usr/src/sys/kern/vfs_cache.c

/*
 * Copyright (c) 1989, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * %sccs.include.redist.c%
 *
 *      @(#)vfs_cache.c 8.1 (Berkeley) %G%
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/time.h>
#include <sys/mount.h>
#include <sys/vnode.h>
#include <sys/namei.h>
#include <sys/errno.h>
#include <sys/malloc.h>

/*
 * Name caching works as follows:
 *
 * Names found by directory scans are retained in a cache
 * for future reference.  It is managed LRU, so frequently
 * used names will hang around.  Cache is indexed by hash value
 * obtained from (vp, name) where vp refers to the directory
 * containing 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.
 *
 * Upon reaching the last segment of a path, if the reference
 * is for DELETE, or NOCACHE is set (rewrite), and the
 * name is located in the cache, it will be dropped.
 */

/*
 * Structures associated with name cacheing.
 */
struct namecache **nchashtbl;
u_long  nchash;                         /* size of hash table - 1 */
long    numcache;                       /* number of cache entries allocated */
struct  namecache *nchhead, **nchtail;  /* LRU chain pointers */
struct  nchstats nchstats;              /* cache effectiveness statistics */

int doingcache = 1;                     /* 1 => enable the cache */

/*
 * Look for a the name in the cache. We don't do this
 * if the segment name is long, simply so the cache can avoid
 * holding long names (which would either waste space, or
 * add greatly to the complexity).
 *
 * Lookup is called with ni_dvp pointing to the directory to search,
 * ni_ptr pointing to the name of the entry being sought, ni_namelen
 * tells the length of the name, and ni_hash contains a hash of
 * the name. If the lookup succeeds, the vnode is returned in ni_vp
 * and a status of -1 is returned. If the lookup determines that
 * the name does not exist (negative cacheing), a status of ENOENT
 * is returned. If the lookup fails, a status of zero is returned.
 */
int
cache_lookup(dvp, vpp, cnp)
        struct vnode *dvp;
        struct vnode **vpp;
        struct componentname *cnp;
{
        register struct namecache *ncp, *ncq, **ncpp;

        if (!doingcache)
                return (0);
        if (cnp->cn_namelen > NCHNAMLEN) {
                nchstats.ncs_long++;
                cnp->cn_flags &= ~MAKEENTRY;
                return (0);
        }
        ncpp = &nchashtbl[cnp->cn_hash & nchash];
        for (ncp = *ncpp; ncp; ncp = ncp->nc_forw) {
                if (ncp->nc_dvp == dvp &&
                    ncp->nc_dvpid == dvp->v_id &&
                    ncp->nc_nlen == cnp->cn_namelen &&
                    !bcmp(ncp->nc_name, cnp->cn_nameptr, (u_int)ncp->nc_nlen))
                        break;
        }
        if (ncp == NULL) {
                nchstats.ncs_miss++;
                return (0);
        }
        if (!(cnp->cn_flags & MAKEENTRY)) {
                nchstats.ncs_badhits++;
        } else if (ncp->nc_vp == NULL) {
                if (cnp->cn_nameiop != CREATE) {
                        nchstats.ncs_neghits++;
                        /*
                         * Move this slot to end of LRU chain,
                         * if not already there.
                         */
                        if (ncp->nc_nxt) {
                                /* remove from LRU chain */
                                *ncp->nc_prev = ncp->nc_nxt;
                                ncp->nc_nxt->nc_prev = ncp->nc_prev;
                                /* and replace at end of it */
                                ncp->nc_nxt = NULL;
                                ncp->nc_prev = nchtail;
                                *nchtail = ncp;
                                nchtail = &ncp->nc_nxt;
                        }
                        return (ENOENT);
                }
        } else if (ncp->nc_vpid != ncp->nc_vp->v_id) {
                nchstats.ncs_falsehits++;
        } else {
                nchstats.ncs_goodhits++;
                /*
                 * move this slot to end of LRU chain, if not already there
                 */
                if (ncp->nc_nxt) {
                        /* remove from LRU chain */
                        *ncp->nc_prev = ncp->nc_nxt;
                        ncp->nc_nxt->nc_prev = ncp->nc_prev;
                        /* and replace at end of it */
                        ncp->nc_nxt = NULL;
                        ncp->nc_prev = nchtail;
                        *nchtail = ncp;
                        nchtail = &ncp->nc_nxt;
                }
                *vpp = ncp->nc_vp;
                return (-1);
        }

        /*
         * Last component and we are renaming or deleting,
         * the cache entry is invalid, or otherwise don't
         * want cache entry to exist.
         */
        /* remove from LRU chain */
        if (ncq = ncp->nc_nxt)
                ncq->nc_prev = ncp->nc_prev;
        else
                nchtail = ncp->nc_prev;
        *ncp->nc_prev = ncq;
        /* remove from hash chain */
        if (ncq = ncp->nc_forw)
                ncq->nc_back = ncp->nc_back;
        *ncp->nc_back = ncq;
        /* and make a dummy hash chain */
        ncp->nc_forw = NULL;
        ncp->nc_back = NULL;
        /* insert at head of LRU list (first to grab) */
        if (ncq = nchhead)
                ncq->nc_prev = &ncp->nc_nxt;
        else
                nchtail = &ncp->nc_nxt;
        nchhead = ncp;
        ncp->nc_nxt = ncq;
        ncp->nc_prev = &nchhead;
        return (0);
}

