VNODE(9)

VNODE(9) | Kernel Developer's Manual | VNODE(9)

# NAME

vnode, vref, vrele, vrele_async, vput, vhold, holdrele, vcache_get, vcache_new, vcache_rekey_enter, vcache_rekey_exit, vrecycle, vgone, vgonel, vdead_check, vflush, vaccess, bdevvp, cdevvp, vfinddev, vdevgone, vwakeup, vflushbuf, vinvalbuf, vtruncbuf, vprint — kernel representation of a file or directory

# SYNOPSIS

#include <sys/param.h>
#include <sys/vnode.h>
void
vref(struct vnode *vp);
void
vrele(struct vnode *vp);
void
vrele_async(struct vnode *vp);
void
vput(struct vnode *vp);
void
vhold(struct vnode *vp);
void
holdrele(struct vnode *vp);
int
vcache_get(struct mount *mp, const void *key, size_t key_len, struct vnode **vpp);
int
vcache_new(struct mount *mp, struct vnode *dvp, struct vattr *vap, kauth_cred_t cred, struct vnode **vpp);
int
vcache_rekey_enter(struct mount *mp, struct vnode *vp, const void *old_key, size_t old_key_len, const void *new_key, size_t new_key_len);
void
vcache_rekey_exit(struct mount *mp, struct vnode *vp, const void *old_key, size_t old_key_len, const void *new_key, size_t new_key_len);
int
vrecycle(struct vnode *vp);
void
vgone(struct vnode *vp);
void
vgonel(struct vnode *vp, struct lwp *l);
int
vdead_check(struct vnode *vp, int flags);
int
vflush(struct mount *mp, struct vnode *skipvp, int flags);
int
vaccess(enum vtype type, mode_t file_mode, uid_t uid, gid_t gid, mode_t acc_mode, kauth_cred_t cred);
int
bdevvp(dev_t dev, struct vnode **vpp);
int
cdevvp(dev_t dev, struct vnode **vpp);
int
vfinddev(dev_t dev, enum vtype, struct vnode **vpp);
void
vdevgone(int maj, int minl, int minh, enum vtype type);
void
vwakeup(struct buf *bp);
int
vflushbuf(struct vnode *vp, int sync);
int
vinvalbuf(struct vnode *vp, int flags, kauth_cred_t cred, struct lwp *l, int slpflag, int slptimeo);
int
vtruncbuf(struct vnode *vp, daddr_t lbn, int slpflag, int slptimeo);
void
vprint(const char *label, struct vnode *vp);

# DESCRIPTION

A vnode represents an on-disk file in use by the system. Each vfs(9) file system provides a set of vnodeops(9) operations on vnodes, invoked by file-system-independent system calls and supported by file-system-independent library routines.
Each mounted file system provides a vnode for the root of the file system, via VFS_ROOT(9). Other vnodes are obtained by VOP_LOOKUP(9). Users of vnodes usually invoke these indirectly via namei(9) to obtain vnodes from paths.
Each file system usually maintains a cache mapping recently used inode numbers, or the equivalent, to vnodes, and a cache mapping recently used file names to vnodes. If memory is scarce, the system may decide to reclaim an unused cached vnode, calling VOP_RECLAIM(9) to remove it from the caches and to free file-system-specific memory associated with it. A file system may also choose to immediately reclaim a cached vnode once it is unused, in VOP_INACTIVE(9), if the vnode has been deleted on disk.
When a file system retrieves a vnode from a cache, the vnode may not have any users, and another thread in the system may be simultaneously deciding to reclaim it. Thus, to retrieve a vnode from a cache, one must use vcache_get(), not vref(), to acquire the first reference.
The vnode has the following structure:

    struct vnode {
    	struct uvm_object v_uobj;		/* the VM object */
    	kcondvar_t	v_cv;			/* synchronization */
    	voff_t		v_size;			/* size of file */
    	voff_t		v_writesize;		/* new size after write */
    	int		v_iflag;		/* VI_* flags */
    	int		v_vflag;		/* VV_* flags */
    	int		v_uflag;		/* VU_* flags */
    	int		v_numoutput;		/* # of pending writes */
    	int		v_writecount;		/* ref count of writers */
    	int		v_holdcnt;		/* page & buffer refs */
    	struct mount	*v_mount;		/* ptr to vfs we are in */
    	int		(**v_op)(void *);	/* vnode operations vector */
    	struct buflists	v_cleanblkhd;		/* clean blocklist head */
    	struct buflists	v_dirtyblkhd;		/* dirty blocklist head */
    	union {
    		struct mount	*vu_mountedhere;/* ptr to vfs (VDIR) */
    		struct socket	*vu_socket;	/* unix ipc (VSOCK) */
    		struct specnode	*vu_specnode;	/* device (VCHR, VBLK) */
    		struct fifoinfo	*vu_fifoinfo;	/* fifo (VFIFO) */
    		struct uvm_ractx *vu_ractx;	/* read-ahead ctx (VREG) */
    	} v_un;
    	enum vtype	v_type;			/* vnode type */
    	enum vtagtype	v_tag;			/* type of underlying data */
    	void 		*v_data;		/* private data for fs */
    	struct klist	v_klist;		/* notes attached to vnode */
    };

