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


NAME

     namecache, cache_lookup, cache_revlookup, cache_enter, cache_purge,
     cache_purgevfs, namecache_print -- name lookup cache


SYNOPSIS

     #include <sys/namei.h>
     #include <sys/proc.h>
     #include <sys/uio.h>
     #include <sys/vnode.h>

     int
     cache_lookup(struct vnode *dvp, struct vnode **vpp,
         struct componentname *cnp);

     int
     cache_revlookup(struct vnode *vp, struct vnode *dvp, char **bpp,
         char *bufp);

     void
     cache_enter(struct vnode *dvp, struct vnode *vp,
         struct componentname *cnp);

     void
     cache_purge(struct vnode *vp);

     void
     cache_purgevfs(struct mount *mp);

     void
     namecache_print(struct vnode *vp, void (*func)(const char *, ...));


DESCRIPTION

     The name lookup cache is the mechanism to allow the file system type
     dependent algorithms to quickly resolve a file's vnode from its pathname.
     The name lookup cache is generally accessed through the higher-level
     namei(9) interface.

     The name of the file is used to lookup an entry associated with the file
     in the name lookup cache.  If no entry is found, one is created for it.
     If an entry is found, the information obtained from the cache lookup con-
     tains information about the file which is useful to the file system type
     dependent functions.

     The name lookup cache is managed by a least recently used (LRU) algorithm
     so frequently used names will hang around.  The cache itself is a hash
     table called nchashtbl, containing namecache entries that are allocated
     dynamically from a kernel memory pool.  Each entry has the following
     structure:

     #define NCHNAMLEN       31      /* maximum name segment length */
     struct  namecache {
             LIST_ENTRY(namecache) nc_hash;  /* hash chain */
             TAILQ_ENTRY(namecache) nc_lru;  /* LRU chain */
             LIST_ENTRY(namecache) nc_vhash; /* directory hash chain */
             LIST_ENTRY(namecache) nc_dvlist;
             struct  vnode *nc_dvp;          /* vnode of parent of name */
             LIST_ENTRY(namecache) nc_vlist;
             struct  vnode *nc_vp;           /* vnode the name refers to */
             int     nc_flags;               /* copy of componentname's ISWHITEOUT */
             char    nc_nlen;                /* length of name */
             char    nc_name[NCHNAMLEN];     /* segment name */
     };

     For simplicity (and economy of storage), names longer than a maximum
     length of NCHNAMLEN are not cached; they occur infrequently in any case,
     and are almost never of interest.

     Each namecache entry can appear on two hash chains in addition to
     nshashtbl: ncvhashtbl (the name cache directory hash chain), and
     nclruhead (the name cache LRU chain).  The hash chains are indexed by a
     hash value obtained from the file's name and the address of its parent
     directory vnode.

     Functions which access to the name cache pass arguments in the partially
     initialised componentname structure.  See vnodeops(9) for details on this
     structure.


FUNCTIONS

     cache_lookup(dvp, vpp, cnp)
              Look for a name in the cache.  cache_lookup() is called with dvp
              pointing to the vnode of the directory to search and cnp point-
              ing to the partially initialised component structure.
              cnp->cn_nameptr points to the name of the entry being sought,
              cnp->cn_namelen tells the length of the name, and cnp->cn_hash
              contains a hash of the name.  If the lookup succeeds, the vnode
              is locked, stored in vpp and a status of zero is returned.  If
              the locking fails for whatever reason, the vnode is unlocked and
              the error is returned.  If the lookup determines that the name
              does not exist any longer, a status of ENOENT is returned.  If
              the lookup fails, a status of -1 is returned.

     cache_revlookup(vp, dvp, bpp, bufp)
              Scan cache looking for name of directory entry pointing at vp
              and fill in dvpp.  If bufp is non-NULL, also place the name in
              the buffer which starts at bufp, immediately before bpp, and
              move bpp backwards to point at the start of it.  If the lookup
              succeeds, the vnode is referenced.  Returns 0 on success, -1 on
              cache miss, positive errno on failure.

     cache_enter(dvp, vp, cnp)
              Add an entry to the cache.  cache_enter() is called with dvp
              pointing to the vnode of the directory to enter and cnp pointing
              to the partially initialised component structure.  If vp is
              NULL, a negative cache entry is created, specifying that the
              entry does not exist in the file system.  cnp->cn_nameptr points
              to the name of the entry being entered, cnp->cn_namelen tells
              the length of the name, and cnp->cn_hash contains a hash of the
              name.

     cache_purge(vp)
              Flush the cache of a particular vnode vp.  cache_purge() is
              called when a vnode is renamed to hide entries that would now be
              invalid.

     cache_purgevfs(mp)
              Flush cache of a whole file system mp.  cache_purgevfs() is
              called when file system is unmounted to remove entries that
              would now be invalid.

     namecache_print(vp, func)
              Print all entries of the name cache.  func is the printf(9) for-
              mat.  namecache_print() is only defined if the kernel option DDB
              is compiled into the kernel.


CODE REFERENCES

     The name lookup cache is implemented within the file
     sys/kern/vfs_cache.c.


SEE ALSO

     intro(9), namei(9), vfs(9), vnode(9)


HISTORY

     The name lookup cache first appeared in 4.2BSD.


AUTHORS

     The original name lookup cache was written by Robert Elz.

NetBSD 6.1.2                     July 21, 2010                    NetBSD 6.1.2