ``thread'' at a
time is calling VOP_ vnode operations on a given file or directory.
Otherwise, the file system's behavior is undefined.
o On rare occasions, code will hold the vnode lock so that a series of
VOP_ operations occurs as an atomic unit. (Of course, this doesn't
work with network file systems like NFSv2 that don't have any notion
of bundling a bunch of operations into an atomic unit.)
o While the vnode lock is held, the vnode will not be reclaimed.
There is a discipline to using the vnode lock. Some VOP_ operations
require that the vnode lock is held before being called. A description
of this rather arcane locking discipline is in sys/kern/vnode_if.src.
The vnode lock is acquired by calling vn_lock(9) and released by calling
VOP_UNLOCK(9).
A process is allowed to sleep while holding the vnode lock.
The implementation of the vnode lock is the responsibility of the
individual file systems. Not all file systems implement it.
To prevent deadlocks, when acquiring locks on multiple vnodes, the lock
of parent directory must be acquired before the lock on the child
directory.
Other vnode synchronization
The vnode reclamation lock (VXLOCK) is used to prevent multiple processes
from entering the vnode reclamation code. It is also used as a flag to
indicate that reclamation is in progress. The VXWANT flag is set by
processes that wish to be woken up when reclamation is finished.
The vwaitforio(9) call is used to wait for all outstanding write I/Os
associated with a vnode to complete.
Version number/capability
The vnode capability, v_id, is a 32-bit version number on the vnode.
Every time a vnode is reassigned to a new file, the vnode capability is
changed. This is used by code that wishes to keep pointers to vnodes but
doesn't want to hold a reference (e.g., caches). The code keeps both a
vnode pointer and a copy of the capability. The code can later compare
the vnode's capability to its copy and see if the vnode still points to
the same file.
Note: for this to work, memory assigned to hold a struct vnode can only
be used for another purpose when all pointers to it have disappeared.
Since the vnode pool has no way of knowing when all pointers have
disappeared, it never frees memory it has allocated for vnodes.
Vnode fields
Most of the fields of the vnode structure should be treated as opaque and
only manipulated through the proper APIs. This section describes the
fields that are manipulated directly.
The v_flag attribute contains random flags related to various functions.
They are summarized in the following table:
VROOT This vnode is the root of its file system.
VTEXT This vnode is a pure text prototype.
VSYSTEM This vnode is being used by kernel.
VISTTY This vnode represents a tty(4).
VXLOCK This vnode is locked to change its underlying type.
VXWANT A process is waiting for this vnode.
VALIASED This vnode has an alias.
VLOCKSWORK This vnode's underlying file system supports locking
discipline.
The v_tag attribute indicates what file system the vnode belongs to.
Very little code actually uses this attribute and its use is deprecated.
Programmers should seriously consider using more object-oriented
approaches (e.g. function tables). There is no safe way of defining new
v_tag's for loadable file systems. The v_tag attribute is read-only.
The v_type attribute indicates what type of file (e.g. directory,
regular, FIFO) this vnode is. This is used by the generic code for
various checks. For example, the read(2) system call returns zero when a
read is attempted on a directory.
Possible types are:
VNON This vnode has no type.
VREG This vnode represents a regular file.
VDIR This vnode represents a directory.
VBLK This vnode represents a block device.
VCHR This vnode represents a character device.
VLNK This vnode represents a symbolic link.
VSOCK This vnode represents a socket.
VFIFO This vnode represents a named pipe.
VBAD This vnode represents a bad or dead file.
The v_data attribute allows a file system to attach a piece of file
system specific memory to the vnode. This contains information about the
file that is specific to the file system (such as an inode pointer in the
case of FFS).
The v_numoutput attribute indicates the number of pending synchronous and
asynchronous writes on the vnode. It does not track the number of dirty
buffers attached to the vnode. The attribute is used by code like
fsync(2) to wait for all writes to complete before returning to the user.
This attribute must be manipulated at splbio(9).
The v_writecount attribute tracks the number of write calls pending on
the vnode.
Rules
The vast majority of vnode functions may not be called from interrupt
context. The exceptions are bgetvp() and brelvp(). The following fields
of the vnode are manipulated at interrupt level: v_numoutput, v_holdcnt,
v_dirtyblkhd, v_cleanblkhd, v_bioflag, v_freelist, and v_synclist. Any
access to these fields should be protected by splbio(9).
SEE ALSOuvm(9), vaccess(9), vclean(9), vcount(9), vdevgone(9), vfinddev(9),
vflush(9), vflushbuf(9), vfs(9), vget(9), vgone(9), vhold(9),
vinvalbuf(9), vn_lock(9), VOP_LOOKUP(9), vput(9), vrecycle(9), vref(9),
vrele(9), vwaitforio(9), vwakeup(9)HISTORY
This document first appeared in OpenBSD 2.9.
OpenBSD 4.9 April 12, 2008 OpenBSD 4.9