Most members of the vnode structure should be treated as opaque and only manipulated using the proper functions. There are some rather common exceptions detailed throughout this page.
Files and file systems are inextricably linked with the virtual memory system and v_uobj contains the data maintained by the virtual memory system. For compatibility with code written before the integration of uvm(9) into NetBSD, C-preprocessor directives are used to alias the members of v_uobj.
Vnode flags are recorded by v_iflag, v_vflag and v_uflag. Valid flags are:

`VV_ROOT`
  This vnode is the root of its file system.
`VV_SYSTEM`
  This vnode is being used by the kernel; only used to skip quota files in vflush().
`VV_ISTTY`
  This vnode represents a tty; used when reading dead vnodes.
`VV_MAPPED`
  This vnode might have user mappings.
`VV_MPSAFE`
  This file system is MP safe.
`VV_LOCKSWORK`
  This vnode's file system supports locking.
`VI_TEXT`
  This vnode is a pure text prototype.
`VI_EXECMAP`
  This vnode has executable mappings.
`VI_WRMAP`
  This vnode might have PROT_WRITE user mappings.
`VI_WRMAPDIRTY`
  This vnode might have dirty pages due to `VWRITEMAP`.
`VI_XLOCK`
  This vnode is currently locked to change underlying type.
`VI_ONWORKLST`
  This vnode is on syncer work-list.
`VI_MARKER`
  A dummy marker vnode.
`VI_CLEAN`
  This vnode has been reclaimed and is no longer attached to a file system.
`VU_DIROP`
  This vnode is involved in a directory operation. This flag is used exclusively by LFS.

The `VI_XLOCK` flag 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. Before v_iflag can be modified, the v_interlock mutex must be acquired. See lock(9) for details on the kernel locking API.
Each vnode has three reference counts: v_usecount, v_writecount and v_holdcnt. The first is the number of active references within the kernel to the vnode. This count is maintained by vref(), vrele(), vrele_async(), and vput(). The second is the number of active references within the kernel to the vnode performing write access to the file. It is maintained by the open(2) and close(2) system calls. The third is the number of references within the kernel requiring the vnode to remain active and not be recycled. This count is maintained by vhold() and holdrele(). When both the v_usecount and v_holdcnt reach zero, the vnode is cached. The transition from the cache is handled by a kernel thread and vrecycle(). Access to v_usecount, v_writecount and v_holdcnt is also protected by the v_interlock mutex.
The number of pending synchronous and asynchronous writes on the vnode are recorded in v_numoutput. It is used by fsync(2) to wait for all writes to complete before returning to the user. Its value must only be modified at splbio (see spl(9)). It does not track the number of dirty buffers attached to the vnode.
The link to the file system which owns the vnode is recorded by v_mount. See vfsops(9) for further information of file system mount status.
The v_op pointer points to its vnode operations vector. This vector describes what operations can be done to the file associated with the vnode. The system maintains one vnode operations vector for each file system type configured into the kernel. The vnode operations vector contains a pointer to a function for each operation supported by the file system. See vnodeops(9) for a description of vnode operations.
When a user wants a new vnode for another file or wants a valid vnode which is cached, vcache_get() or vcache_new() is invoked to allocate a vnode and initialize it for the new file.
The type of object the vnode represents is recorded by v_type. It is used by generic code to perform checks to ensure operations are performed on valid file system objects. Valid types are:

`VNON`
  The vnode has no type.
`VREG`
  The vnode represents a regular file.
`VDIR`
  The vnode represents a directory.
`VBLK`
  The vnode represents a block special device.
`VCHR`
  The vnode represents a character special device.
`VLNK`
  The vnode represents a symbolic link.
`VSOCK`
  The vnode represents a socket.
`VFIFO`
  The vnode represents a pipe.
`VBAD`
  The vnode represents a bad file (not currently used).

