Re: [PATCH 15/18] btrfs: simple quota auto hierarchy for nested subvols

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Jul 05, 2023 at 04:20:52PM -0700, Boris Burkov wrote:
> Consider the following sequence:
> - enable quotas
> - create subvol S id 256 at dir outer/
> - create a qgroup 1/100
> - add 0/256 (S's auto qgroup) to 1/100
> - create subvol T id 257 at dir outer/inner/
> 
> With full qgroups, there is no relationship between 0/257 and either of
> 0/256 or 1/100. There is an inherit feature that the creator of inner/
> can use to specify it ought to be in 1/100.
> 
> Simple quotas are targeted at container isolation, where such automatic
> inheritance for not necessarily trusted/controlled nested subvol
> creation would be quite helpful. Therefore, add a new default behavior
> for simple quotas: when you create a nested subvol, automatically
> inherit as parents any parents of the qgroup of the subvol the new inode
> is going in.
> 
> In our example, 257/0 would also be under 1/100, allowing easy control
> of a total quota over an arbitrary hierarchy of subvolumes.
> 
> I think this _might_ be a generally useful behavior, so it could be
> interesting to put it behind a new inheritance flag that simple quotas
> always use while traditional quotas let the user specify, but this is a
> minimally intrusive change to start.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>
> ---
>  fs/btrfs/ioctl.c       |  2 +-
>  fs/btrfs/qgroup.c      | 46 +++++++++++++++++++++++++++++++++++++++---
>  fs/btrfs/qgroup.h      |  6 +++---
>  fs/btrfs/transaction.c | 13 ++++++++----
>  4 files changed, 56 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 63c1a9258a1b..23f5142ef18b 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -652,7 +652,7 @@ static noinline int create_subvol(struct mnt_idmap *idmap,
>  	/* Tree log can't currently deal with an inode which is a new root. */
>  	btrfs_set_log_full_commit(trans);
>  
> -	ret = btrfs_qgroup_inherit(trans, 0, objectid, inherit);
> +	ret = btrfs_qgroup_inherit(trans, 0, objectid, root->root_key.objectid, inherit);
>  	if (ret)
>  		goto out;
>  
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 97c00697b475..6714c5aeb4e1 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -1557,8 +1557,7 @@ static int quick_update_accounting(struct btrfs_fs_info *fs_info,
>  	return ret;
>  }
>  
> -int btrfs_add_qgroup_relation(struct btrfs_trans_handle *trans, u64 src,
> -			      u64 dst)
> +int btrfs_add_qgroup_relation(struct btrfs_trans_handle *trans, u64 src, u64 dst)
>  {
>  	struct btrfs_fs_info *fs_info = trans->fs_info;
>  	struct btrfs_qgroup *parent;
> @@ -2998,6 +2997,42 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans)
>  	return ret;
>  }
>  
> +static int qgroup_auto_inherit(struct btrfs_fs_info *fs_info,
> +			       u64 inode_rootid,
> +			       struct btrfs_qgroup_inherit **inherit)
> +{
> +	int i = 0;
> +	u64 num_qgroups = 0;
> +	struct btrfs_qgroup *inode_qg;
> +	struct btrfs_qgroup_list *qg_list;
> +
> +	if (*inherit)
> +		return -EEXIST;
> +
> +	inode_qg = find_qgroup_rb(fs_info, inode_rootid);
> +	if (!inode_qg)
> +		return -ENOENT;
> +
> +	list_for_each_entry(qg_list, &inode_qg->groups, next_group) {
> +		++num_qgroups;
> +	}

You can simplify this with

num_qgroups = list_count_nodes(&inode_qg->groups);

Thanks,

Josef