btrfs-progs: check: add repair ability for missing orphan root item

There is a known bug in older kernels that orphan items can be missing
for dropped subvolumes.

This makes the subvolumes unable to be removed on the next mount, and
recent kernel commit 4289b494ac55 ("btrfs: do not allow relocation of
partially dropped subvolumes") introduced one extra safe net to catch
such problem.

But unfortunately there is no way to repair it.

Add the repair ability to both the original and lowmem modes.

Reviewed-by: Boris Burkov <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adam900710

check/main.c

@@ -3550,6 +3550,11 @@ static int check_root_refs(struct btrfs_root *root,
 				 */
 				if (!rec->found_root_item)
 					continue;
+				if (opt_check_repair) {
+					ret = repair_subvol_orphan_item(gfs_info, rec->objectid);
+					if (!ret)
+						continue;
+				}
 				errors++;
 				fprintf(stderr, "fs tree %llu missing orphan item\n", rec->objectid);
 			}

check/mode-common.c

@@ -1672,3 +1672,36 @@ int check_and_repair_super_num_devs(struct btrfs_fs_info *fs_info)
 	printf("Successfully reset super num devices to %u\n", found_devs);
 	return 0;
 }
+
+int repair_subvol_orphan_item(struct btrfs_fs_info *fs_info, u64 rootid)
+{
+	struct btrfs_root *tree_root = fs_info->tree_root;
+	struct btrfs_trans_handle *trans;
+	struct btrfs_path path = { 0 };
+	int ret;
+
+	trans = btrfs_start_transaction(tree_root, 1);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		errno = -ret;
+		error_msg(ERROR_MSG_START_TRANS, "%m");
+		return ret;
+	}
+	ret = btrfs_add_orphan_item(trans, tree_root, &path, rootid);
+	btrfs_release_path(&path);
+	if (ret < 0) {
+		errno = -ret;
+		error("failed to insert orphan item for subvolume %llu: %m", rootid);
+		btrfs_abort_transaction(trans, ret);
+		btrfs_commit_transaction(trans, tree_root);
+		return ret;
+	}
+	ret = btrfs_commit_transaction(trans, tree_root);
+	if (ret < 0) {
+		errno = -ret;
+		error_msg(ERROR_MSG_COMMIT_TRANS, "%m");
+		return ret;
+	}
+	pr_verbose(LOG_DEFAULT, "Added back missing orphan item for subvolume %llu\n", rootid);
+	return 0;
+}

check/mode-common.h

@@ -197,5 +197,6 @@ int repair_dev_item_bytes_used(struct btrfs_fs_info *fs_info,
 int fill_csum_tree(struct btrfs_trans_handle *trans, bool search_fs_tree);
 
 int check_and_repair_super_num_devs(struct btrfs_fs_info *fs_info);
+int repair_subvol_orphan_item(struct btrfs_fs_info *fs_info, u64 rootid);
 
 #endif

check/mode-lowmem.c

@@ -5569,9 +5569,16 @@ static int check_btrfs_root(struct btrfs_root *root, int check_all)
 	 * If this tree is a subvolume (not a reloc tree) and has no refs, there
 	 * should be an orphan item for it, or this subvolume will never be deleted.
 	 */
-	if (btrfs_root_refs(root_item) == 0 && is_fstree(btrfs_root_id(root))) {
-		if (!has_orphan_item(root->fs_info->tree_root,
-				     btrfs_root_id(root))) {
+	if (btrfs_root_refs(root_item) == 0 && is_fstree(btrfs_root_id(root)) &&
+	    !has_orphan_item(root->fs_info->tree_root, btrfs_root_id(root))) {
+		bool repaired = false;
+
+		if (opt_check_repair) {
+			ret = repair_subvol_orphan_item(root->fs_info, btrfs_root_id(root));
+			if (!ret)
+				repaired = true;
+		}
+		if (!repaired) {
 			error("missing orphan item for root %lld", btrfs_root_id(root));
 			err |= REFERENCER_MISSING;
 		}