From f0696c5e4b536bed69c6aa8dcccf459153f817fc Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Mon, 24 Aug 2020 08:55:55 +0000
Subject: [PATCH] cache: perform reverse lookup using v_cache_dd if possible

Tested by:	pho
---
 sys/kern/vfs_cache.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/sys/kern/vfs_cache.c b/sys/kern/vfs_cache.c
index e917881975c..69d562a21d8 100644
--- a/sys/kern/vfs_cache.c
+++ b/sys/kern/vfs_cache.c
@@ -2572,6 +2572,19 @@ vn_fullpath_global(struct thread *td, struct vnode *vn,
 	return (error);
 }
 
+static struct namecache *
+vn_dd_from_dst(struct vnode *vp)
+{
+	struct namecache *ncp;
+
+	cache_assert_vnode_locked(vp);
+	TAILQ_FOREACH(ncp, &vp->v_cache_dst, nc_dst) {
+		if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
+			return (ncp);
+	}
+	return (NULL);
+}
+
 int
 vn_vptocnp(struct vnode **vp, struct ucred *cred, char *buf, size_t *buflen)
 {
@@ -2582,9 +2595,13 @@ vn_vptocnp(struct vnode **vp, struct ucred *cred, char *buf, size_t *buflen)
 
 	vlp = VP2VNODELOCK(*vp);
 	mtx_lock(vlp);
-	TAILQ_FOREACH(ncp, &((*vp)->v_cache_dst), nc_dst) {
-		if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
-			break;
+	ncp = (*vp)->v_cache_dd;
+	if (ncp != NULL && (ncp->nc_flag & NCF_ISDOTDOT) == 0) {
+		KASSERT(ncp == vn_dd_from_dst(*vp),
+		    ("%s: mismatch for dd entry (%p != %p)", __func__,
+		    ncp, vn_dd_from_dst(*vp)));
+	} else {
+		ncp = vn_dd_from_dst(*vp);
 	}
 	if (ncp != NULL) {
 		if (*buflen < ncp->nc_nlen) {