VNODE(9) BSD Kernel Developer's Manual VNODE(9)NAME
vnode, vref, vrele, vrele_async, vget, vput, vhold, holdrele,
getnewvnode, ungetnewvnode, vrecycle, vgone, vgonel, 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);
int
vget(struct vnode *vp, int lockflag);
void
vput(struct vnode *vp);
void
vhold(struct vnode *vp);
void
holdrele(struct vnode *vp);
int
getnewvnode(enum vtagtype tag, struct mount *mp, int (**vops)(void *),
kmutex_t *slock, struct vnode **vpp);
void
ungetnewvnode(struct vnode *vp);
int
vrecycle(struct vnode *vp, struct simplelock *inter_lkp, struct lwp *l);
void
vgone(struct vnode *vp);
void
vgonel(struct vnode *vp, struct lwp *l);
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
The vnode is the focus of all file activity in NetBSD. There is a unique
vnode allocated for each active file, directory, mounted-on file, fifo,
domain socket, symbolic link and device. The kernel has no concept of a
file's underlying structure and so it relies on the information stored in
the vnode to describe the file. Thus, the vnode associated with a file
holds all the administration information pertaining to it.
When a process requests an operation on a file, the vfs(9) interface
passes control to a file system type dependent function to carry out the
operation. If the file system type dependent function finds that a vnode
representing the file is not in main memory, it dynamically allocates a
new vnode from the system main memory pool. Once allocated, the vnode is
attached to the data structure pointer associated with the cause of the
vnode allocation and it remains resident in the main memory until the
system decides that it is no longer needed and can be recycled.
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 */
int v_synclist_slot; /* synclist slot index */
struct mount *v_mount; /* ptr to vfs we are in */
int (**v_op)(void *); /* vnode operations vector */
TAILQ_ENTRY(vnode) v_freelist; /* vnode freelist */
struct vnodelst *v_freelisthd; /* which freelist? */
TAILQ_ENTRY(vnode) v_mntvnodes; /* vnodes for mount point */
struct buflists v_cleanblkhd; /* clean blocklist head */
struct buflists v_dirtyblkhd; /* dirty blocklist head */
TAILQ_ENTRY(vnode) v_synclist; /* vnodes with dirty bufs */
LIST_HEAD(, namecache) v_dnclist; /* namecaches (children) */
LIST_HEAD(, namecache) v_nclist; /* namecaches (parent) */
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 */
struct vnlock v_lock; /* lock for this vnode */
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 sys‐
tem. For compatibility with code written before the integration of
uvm(9) into NetBSD, C-preprocessor directives are used to alias the mem‐
bers of v_uobj.
Vnode flags are recorded by v_flag. Valid flags are:
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 the kernel; only used to
skip quota files in vflush().
VISTTY This vnode represents a tty; used when reading dead
vnodes.
VEXECMAP This vnode has executable mappings.
VWRITEMAP This vnode might have PROT_WRITE user mappings.
VWRITEMAPDIRTY
This vnode might have dirty pages due to VWRITEMAP
VLOCKSWORK This vnode's file system supports locking.
VXLOCK This vnode is currently locked to change underlying
type.
VXWANT A process is waiting for this vnode.
VBWAIT Waiting for output associated with this vnode to com‐
plete.
VALIASED This vnode has an alias.
VDIROP This vnode is involved in a directory operation. This
flag is used exclusively by LFS.
VLAYER This vnode is on a layered file system.
VONWORKLST This vnode is on syncer work-list.
VFREEING This vnode is being freed.
VMAPPED This vnode might have user mappings.
The VXLOCK 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. The VXWANT flag is set by threads that wish
to be awakened when reclamation is finished. Before v_flag can be modi‐
fied, the v_interlock simplelock 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 ker‐
nel 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 recycled to the freelist and may be reused for another file. The
transition to and from the freelist is handled by getnewvnode(),
ungetnewvnode() and vrecycle(). Access to v_usecount, v_writecount and
v_holdcnt is also protected by the v_interlock simplelock.
