[PATCH] btrfs: Use default subvolume as filesystem root

[Qu Wenruo]

On 2021/8/2 上午4:52, Matwey V. Kornilov wrote:
> 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).

I also noticed the problem in the past, but forgot to fix it....

> 
> 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>

Reviewed-by: Qu Wenruo <wqu at suse.com>

Thanks,
Qu

> ---
>   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))
>