.\" $OpenBSD: vnode.9,v 1.28 2011/07/18 12:03:45 thib Exp $ .\" .\" Copyright (c) 2001 Constantine Sapuntzakis .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, .\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY .\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL .\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, .\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, .\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; .\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, .\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR .\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF .\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: July 18 2011 $ .Dt VNODE 9 .Os .Sh NAME .Nm vnode .Nd an overview of vnodes .Sh DESCRIPTION A .Em vnode is an object in kernel memory that speaks the .Ux file interface (open, read, write, close, readdir, etc.). Vnodes can represent files, directories, FIFOs, domain sockets, block devices, character devices. .Pp Each vnode has a set of methods which start with the string .Dq VOP_ . These methods include .Fn VOP_OPEN , .Fn VOP_READ , .Fn VOP_WRITE , .Fn VOP_RENAME , .Fn VOP_CLOSE , and .Fn VOP_MKDIR . Many of these methods correspond closely to the equivalent file system call \- .Xr open 2 , .Xr read 2 , .Xr write 2 , .Xr rename 2 , etc. Each file system (FFS, NFS, etc.) provides implementations for these methods. .Pp The Virtual File System library (see .Xr 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. .Pp The definition of a vnode is as follows: .Bd -literal 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 .Ed .Ss 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 .Pq Va 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 .Em active . .Pp When a vnode's reference count drops to zero, it becomes .Em inactive , that is, a candidate for reuse. An inactive vnode still refers to a valid file and one can try to reactivate it using .Xr vget 9 (this is used a lot by caches). .Pp 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 .Em reclaimed vnode. .Pp To support forceable unmounts and the .Xr 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. .Ss Vnode pool The .Xr 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 .Pq Va v_usecount of 1. .Pp The .Xr 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 .Xr vrele 9 and .Xr vput 9 calls decrement the reference count. In addition, the .Xr vput 9 call also releases the vnode lock. .Pp The .Xr 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, .Fn vget increases the reference count by one. However, if the vnode is being reclaimed concurrently, then .Fn vget will fail and return an error. .Pp The .Xr vgone 9 and .Xr vgonel 9 calls orchestrate the reclamation of a vnode. They can be called on both active and inactive vnodes. .Pp When transitioning a vnode to the reclaimed state, the VFS will call the .Xr VOP_RECLAIM 9 method. File systems use this method to free any file-system-specific data they attached to the vnode. .Ss Vnode locks The vnode actually has two different types of locks: the vnode lock and the vnode reclamation lock .Pq Dv VXLOCK . .Ss The vnode lock The vnode lock and its consistent use accomplishes the following: .Bl -bullet .It 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 .Xr VOP_LOOKUP 9 call and a .Xr VOP_CREATE 9 . The .Fn 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 .Fn VOP_CREATE call can then go ahead and create the file without checking if it already exists or looking for free space. .It Some file systems rely on it to ensure that only one .Dq thread at a time is calling VOP_ vnode operations on a given file or directory. Otherwise, the file system's behavior is undefined. .It 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.) .It While the vnode lock is held, the vnode will not be reclaimed. .El .Pp There is a discipline to using the vnode lock. Some VOP_ operations require that the vnode lock is held before being called. .Pp The vnode lock is acquired by calling .Xr vn_lock 9 and released by calling .Xr VOP_UNLOCK 9 . .Pp A process is allowed to sleep while holding the vnode lock. .Pp The implementation of the vnode lock is the responsibility of the individual file systems. Not all file systems implement it. .Pp To prevent deadlocks, when acquiring locks on multiple vnodes, the lock of parent directory must be acquired before the lock on the child directory. .Ss Other vnode synchronization The vnode reclamation lock .Pq Dv 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 .Dv VXWANT flag is set by processes that wish to be woken up when reclamation is finished. .Pp The .Xr vwaitforio 9 call is used to wait for all outstanding write I/Os associated with a vnode to complete. .Ss Version number/capability The vnode capability, .Va 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. .Pp Note: for this to work, memory assigned to hold a .Vt 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. .Ss 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. .Pp The .Va v_flag attribute contains random flags related to various functions. They are summarized in the following table: .Pp .Bl -tag -width 10n -compact -offset indent .It Dv VROOT This vnode is the root of its file system. .It Dv VTEXT This vnode is a pure text prototype. .It Dv VSYSTEM This vnode is being used by kernel. .It Dv VISTTY This vnode represents a .Xr tty 4 . .It Dv VXLOCK This vnode is locked to change its underlying type. .It Dv VXWANT A process is waiting for this vnode. .It Dv VALIASED This vnode has an alias. .It Dv VLOCKSWORK This vnode's underlying file system supports locking discipline. .El .Pp The .Va 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 .Va v_tag Ns 's for loadable file systems. The .Va v_tag attribute is read-only. .Pp The .Va 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 .Xr read 2 system call returns zero when a read is attempted on a directory. .Pp Possible types are: .Pp .Bl -tag -width 10n -offset indent -compact .It Dv VNON This vnode has no type. .It Dv VREG This vnode represents a regular file. .It Dv VDIR This vnode represents a directory. .It Dv VBLK This vnode represents a block device. .It Dv VCHR This vnode represents a character device. .It Dv VLNK This vnode represents a symbolic link. .It Dv VSOCK This vnode represents a socket. .It Dv VFIFO This vnode represents a named pipe. .It Dv VBAD This vnode represents a bad or dead file. .El .Pp The .Va 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). .Pp The .Va 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 .Xr fsync 2 to wait for all writes to complete before returning to the user. This attribute must be manipulated at .Xr splbio 9 . .Pp The .Va v_writecount attribute tracks the number of write calls pending on the vnode. .Ss Rules The vast majority of vnode functions may not be called from interrupt context. The exceptions are .Fn bgetvp and .Fn brelvp . The following fields of the vnode are manipulated at interrupt level: .Va v_numoutput , v_holdcnt , v_dirtyblkhd , .Va v_cleanblkhd , v_bioflag , v_freelist , and .Va v_synclist . Any access to these fields should be protected by .Xr splbio 9 . .Sh SEE ALSO .Xr uvm 9 , .Xr vaccess 9 , .Xr vclean 9 , .Xr vcount 9 , .Xr vdevgone 9 , .Xr vfinddev 9 , .Xr vflush 9 , .Xr vflushbuf 9 , .Xr vfs 9 , .Xr vget 9 , .Xr vgone 9 , .Xr vhold 9 , .Xr vinvalbuf 9 , .Xr vn_lock 9 , .Xr VOP_LOOKUP 9 , .Xr vput 9 , .Xr vrecycle 9 , .Xr vref 9 , .Xr vrele 9 , .Xr vwaitforio 9 , .Xr vwakeup 9 .Sh HISTORY This document first appeared in .Ox 2.9 .