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.
v_dnclist and v_nclist are used by namecache(9) to maintain the list of
associated entries so that cache_purge(9) can purge them.
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 con‐
tains a pointer to a function for each operation supported by the file
system. See vnodeops(9) for a description of vnode operations.
When not in use, vnodes are kept on the freelist through v_freelist. The
vnodes still reference valid files but may be reused to refer to a new
file at any time. When a valid vnode which is on the freelist is used
again, the user must call vget() to increment the reference count and
retrieve it from the freelist. When a user wants a new vnode for another
file, getnewvnode() is invoked to remove a vnode from the freelist 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 mem‐
ber 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
All vnode locking operations use v_lock. This lock is acquired by call‐
ing 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 struc‐
tures 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 lock‐
ing 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 v_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, v_cleanblkhd, v_freelist, and v_synclist 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.
FUNCTIONSvref(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 sys‐
tem which is using a vnode should call vrele() when it is fin‐
ished 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 placed on the freelist.
vrele_async(vp)
Will asychronously release the vnode in different context than
the caller, sometime after the call.
vget(vp, lockflags)
Reclaim vnode vp from the freelist, increment its reference
count and lock it. The argument lockflags specifies the
rwlock(9) flags used to lock the vnode. If the VXLOCK is set in
vp's v_flag, vnode vp is being recycled in vgone() and the call‐
ing thread sleeps until the transition is complete. When it is
awakened, an error is returned to indicate that the vnode is no
longer usable (possibly having been recycled to a new file sys‐
tem type).
vput(vp)
Unlock vnode vp and decrement its v_usecount. Depending on the
reference counts, move the vnode to the holdlist or the freel‐
ist. This operation is functionally equivalent to calling
VOP_UNLOCK(9) followed by vrele().
vhold(vp)
Mark the vnode vp as active by incrementing vp->v_holdcnt and
moving the vnode from the freelist to the holdlist. Once on the
holdlist, 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 and
moving the vnode from the holdlist to the freelist.
getnewvnode(tag, mp, vops, slock, vpp)
Retrieve the next vnode from the freelist. getnewvnode() must
choose whether to allocate a new vnode or recycle an existing
one. The criterion for allocating a new one is that the total
number of vnodes is less than the number desired or there are no
vnodes on either free list. Generally only vnodes that have no
buffers associated with them are recycled and the next vnode
from the freelist is retrieved. If the freelist is empty,
vnodes on the holdlist are considered. The new vnode is
returned in the address specified by vpp.
The argument mp is the mount point for the file system requested
the new vnode. Before retrieving the new vnode, the file system
is checked if it is busy (such as currently unmounting). An
error is returned if the file system is unmounted.
The argument tag is the vnode tag assigned to *vpp->v_tag. The
argument vops is the vnode operations vector of the file system
requesting the new vnode. If a vnode is successfully retrieved
zero is returned, otherwise an appropriate error code is
returned. If slock is not NULL, it specifies the lock to share
for v_interlock. The reference will be held on the lock and
sharing noted. Reference will be released and lock unshared
when the vnode gets recycled. If NULL (regular case), vnode
will use its own interlock.
ungetnewvnode(vp)
Undo the operation of getnewvnode(). The argument vp is the
vnode to return to the freelist. This function is needed for
VFS_VGET(9) which may need to push back a vnode in case of a
locking race condition.
vrecycle(vp, inter_lkp, l)
Recycle the unused vnode vp to the front of the freelist.
vrecycle() is a null operation if the reference count is greater
than zero.
vgone(vp)
Eliminate all activity associated with the unlocked vnode vp in
preparation for recycling.
vgonel(vp, p)
Eliminate all activity associated with the locked vnode vp in
preparation for recycling.
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 speci‐
fied by the arguments slpflag and slptimeo respectively. If the
operation is successful zero is returned, otherwise an appropri‐
ate 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 success‐
ful 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 ALSOintro(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 lock‐
ing protocol is used in some places to attempt to make a series of opera‐
tions 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.
BSD February 8, 2012 BSD
