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btrfs: Use default subvolume as filesystem root

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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>
Fixes: f06bfcf54d ("fs: btrfs: Crossport open_ctree_fs_info() from btrfs-progs")
Reviewed-by: Qu Wenruo <wqu@suse.com>
This commit is contained in:
Matwey V. Kornilov 2021-08-01 23:52:16 +03:00 committed by Tom Rini
parent 48cf96fbdf
commit 94509b79b1
1 changed files with 35 additions and 3 deletions
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38
fs/btrfs/disk-io.c
View file

@ -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))