/*
 * Add an entry to the cache
 */
cache_enter(dvp, vp, cnp)
        struct vnode *dvp;
        struct vnode *vp;
        struct componentname *cnp;
{
        register struct namecache *ncp, *ncq, **ncpp;

#ifdef DIAGNOSTIC
        if (cnp->cn_namelen > NCHNAMLEN)
                panic("cache_enter: name too long");
#endif
        if (!doingcache)
                return;
        /*
         * Free the cache slot at head of lru chain.
         */
        if (numcache < desiredvnodes) {
                ncp = (struct namecache *)
                        malloc((u_long)sizeof *ncp, M_CACHE, M_WAITOK);
                bzero((char *)ncp, sizeof *ncp);
                numcache++;
        } else if (ncp = nchhead) {
                /* remove from lru chain */
                if (ncq = ncp->nc_nxt)
                        ncq->nc_prev = ncp->nc_prev;
                else
                        nchtail = ncp->nc_prev;
                *ncp->nc_prev = ncq;
                /* remove from old hash chain, if on one */
                if (ncp->nc_back) {
                        if (ncq = ncp->nc_forw)
                                ncq->nc_back = ncp->nc_back;
                        *ncp->nc_back = ncq;
                        ncp->nc_forw = NULL;
                        ncp->nc_back = NULL;
                }
        } else
                return;
        /* grab the vnode we just found */
        ncp->nc_vp = vp;
        if (vp)
                ncp->nc_vpid = vp->v_id;
        else
                ncp->nc_vpid = 0;
        /* fill in cache info */
        ncp->nc_dvp = dvp;
        ncp->nc_dvpid = dvp->v_id;
        ncp->nc_nlen = cnp->cn_namelen;
        bcopy(cnp->cn_nameptr, ncp->nc_name, (unsigned)ncp->nc_nlen);
        /* link at end of lru chain */
        ncp->nc_nxt = NULL;
        ncp->nc_prev = nchtail;
        *nchtail = ncp;
        nchtail = &ncp->nc_nxt;
        /* and insert on hash chain */
        ncpp = &nchashtbl[cnp->cn_hash & nchash];
        if (ncq = *ncpp)
                ncq->nc_back = &ncp->nc_forw;
        ncp->nc_forw = ncq;
        ncp->nc_back = ncpp;
        *ncpp = ncp;
}

/*
 * Name cache initialization, from vfs_init() when we are booting
 */
nchinit()
{

        nchtail = &nchhead;
        nchashtbl = hashinit(desiredvnodes, M_CACHE, &nchash);
}

/*
 * Cache flush, a particular vnode; called when a vnode is renamed to
 * hide entries that would now be invalid
 */
cache_purge(vp)
        struct vnode *vp;
{
        struct namecache *ncp, **ncpp;

        vp->v_id = ++nextvnodeid;
        if (nextvnodeid != 0)
                return;
        for (ncpp = &nchashtbl[nchash]; ncpp >= nchashtbl; ncpp--) {
                for (ncp = *ncpp; ncp; ncp = ncp->nc_forw) {
                        ncp->nc_vpid = 0;
                        ncp->nc_dvpid = 0;
                }
        }
        vp->v_id = ++nextvnodeid;
}

/*
 * Cache flush, a whole filesystem; called when filesys is umounted to
 * remove entries that would now be invalid
 *
 * The line "nxtcp = nchhead" near the end is to avoid potential problems
 * if the cache lru chain is modified while we are dumping the
 * inode.  This makes the algorithm O(n^2), but do you think I care?
 */
cache_purgevfs(mp)
        struct mount *mp;
{
        register struct namecache *ncp, *nxtcp;

        for (ncp = nchhead; ncp; ncp = nxtcp) {
                if (ncp->nc_dvp == NULL || ncp->nc_dvp->v_mount != mp) {
                        nxtcp = ncp->nc_nxt;
                        continue;
                }
                /* free the resources we had */
                ncp->nc_vp = NULL;
                ncp->nc_dvp = NULL;
                /* remove from old hash chain, if on one */
                if (ncp->nc_back) {
                        if (nxtcp = ncp->nc_forw)
                                nxtcp->nc_back = ncp->nc_back;
                        *ncp->nc_back = nxtcp;
                        ncp->nc_forw = NULL;
                        ncp->nc_back = NULL;
                }
                /* delete this entry from LRU chain */
                if (nxtcp = ncp->nc_nxt)
                        nxtcp->nc_prev = ncp->nc_prev;
                else
                        nchtail = ncp->nc_prev;
                *ncp->nc_prev = nxtcp;
                /* cause rescan of list, it may have altered */
                /* also put the now-free entry at head of LRU */
                if (nxtcp = nchhead)
                        nxtcp->nc_prev = &ncp->nc_nxt;
                else
                        nchtail = &ncp->nc_nxt;
                nchhead = ncp;
                ncp->nc_nxt = nxtcp;
                ncp->nc_prev = &nchhead;
        }
}