[PATCH] btrfs: Use default subvolume as filesystem root

[Matwey V. Kornilov]

BTRFS volume consists of a number of subvolumes which can be mounted separately
from each other. The top-level subvolume always exists even if no subvolumes
were created manually. A subvolume can be denoted as the default subvolume i.e.
the subvolume which is mounted by default.

The default "default subvolume" is the top-level one, but this is far from the
common practices used in the wild. For instance, openSUSE provides an OS
snapshot/rollback feature based on BTRFS. To achieve this, the actual OS root
filesystem is located into a separate subvolume which is "default" but not
"top-level". That means that the /boot/dtb/ directory is also located inside
this default subvolume instead of top-level one.

However, the existing btrfs u-boot driver always uses the top-level subvolume
as the filesystem root. This behaviour 1) is inconsistent with

    mount /dev/sda1 /target

command, which mount the default subvolume 2) leads to the issues when
/boot/dtb cannot be found properly (see the reference).

This patch uses the default subvolume as the filesystem root to overcome
mentioned issues.

Reference: https://bugzilla.suse.com/show_bug.cgi?id=1185656
Signed-off-by: Matwey V. Kornilov <matwey.kornilov at gmail.com>
---
 fs/btrfs/disk-io.c | 38 +++++++++++++++++++++++++++++++++++---
 1 file changed, 35 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 349411c3cc..12f9579fcf 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -804,6 +804,30 @@ static int setup_root_or_create_block(struct btrfs_fs_info *fs_info,
 	return 0;
 }
 
+static int get_default_subvolume(struct btrfs_fs_info *fs_info,
+				 struct btrfs_key *key_ret)
+{
+	struct btrfs_root *root = fs_info->tree_root;
+	struct btrfs_dir_item *dir_item;
+	struct btrfs_path path;
+	int ret = 0;
+
+	btrfs_init_path(&path);
+
+	dir_item = btrfs_lookup_dir_item(NULL, root, &path,
+					 BTRFS_ROOT_TREE_DIR_OBJECTID,
+					 "default", 7, 0);
+	if (IS_ERR(dir_item)) {
+		ret = PTR_ERR(dir_item);
+		goto out;
+	}
+
+	btrfs_dir_item_key_to_cpu(path.nodes[0], dir_item, key_ret);
+out:
+	btrfs_release_path(&path);
+	return ret;
+}
+
 int btrfs_setup_all_roots(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_super_block *sb = fs_info->super_copy;
@@ -833,9 +857,17 @@ int btrfs_setup_all_roots(struct btrfs_fs_info *fs_info)
 
 	fs_info->last_trans_committed = generation;
 
-	key.objectid = BTRFS_FS_TREE_OBJECTID;
-	key.type = BTRFS_ROOT_ITEM_KEY;
-	key.offset = (u64)-1;
+	ret = get_default_subvolume(fs_info, &key);
+	if (ret) {
+		/*
+		 * The default dir item isn't there. Linux kernel behaviour is
+		 * to silently use the top-level subvolume in this case.
+		 */
+		key.objectid = BTRFS_FS_TREE_OBJECTID;
+		key.type = BTRFS_ROOT_ITEM_KEY;
+		key.offset = (u64)-1;
+	}
+
 	fs_info->fs_root = btrfs_read_fs_root(fs_info, &key);
 
 	if (IS_ERR(fs_info->fs_root))
-- 
2.31.1