VFS_CACHE(9) OpenBSD Kernel Manual VFS_CACHE(9)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 {
LIST_ENTRY(namecache) nc_hash; /* hash chain */
LIST_ENTRY(namecache) nc_vhash; /* (reverse) dir hash chain */
TAILQ_ENTRY(namecache) nc_lru; /* LRU chain */
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[NCHNAMLEN]; /* 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 NCHNAMLEN 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 DELETE or the NOCACHE flag 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, ENOENT is
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 NULL and -1 will be returned. Otherwise, failure
has occurred, dvpp will be set to NULL, 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 ALSOvmstat(8), namei(9), vfs(9), vnode(9)HISTORY
The vfs_cache API first appeared in 4.2BSD.
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9
