Enable caching of negative pathname lookups in the NFS client. To avoid

stale entries, we save a copy of the directory's modification time when
the first negative cache entry was added in the directory's NFS node.
When a negative cache entry is hit during a pathname lookup, the parent
directory's modification time is checked.  If it has changed, all of the
negative cache entries for that parent are purged and the lookup falls
back to using the RPC.  This required adding a new cache_purge_negative()
method to the name cache to purge only negative cache entries for a given
directory.

Submitted by:	mohans, Rick Macklem, Ricardo Labiaga @ NetApp
Reviewed by:	mohans

sys/kern/vfs_cache.c

@@ -662,6 +662,24 @@ cache_purge(vp)
 	CACHE_WUNLOCK();
 }
 
+/*
+ * Invalidate all negative entries for a particular directory vnode.
+ */
+void
+cache_purge_negative(vp)
+	struct vnode *vp;
+{
+	struct namecache *cp, *ncp;
+
+	CTR1(KTR_VFS, "cache_purge_negative(%p)", vp);
+	CACHE_WLOCK();
+	LIST_FOREACH_SAFE(cp, &vp->v_cache_src, nc_src, ncp) {
+		if (cp->nc_vp == NULL)
+			cache_zap(cp);
+	}
+	CACHE_WUNLOCK();
+}
+
 /*
  * Flush all entries referencing a particular filesystem.
  */

sys/nfsclient/nfs_vnops.c

@@ -852,6 +852,7 @@ nfs_lookup(struct vop_lookup_args *ap)
 	struct componentname *cnp = ap->a_cnp;
 	struct vnode *dvp = ap->a_dvp;
 	struct vnode **vpp = ap->a_vpp;
+	struct vattr vattr;
 	int flags = cnp->cn_flags;
 	struct vnode *newvp;
 	struct nfsmount *nmp;
@@ -880,8 +881,12 @@ nfs_lookup(struct vop_lookup_args *ap)
 	if (error > 0 && error != ENOENT)
 		return (error);
 	if (error == -1) {
-		struct vattr vattr;
-
+		/*
+		 * We only accept a positive hit in the cache if the
+		 * change time of the file matches our cached copy.
+		 * Otherwise, we discard the cache entry and fallback
+		 * to doing a lookup RPC.
+		 */
 		newvp = *vpp;
 		if (!VOP_GETATTR(newvp, &vattr, cnp->cn_cred)
 		    && vattr.va_ctime.tv_sec == VTONFS(newvp)->n_ctime) {
@@ -897,6 +902,24 @@ nfs_lookup(struct vop_lookup_args *ap)
 		else 
 			vrele(newvp);
 		*vpp = NULLVP;
+	} else if (error == ENOENT) {
+		/*
+		 * We only accept a negative hit in the cache if the
+		 * modification time of the parent directory matches
+		 * our cached copy.  Otherwise, we discard all of the
+		 * negative cache entries for this directory.
+		 */
+		if (VOP_GETATTR(dvp, &vattr, cnp->cn_cred) == 0 &&
+		    vattr.va_mtime.tv_sec == np->n_dmtime) {
+			nfsstats.lookupcache_hits++;
+			if ((cnp->cn_nameiop == CREATE ||
+			    cnp->cn_nameiop == RENAME) &&
+			    (flags & ISLASTCN))
+				cnp->cn_flags |= SAVENAME;
+			return (ENOENT);
+		}
+		cache_purge_negative(dvp);
+		np->n_dmtime = 0;
 	}
 	error = 0;
 	newvp = NULLVP;
@@ -982,16 +1005,38 @@ nfs_lookup(struct vop_lookup_args *ap)
 			vput(newvp);
 			*vpp = NULLVP;
 		}
+
+		if (error != ENOENT)
+			goto done;
+
+		/* The requested file was not found. */
 		if ((cnp->cn_nameiop == CREATE || cnp->cn_nameiop == RENAME) &&
-		    (flags & ISLASTCN) && error == ENOENT) {
+		    (flags & ISLASTCN)) {
+			/*
+			 * XXX: UFS does a full VOP_ACCESS(dvp,
+			 * VWRITE) here instead of just checking
+			 * MNT_RDONLY.
+			 */
 			if (dvp->v_mount->mnt_flag & MNT_RDONLY)
-				error = EROFS;
-			else
-				error = EJUSTRETURN;
-		}
-		if (cnp->cn_nameiop != LOOKUP && (flags & ISLASTCN))
+				return (EROFS);
 			cnp->cn_flags |= SAVENAME;
+			return (EJUSTRETURN);
+		}
+
+		if ((cnp->cn_flags & MAKEENTRY) && cnp->cn_nameiop != CREATE) {
+			/*
+			 * Maintain n_dmtime as the modification time
+			 * of the parent directory when the oldest -ve
+			 * name cache entry for this directory was
+			 * added.
+			 */
+			if (np->n_dmtime == 0)
+				np->n_dmtime = np->n_vattr.va_mtime.tv_sec;
+			cache_enter(dvp, NULL, cnp);
+		}
+		return (ENOENT);
 	}
+done:
 	return (error);
 }
 

sys/nfsclient/nfsnode.h

@@ -109,6 +109,7 @@ struct nfsnode {
 	time_t			n_modestamp;	/* mode cache timestamp */
 	struct timespec		n_mtime;	/* Prev modify time. */
 	time_t			n_ctime;	/* Prev create time. */
+	time_t			n_dmtime;	/* Prev dir modify time. */
 	time_t			n_expiry;	/* Lease expiry time */
 	nfsfh_t			*n_fhp;		/* NFS File Handle */
 	struct vnode		*n_vnode;	/* associated vnode */

sys/sys/vnode.h

@@ -562,6 +562,7 @@ void	cache_enter(struct vnode *dvp, struct vnode *vp,
 int	cache_lookup(struct vnode *dvp, struct vnode **vpp,
 	    struct componentname *cnp);
 void	cache_purge(struct vnode *vp);
+void	cache_purge_negative(struct vnode *vp);
 void	cache_purgevfs(struct mount *mp);
 int	change_dir(struct vnode *vp, struct thread *td);
 int	change_root(struct vnode *vp, struct thread *td);