Vnode tag types are used by external programs only (e.g., pstat(8)), and should never be inspected by the kernel. Its use is deprecated since new v_tag values cannot be defined for loadable file systems. The v_tag member is read-only. Valid tag types are:

`VT_NON`
  non file system
`VT_UFS`
  universal file system
`VT_NFS`
  network file system
`VT_MFS`
  memory file system
`VT_MSDOSFS`
  FAT file system
`VT_LFS`
  log-structured file system
`VT_LOFS`
  loopback file system
`VT_FDESC`
  file descriptor file system
`VT_NULL`
  null file system layer
`VT_UMAP`
  uid/gid remapping file system layer
`VT_KERNFS`
  kernel interface file system
`VT_PROCFS`
  process interface file system
`VT_AFS`
  AFS file system
`VT_ISOFS`
  ISO 9660 file system(s)
`VT_UNION`
  union file system
`VT_ADOSFS`
  Amiga file system
`VT_EXT2FS`
  Linux's ext2 file system
`VT_CODA`
  Coda file system
`VT_FILECORE`
  filecore file system
`VT_NTFS`
  Microsoft NT's file system
`VT_VFS`
  virtual file system
`VT_OVERLAY`
  overlay file system
`VT_SMBFS`
  SMB file system
`VT_PTYFS`
  pseudo-terminal device file system
`VT_TMPFS`
  efficient memory file system
`VT_UDF`
  universal disk format file system
`VT_SYSVBFS`
  systemV boot file system

The vnode lock is acquired by calling vn_lock(9) and released by calling VOP_UNLOCK(9). The reason for this asymmetry is that vn_lock(9) is a wrapper for VOP_LOCK(9) with extra checks, while the unlocking step usually does not need additional checks and thus has no wrapper.
The vnode locking operation is complicated because it is used for many purposes. Sometimes it is used to bundle a series of vnode operations (see vnodeops(9)) into an atomic group. Many file systems rely on it to prevent race conditions in updating file system type specific data structures rather than using their own private locks. The vnode lock can operate as a multiple-reader (shared-access lock) or single-writer lock (exclusive access lock), however many current file system implementations were written assuming only single-writer locking. Multiple-reader locking functions equivalently only in the presence of big-lock SMP locking or a uni-processor machine. The lock may be held while sleeping. While the vnode lock is acquired, the holder is guaranteed that the vnode will not be reclaimed or invalidated. Most file system functions require that you hold the vnode lock on entry. See lock(9) for details on the kernel locking API.
Each file system underlying a vnode allocates its own private area and hangs it from v_data.
Most functions discussed in this page that operate on vnodes cannot be called from interrupt context. The members v_numoutput, v_holdcnt, v_dirtyblkhd, and v_cleanblkhd are modified in interrupt context and must be protected by splbio(9) unless it is certain that there is no chance an interrupt handler will modify them. The vnode lock must not be acquired within interrupt context.

# FUNCTIONS

vref(vp)
  Increment v_usecount of the vnode vp. Any kernel thread system which uses a vnode (e.g., during the operation of some algorithm or to store in a data structure) should call vref().
vrele(vp)
  Decrement v_usecount of unlocked vnode vp. Any code in the system which is using a vnode should call vrele() when it is finished with the vnode. If v_usecount of the vnode reaches zero and v_holdcnt is greater than zero, the vnode is placed on the holdlist. If both v_usecount and v_holdcnt are zero, the vnode is cached.
vrele_async(vp)
  Will asynchronously release the vnode in different context than the caller, sometime after the call.
vput(vp)
  Legacy convenience routine for unlocking and releasing vp. Equivalent to:

      VOP_UNLOCK(vp);
      vrele(vp);

