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

NAME
     vnode, vref, vrele, vrele_async, vget, vput, vhold, holdrele,
     getnewvnode, ungetnewvnode, vrecycle, vgone, vgonel, vdead_check, vflush,
     vaccess, bdevvp, cdevvp, vfinddev, vdevgone, vwakeup, vflushbuf,
     vinvalbuf, vtruncbuf, vprint -- kernel representation of a file or direc-
     tory

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);

     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
     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_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_LAYER       This vnode is on a layered file system.
           VI_LOCKSHARE   This vnode shares its v_interlock with other vnodes.
           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 ker-
     nel 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 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.

     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 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 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.

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 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 VI_XLOCK flag is
              set in vp's v_flag, vnode vp is being recycled in vgone() and
              the calling thread sleeps until the transition is complete.
              When it is awakened, an error is returned to indicate that the
              vnode is no longer usable.

     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() allo-
              cates a new vnode.  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)
              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.

     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 func-
              tion 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 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 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 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.

NetBSD 7.0                      March 24, 2014                      NetBSD 7.0

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