NAME
vfs_cache,
cache_enter, cache_lookup,
cache_purge, cache_purgevfs,
cache_revlookup —
name lookup cache
SYNOPSIS
#include
<sys/vnode.h>
#include <sys/namei.h>
int
cache_lookup(struct
vnode *dvp, struct vnode
**vpp, struct
componentname *cnp);
void
cache_enter(struct
vnode *dvp, struct vnode
*vp, struct componentname
*cnp);
void
cache_purge(struct
vnode *vp);
void
cache_purgevfs(struct
mount *mp);
int
cache_revlookup(struct
vnode *vp, struct vnode
**dvpp, char **bpp,
char *bufp);
DESCRIPTION
In order to speed up file name look-up operations (see VOP_LOOKUP(9)), the kernel provides an interface for maintaining a cache of the most recently looked-up file name translations. Entries in this cache have the following definition:
struct namecache {
TAILQ_ENTRY(namecache) nc_lru; /* Regular Entry LRU chain */
TAILQ_ENTRY(namecache) nc_neg; /* Negative Entry LRU chain */
RBT_ENTRY(namecache) n_rbcache; /* Namecache rb tree from vnode */
TAILQ_ENTRY(namecache) nc_me; /* ncp's referring to me */
struct vnode *nc_dvp; /* vnode of parent of name */
u_long nc_dvpid; /* capability number of nc_dvp */
struct vnode *nc_vp; /* vnode the name refers to */
u_long nc_vpid; /* capability number of nc_vp */
char nc_nlen; /* length of name */
char nc_name[NAMECACHE_MAXLEN]; /* segment name */
};
The cache is indexed by a hash value based on the file's base name and its encompassing directory's vnode generation number. Negative caching is also performed so that frequently accessed path names of files that do not exist do not result in expensive lookups.
File names with length longer than
NAMECACHE_MAXLEN are not cached to simplify lookups
and to save space. Such names are rare and are generally not worth
caching.
The vfs_cache API contains the following
routines:
cache_lookup(dvp, vpp, cnp)- Look up the given name in the cache. dvp points to
the directory to search, vpp points to a pointer
where the vnode of the name being sought will be stored, and
cnp contains the last component of the path name.
cnp must have the cn_nameptr,
cn_namelen, and cn_hash fields
filled in. If no entry is found for the given name, a new one will be
created, even if the path name fails (i.e. it will be negative cached),
unless the namei(9) lookup operation was
DELETEor theNOCACHEflag was set for the call to namei(9).Upon success, a pointer to a locked vnode is stored in vpp and a zero value is returned. If locking the vnode fails, the vnode will remain unlocked, *vpp will be set to
NULL, and the corresponding error will be returned. If the cache entry is negative cached, meaning the name is no longer valid,ENOENTis returned. Otherwise, the cache lookup has failed and a -1 value is returned. cache_enter(dvp, vp, cnp)- Add a new entry for the translation in the directory dvp for the vnode vp with name cnp to the cache. cnp must have the cn_nameptr, cn_namelen, and cn_hash fields filled in.
cache_purge(vp)- Flush all cache entries corresponding with the given vnode vp. This is called after rename operations to hide entries that would no longer be valid.
cache_purgevfs(mp)- Flush all cache entries for name translations associated with the file system mount described by mp. This is called when unmounting file systems, which would make all name translations pertaining to the mount invalid.
cache_revlookup(vp, dvpp, bpp, bufp)- Scan the cache for the name of the directory entry that points to
vp. dvpp points to where a
pointer to the encompassing directory will be stored. If
bufp is not
NULL, the name will be written to the end of the space between this pointer and the value in bpp, and bpp will be updated on return to point to the start of the copied name.On success, *dvpp will be set to point to the encompassing directory and zero will be returned. If the cache misses, dvpp will be set to
NULLand -1 will be returned. Otherwise, failure has occurred, dvpp will be set toNULL, and an appropriate error code will be returned.
CODE REFERENCES
The vfs_cache API is implemented in the
file sys/kern/vfs_cache.c.
SEE ALSO
HISTORY
The vfs_cache API first appeared in
4.2BSD.