ovl: allow remote upper

No reason to prevent upper layer being a remote filesystem.  Do the
revalidation in that case, just as we already do for lower layers.

This lets virtiofs be used as upper layer, which appears to be a real use
case.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/namei.c

@@ -845,8 +845,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		if (err)
 			goto out;
 
-		if (upperdentry && (upperdentry->d_flags & DCACHE_OP_REAL ||
-				    unlikely(ovl_dentry_remote(upperdentry)))) {
+		if (upperdentry && upperdentry->d_flags & DCACHE_OP_REAL) {
 			dput(upperdentry);
 			err = -EREMOTE;
 			goto out;

fs/overlayfs/super.c

@@ -135,9 +135,14 @@ static int ovl_dentry_revalidate_common(struct dentry *dentry,
 					unsigned int flags, bool weak)
 {
 	struct ovl_entry *oe = dentry->d_fsdata;
+	struct dentry *upper;
 	unsigned int i;
 	int ret = 1;
 
+	upper = ovl_dentry_upper(dentry);
+	if (upper)
+		ret = ovl_revalidate_real(upper, flags, weak);
+
 	for (i = 0; ret > 0 && i < oe->numlower; i++) {
 		ret = ovl_revalidate_real(oe->lowerstack[i].dentry, flags,
 					  weak);
@@ -747,8 +752,7 @@ static int ovl_mount_dir(const char *name, struct path *path)
 		ovl_unescape(tmp);
 		err = ovl_mount_dir_noesc(tmp, path);
 
-		if (!err && (ovl_dentry_remote(path->dentry) ||
-			     path->dentry->d_flags & DCACHE_OP_REAL)) {
+		if (!err && path->dentry->d_flags & DCACHE_OP_REAL) {
 			pr_err("filesystem on '%s' not supported as upperdir\n",
 			       tmp);
 			path_put_init(path);

fs/overlayfs/util.c

@@ -102,6 +102,8 @@ void ovl_dentry_update_reval(struct dentry *dentry, struct dentry *upperdentry,
 	struct ovl_entry *oe = OVL_E(dentry);
 	unsigned int i, flags = 0;
 
+	if (upperdentry)
+		flags |= upperdentry->d_flags;
 	for (i = 0; i < oe->numlower; i++)
 		flags |= oe->lowerstack[i].dentry->d_flags;