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@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))