NAMECACHE(9) | Kernel Developer's Manual | NAMECACHE(9) |
int
cache_lookup(struct vnode *dvp, const char *name, size_t namelen, uint32_t nameiop, uint32_t nameiflags, int *iswhiteout, struct vnode **vpp);
int
cache_revlookup(struct vnode *vp, struct vnode *dvp, char **bpp, char *bufp);
void
cache_enter(struct vnode *dvp, struct vnode *vp, const char *name, size_t namelen, uint32_t nameiflags);
void
cache_purge(struct vnode *vp);
void
cache_purgevfs(struct mount *mp);
void
namecache_print(struct vnode *vp, void (*func)(const char *, ...));
The name of the file is used to look up 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.
The lookup can produce either a cache miss or a cache hit, and a cache hit can either be a positive hit, where the name looked up refers to some existing object, or a negative hit, where the name looked up is known to refer to no existing object. (The lookup cannot fail, in the sense of generating an error condition that requires aborting the operation in progress.)
On a cache miss, cache_lookup() returns zero (false). On a positive hit, the unlocked vnode for the object found is stored in vpp, and a nonzero (true) value is returned. On a negative hit, vpp is set to contain a null pointer and a nonzero value is returned. Usually a negative hit will prompt the caller to fail with ENOENT.
The iswhiteout argument is a pointer to an integer result that will be set to nonzero if the entry represents a whiteout, and zero if it does not. This pointer may be NULL if the caller does not support whiteouts. According to the current scheme for handling whiteouts, if cache_lookup() sets iswhiteout the caller should add ISWHITEOUT to the cn_flags field of its struct componentname.
If vp is NULL, a negative cache entry is created, specifying that the entry does not exist in the file system.
February 7, 2014 | NetBSD 7.2 |