  New code should prefer using VOP_UNLOCK(9) and vrele() directly.
vhold(vp)
  Mark the vnode vp as active by incrementing vp->v_holdcnt. Once held, the vnode will not be recycled until it is released with holdrele().
holdrele(vp)
  Mark the vnode vp as inactive by decrementing vp->v_holdcnt.
vcache_get(mp, key, key_len, vpp)
  Allocate a new vnode. The new vnode is returned referenced in the address specified by vpp.
  The argument mp is the mount point for the file system to lookup the file in.
  The arguments key and key_len uniquely identify the file in the file system.
  If a vnode is successfully retrieved zero is returned, otherwise an appropriate error code is returned.
vcache_new(mp, dvp, vap, cred, vpp)
  Allocate a new vnode with a new file. The new vnode is returned referenced in the address specified by vpp.
  The argument mp is the mount point for the file system to create the file in.
  The argument dvp points to the directory to create the file in.
  The argument vap points to the attributes for the file to create.
  The argument cred holds the credentials for the file to create.
  If a vnode is successfully created zero is returned, otherwise an appropriate error code is returned.
vcache_rekey_enter(mp, vp, old_key, old_key_len, new_key, new_key_len)
  Prepare to change the key of a cached vnode.
  The argument mp is the mount point for the file system the vnode vp resides in.
  The arguments old_key and old_key_len identify the cached vnode.
  The arguments new_key and new_key_len will identify the vnode after rename.
  If the new key already exists `EEXIST` is returned, otherwise zero is returned.
vcache_rekey_exit(mp, vp, old_key, old_key_len, new_key, new_key_len)
  Finish rename after calling vcache_rekey_enter().
vrecycle(vp)
  Recycle the referenced vnode vp if this is the last reference. vrecycle() is a null operation if the reference count is greater than one.
vgone(vp)
  Eliminate all activity associated with the unlocked vnode vp in preparation for recycling. This operation is restricted to suspended file systems. See vfs_suspend(9).
vgonel(vp, p)
  Eliminate all activity associated with the locked vnode vp in preparation for recycling.
vdead_check(vp, flags)
  Check the vnode vp for being or becoming dead. Returns `ENOENT` for a dead vnode and zero otherwise. If flags is `VDEAD_NOWAIT` it will return `EBUSY` if the vnode is becoming dead and the function will not sleep.
  Whenever this function returns a non-zero value all future calls for this vp will also return a non-zero value.
vflush(mp, skipvp, flags)
  Remove any vnodes in the vnode table belonging to mount point mp. If skipvp is not `NULL` it is exempt from being flushed. The argument flags is a set of flags modifying the operation of vflush(). If `FORCECLOSE` is not specified, there should not be any active vnodes and the error `EBUSY` is returned if any are found (this is a user error, not a system error). If `FORCECLOSE` is specified, active vnodes that are found are detached. If `WRITECLOSE` is set, only flush out regular file vnodes open for writing. SKIPSYSTEM causes any vnodes marked `V_SYSTEM` to be skipped.
vaccess(type, file_mode, uid, gid, acc_mode, cred)
  Do access checking by comparing the file's permissions to the caller's desired access type acc_mode and credentials cred.
bdevvp(dev, vpp)
  Create a vnode for a block device. bdevvp() is used for root file systems, swap areas and for memory file system special devices.
cdevvp(dev, vpp)
  Create a vnode for a character device. cdevvp() is used for the console and kernfs special devices.
vfinddev(dev, vtype, vpp)
  Lookup a vnode by device number. The vnode is referenced and returned in the address specified by vpp.
vdevgone(int maj, int min, int minh, enum vtype type)
  Reclaim all vnodes that correspond to the specified minor number range minl to minh (endpoints inclusive) of the specified major maj.
vwakeup(bp)
  Update outstanding I/O count vp->v_numoutput for the vnode bp->b_vp and do a wakeup if requested and vp->vflag has `VBWAIT` set.
vflushbuf(vp, sync)
  Flush all dirty buffers to disk for the file with the locked vnode vp. The argument sync specifies whether the I/O should be synchronous and vflushbuf() will sleep until vp->v_numoutput is zero and vp->v_dirtyblkhd is empty.
vinvalbuf(vp, flags, cred, l, slpflag, slptimeo)
  Flush out and invalidate all buffers associated with locked vnode vp. The argument l and cred specified the calling process and its credentials. The ltsleep(9) flag and timeout are specified by the arguments slpflag and slptimeo respectively. If the operation is successful zero is returned, otherwise an appropriate error code is returned.
vtruncbuf(vp, lbn, slpflag, slptimeo)
  Destroy any in-core buffers past the file truncation length for the locked vnode vp. The truncation length is specified by lbn. vtruncbuf() will sleep while the I/O is performed, The ltsleep(9) flag and timeout are specified by the arguments slpflag and slptimeo respectively. If the operation is successful zero is returned, otherwise an appropriate error code is returned.
vprint(label, vp)
  This function is used by the kernel to dump vnode information during a panic. It is only used if the kernel option DIAGNOSTIC is compiled into the kernel. The argument label is a string to prefix the information dump of vnode vp.

# CODE REFERENCES

The vnode framework is implemented within the file sys/kern/vfs_subr.c.

# SEE ALSO

intro(9), lock(9), namecache(9), namei(9), uvm(9), vattr(9), vfs(9), vfsops(9), vnodeops(9), vnsubr(9)

# BUGS

The locking protocol is inconsistent. Many vnode operations are passed locked vnodes on entry but release the lock before they exit. The locking protocol is used in some places to attempt to make a series of operations atomic (e.g., access check then operation). This does not work for non-local file systems that do not support locking (e.g., NFS). The vnode interface would benefit from a simpler locking protocol.

May 28, 2017 | NetBSD 8.0