man/man9/namecache.9

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.Dd November 5, 2012
.Dt NAMECACHE 9
.Os
.Sh NAME
.Nm namecache ,
.Nm cache_lookup ,
.Nm cache_revlookup ,
.Nm cache_enter ,
.Nm cache_purge ,
.Nm cache_purgevfs ,
.Nm namecache_print
.Nd name lookup cache
.Sh SYNOPSIS
.In sys/namei.h
.In sys/proc.h
.In sys/uio.h
.In sys/vnode.h
.Ft int
.Fn cache_lookup "struct vnode *dvp" "const char *name" "size_t namelen" \
"uint32_t nameiop" "uint32_t nameiflags" "int *iswhiteout" "struct vnode **vpp"
.Ft int
.Fn cache_revlookup "struct vnode *vp" "struct vnode *dvp" \
"char **bpp" "char *bufp"
.Ft void
.Fn cache_enter "struct vnode *dvp" "struct vnode *vp" \
"const char *name" "size_t namelen" "uint32_t nameiflags"
.Ft void
.Fn cache_purge "struct vnode *vp"
.Ft void
.Fn cache_purgevfs "struct mount *mp"
.Ft void
.Fn namecache_print "struct vnode *vp" "void (*func)(const char *, ...)"
.Sh 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
.Xr namei 9
interface.
.Pp
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.
.Pp
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
.Va nchashtbl ,
containing
.Em namecache
entries that are allocated dynamically from a kernel memory pool.
Each entry has the following structure:
.Bd -literal
#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 */
};
.Ed
.Pp
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.
.Pp
Each
.Em namecache
entry can appear on two hash chains in addition to
.Va nshashtbl :
.Em ncvhashtbl
(the name cache directory hash chain), and
.Em 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.
.Pp
Functions which access to the name cache pass arguments in the
partially initialised
.Em componentname
structure.
See
.Xr vnodeops 9
for details on this structure.
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn cache_lookup "dvp" "name" "namelen" "nameiop" "nameiflags" \
"iswhiteout" "vpp"
Look for a name in the cache.
.Fn cache_lookup
is called with
.Fa dvp
pointing to the vnode of the directory to search.
The
.Fa name
and
.Fa namelen
arguments specify the name to look for.
The
.Fa nameiop
and
.Fa nameiflags
should be taken from the
.Fa cn_nameiop
and
.Fa cn_flags
fields of struct componentname.
.Pp
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
.Em no
existing object.
(The lookup cannot fail, in the sense of generating an error condition
that requires aborting the operation in progress.)
.Pp
On a cache miss,
.Fn cache_lookup
returns zero
.Pq false .
On a positive hit, the vnode for the object found is locked and stored
in
.Fa vpp ,
and a nonzero
.Pq true
value is returned.
On a negative hit,
.Fa 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
.Er ENOENT .
.Pp
The
.Fa 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
.Dv NULL
if the caller does not support whiteouts.
According to the current scheme for handling whiteouts, if
.Fn cache_lookup
sets
.Fa iswhiteout
the caller should add
.Dv ISWHITEOUT
to the
.Fa cn_flags
field of its struct componentname.
.It Fn cache_revlookup "vp" "dvp" "bpp" "bufp"
Scan cache looking for name of directory entry pointing at
.Fa vp
and fill in
.Fa dvpp .
If
.Fa bufp
is
.Pf non- Dv NULL ,
also place the name in the buffer which starts at
.Fa bufp ,
immediately before
.Fa 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.
.It Fn cache_enter "dvp" "vp" "name" "namelen" "nameiflags"
Add an entry to the cache.
The
.Fa name
and
.Fa namelen
arguments specify the name to add to the cache.
The
.Fa dvp
argument specifies the directory the name appears in.
The
.Fa vp
argument specifies the object to enter in the cache.
The
.Fa nameiflags
argument should come from the
.Fa cn_flags
member of struct componentname.
.Pp
If
.Fa vp
is
.Dv NULL ,
a negative cache entry is created, specifying that the entry
does not exist in the file system.
.It Fn cache_purge "vp"
Flush the cache of a particular vnode
.Fa vp .
.Fn cache_purge
is called when a vnode is renamed to hide entries that would now be
invalid.
.It Fn cache_purgevfs "mp"
Flush cache of a whole file system
.Fa mp .
.Fn cache_purgevfs
is called when file system is unmounted to remove entries that would
now be invalid.
.It Fn namecache_print "vp" "func"
Print all entries of the name cache.
.Fa func
is the
.Xr printf 9
format.
.Fn namecache_print
is only defined if the kernel option DDB is compiled into the kernel.
.El
.Sh CODE REFERENCES
The name lookup cache is implemented within the file
.Pa sys/kern/vfs_cache.c .
.Sh SEE ALSO
.Xr intro 9 ,
.Xr namei 9 ,
.Xr vfs 9 ,
.Xr vnode 9
.Sh HISTORY
The name lookup cache first appeared in
.Bx 4.2 .
.Sh AUTHORS
The original name lookup cache was written by Robert Elz.