ovl: do not try to reconnect a disconnected origin dentry

On lookup of non directory, we try to decode the origin file handle
stored in upper inode. The origin file handle is supposed to be decoded
to a disconnected non-dir dentry, which is fine, because we only need
the lower inode of a copy up origin.

However, if the origin file handle somehow turns out to be a directory
we pay the expensive cost of reconnecting the directory dentry, only to
get a mismatch file type and drop the dentry.

Optimize this case by explicitly opting out of reconnecting the dentry.
Opting-out of reconnect is done by passing a NULL acceptable callback
to exportfs_decode_fh().

While the case described above is a strange corner case that does not
really need to be optimized, the API added for this optimization will
be used by a following patch to optimize a more common case of decoding
an overlayfs file handle.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/exportfs/expfs.c

@@ -435,6 +435,15 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 	if (IS_ERR_OR_NULL(result))
 		return ERR_PTR(-ESTALE);
 
+	/*
+	 * If no acceptance criteria was specified by caller, a disconnected
+	 * dentry is also accepatable. Callers may use this mode to query if
+	 * file handle is stale or to get a reference to an inode without
+	 * risking the high overhead caused by directory reconnect.
+	 */
+	if (!acceptable)
+		return result;
+
 	if (d_is_dir(result)) {
 		/*
 		 * This request is for a directory.

fs/overlayfs/export.c

@@ -685,7 +685,7 @@ static struct dentry *ovl_upper_fh_to_d(struct super_block *sb,
 	if (!ofs->upper_mnt)
 		return ERR_PTR(-EACCES);
 
-	upper = ovl_decode_real_fh(fh, ofs->upper_mnt);
+	upper = ovl_decode_real_fh(fh, ofs->upper_mnt, true);
 	if (IS_ERR_OR_NULL(upper))
 		return upper;
 
@@ -735,7 +735,7 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 	}
 
 	/* Then lookup origin by fh */
-	err = ovl_check_origin_fh(ofs, fh, NULL, &stack);
+	err = ovl_check_origin_fh(ofs, fh, true, NULL, &stack);
 	if (err) {
 		goto out_err;
 	} else if (index) {

fs/overlayfs/namei.c

@@ -180,7 +180,8 @@ invalid:
 	goto out;
 }
 
-struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt)
+struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
+				  bool connected)
 {
 	struct dentry *real;
 	int bytes;
@@ -195,7 +196,7 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt)
 	bytes = (fh->len - offsetof(struct ovl_fh, fid));
 	real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
 				  bytes >> 2, (int)fh->type,
-				  ovl_acceptable, mnt);
+				  connected ? ovl_acceptable : NULL, mnt);
 	if (IS_ERR(real)) {
 		/*
 		 * Treat stale file handle to lower file as "origin unknown".
@@ -319,14 +320,15 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 }
 
 
-int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
+int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
 			struct dentry *upperdentry, struct ovl_path **stackp)
 {
 	struct dentry *origin = NULL;
 	int i;
 
 	for (i = 0; i < ofs->numlower; i++) {
-		origin = ovl_decode_real_fh(fh, ofs->lower_layers[i].mnt);
+		origin = ovl_decode_real_fh(fh, ofs->lower_layers[i].mnt,
+					    connected);
 		if (origin)
 			break;
 	}
@@ -370,7 +372,7 @@ static int ovl_check_origin(struct ovl_fs *ofs, struct dentry *upperdentry,
 	if (IS_ERR_OR_NULL(fh))
 		return PTR_ERR(fh);
 
-	err = ovl_check_origin_fh(ofs, fh, upperdentry, stackp);
+	err = ovl_check_origin_fh(ofs, fh, false, upperdentry, stackp);
 	kfree(fh);
 
 	if (err) {
@@ -460,7 +462,7 @@ struct dentry *ovl_index_upper(struct ovl_fs *ofs, struct dentry *index)
 	if (IS_ERR_OR_NULL(fh))
 		return ERR_CAST(fh);
 
-	upper = ovl_decode_real_fh(fh, ofs->upper_mnt);
+	upper = ovl_decode_real_fh(fh, ofs->upper_mnt, true);
 	kfree(fh);
 
 	if (IS_ERR_OR_NULL(upper))
@@ -567,7 +569,7 @@ int ovl_verify_index(struct ovl_fs *ofs, struct dentry *index)
 
 	/* Check if non-dir index is orphan and don't warn before cleaning it */
 	if (!d_is_dir(index) && d_inode(index)->i_nlink == 1) {
-		err = ovl_check_origin_fh(ofs, fh, index, &stack);
+		err = ovl_check_origin_fh(ofs, fh, false, index, &stack);
 		if (err)
 			goto fail;
 

fs/overlayfs/overlayfs.h

@@ -266,8 +266,9 @@ static inline bool ovl_is_impuredir(struct dentry *dentry)
 
 /* namei.c */
 int ovl_check_fh_len(struct ovl_fh *fh, int fh_len);
-struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt);
-int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
+struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
+				  bool connected);
+int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
 			struct dentry *upperdentry, struct ovl_path **stackp);
 int ovl_verify_set_fh(struct dentry *dentry, const char *name,
 		      struct dentry *real, bool is_upper, bool set);