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 contains 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 pointing 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) format. 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.