btrfs: don't save block group root into super block

The extent tree v2 needs a new root for storing all block group items,
the whole feature hasn't been finished yet so we can afford to do some
changes.

My initial proposal years ago just added a new tree rootid, and load it
from tree root, just like what we did for quota/free space tree/uuid/extent
roots.

But the extent tree v2 patches introduced a completely new way to store
block group tree root into super block which is arguably wasteful.

Currently there are only 3 trees stored in super blocks, and they all
have their valid reasons:

- Chunk root
  Needed for bootstrap.

- Tree root
  Really the entry point for all trees.

- Log root
  This is special as log root has to be updated out of existing
  transaction mechanism.

There is not even any reason to put block group root into super blocks,
the block group tree is updated at the same time as the old extent tree,
no need for extra bootstrap/out-of-transaction update.

So just move block group root from super block into tree root.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 20 insertions, 20 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index bb87b5697aff..dfc9b3aea76a 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1522,6 +1522,9 @@ static struct btrfs_root *btrfs_get_global_root(struct btrfs_fs_info *fs_info,
 	if (objectid == BTRFS_UUID_TREE_OBJECTID)
 		return btrfs_grab_root(fs_info->uuid_root) ?
 			fs_info->uuid_root : ERR_PTR(-ENOENT);
+	if (objectid == BTRFS_BLOCK_GROUP_TREE_OBJECTID)
+		return btrfs_grab_root(fs_info->block_group_root) ?
+			fs_info->block_group_root : ERR_PTR(-ENOENT);
 	if (objectid == BTRFS_FREE_SPACE_TREE_OBJECTID) {
 		struct btrfs_root *root = btrfs_global_root(fs_info, &key);
 
@@ -1978,14 +1981,7 @@ static void backup_super_roots(struct btrfs_fs_info *info)
 	btrfs_set_backup_chunk_root_level(root_backup,
 			       btrfs_header_level(info->chunk_root->node));
 
-	if (btrfs_fs_incompat(info, EXTENT_TREE_V2)) {
-		btrfs_set_backup_block_group_root(root_backup,
-					info->block_group_root->node->start);
-		btrfs_set_backup_block_group_root_gen(root_backup,
-			btrfs_header_generation(info->block_group_root->node));
-		btrfs_set_backup_block_group_root_level(root_backup,
-			btrfs_header_level(info->block_group_root->node));
-	} else {
+	if (!btrfs_fs_incompat(info, EXTENT_TREE_V2)) {
 		struct btrfs_root *extent_root = btrfs_extent_root(info, 0);
 		struct btrfs_root *csum_root = btrfs_csum_root(info, 0);
 
@@ -2527,10 +2523,24 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
 	if (ret)
 		return ret;
 
-	location.objectid = BTRFS_DEV_TREE_OBJECTID;
 	location.type = BTRFS_ROOT_ITEM_KEY;
 	location.offset = 0;
 
+	if (btrfs_fs_incompat(fs_info, EXTENT_TREE_V2)) {
+		location.objectid = BTRFS_BLOCK_GROUP_TREE_OBJECTID;
+		root = btrfs_read_tree_root(tree_root, &location);
+		if (IS_ERR(root)) {
+			if (!btrfs_test_opt(fs_info, IGNOREBADROOTS)) {
+				ret = PTR_ERR(root);
+				goto out;
+			}
+		} else {
+			set_bit(BTRFS_ROOT_TRACK_DIRTY, &root->state);
+			fs_info->block_group_root = root;
+		}
+	}
+
+	location.objectid = BTRFS_DEV_TREE_OBJECTID;
 	root = btrfs_read_tree_root(tree_root, &location);
 	if (IS_ERR(root)) {
 		if (!btrfs_test_opt(fs_info, IGNOREBADROOTS)) {
@@ -2858,17 +2868,7 @@ static int load_important_roots(struct btrfs_fs_info *fs_info)
 		btrfs_warn(fs_info, "couldn't read tree root");
 		return ret;
 	}
-
-	if (!btrfs_fs_incompat(fs_info, EXTENT_TREE_V2))
-		return 0;
-
-	bytenr = btrfs_super_block_group_root(sb);
-	gen = btrfs_super_block_group_root_generation(sb);
-	level = btrfs_super_block_group_root_level(sb);
-	ret = load_super_root(fs_info->block_group_root, bytenr, gen, level);
-	if (ret)
-		btrfs_warn(fs_info, "couldn't read block group root");
-	return ret;
+	return 0;
 }
 
 static int __cold init_tree_roots(struct btrfs_fs_info *fs_info)