Re: [RFC PATCH] btrfs: qgroup: Deprecate the ability to manually inherit rfer/excl numbers

[Nikolay Borisov <nborisov@suse.com>]

On 19.12.2017 12:45, Qu Wenruo wrote:
> btrfs_qgroup_inherit structure has two members, num_ref_copies and
> num_excl_copies, to info btrfs kernel modules to inherit (copy)
> rfer/excl numbers at snapshot/subvolume creation time.
> 
> Since qgroup number is already hard to maintain for multi-level qgroup
> scenario, allowing user to manually manipulate qgroup inherit is quite
> easy to screw up qgroup numbers.
> 
> Although btrfs-progs supports such inheritance specification, the
> options are hidden from user and not documented.
> So there is no need to allow user to manually specify inheritance in
> kernel.
> 
> Reported-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> The only concern is, currently we don't have good tool to handle
> inheritance of multi-level qgroups.
> The only method to get qgroup numbers correct is to run a quota rescan.
> 
> So there may be some case where experienced (well, mostly a developer)
> user can use the hidden btrfs-progs options or manually craft an ioctl
> to handle multi-level qgroups without costly rescan.
> ---
>  fs/btrfs/qgroup.c          | 56 ++++++++++++++--------------------------------
>  include/uapi/linux/btrfs.h |  4 ++--
>  2 files changed, 19 insertions(+), 41 deletions(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 168fd03ca3ac..d8a2413272f9 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -2158,9 +2158,24 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
>  	}
>  
>  	if (inherit) {
> +		/*
> +		 * num_excl/rfer_copies indicate how many qgroup pairs needs
> +		 * to be manually inherited (copy rfer or excl from src
> +		 * qgroup to dst)
> +		 *
> +		 * Allowing user to manipulate inheritance can easily cause
> +		 * problem in multi-level qgroup scenario.
> +		 * And the ioctl interface is hidden in btrfs-progs for a long
> +		 * time, deprecate them should not be a big problem.
> +		 */
> +		if (inherit->__num_excl_copies || inherit->__num_ref_copies) {
> +			ret = -ENOTTY;
> +			btrfs_warn(fs_info,
> +			"manually inherit excl/rfer is no longer supported");
> +			goto out;
> +		}
>  		i_qgroups = (u64 *)(inherit + 1);
> -		nums = inherit->num_qgroups + 2 * inherit->num_ref_copies +
> -		       2 * inherit->num_excl_copies;
> +		nums = inherit->num_qgroups;
>  		for (i = 0; i < nums; ++i) {
>  			srcgroup = find_qgroup_rb(fs_info, *i_qgroups);
>  
> @@ -2286,43 +2301,6 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
>  		++i_qgroups;
>  	}
>  
> -	for (i = 0; i <  inherit->num_ref_copies; ++i, i_qgroups += 2) {
> -		struct btrfs_qgroup *src;
> -		struct btrfs_qgroup *dst;
> -
> -		if (!i_qgroups[0] || !i_qgroups[1])
> -			continue;
> -
> -		src = find_qgroup_rb(fs_info, i_qgroups[0]);
> -		dst = find_qgroup_rb(fs_info, i_qgroups[1]);
> -
> -		if (!src || !dst) {
> -			ret = -EINVAL;
> -			goto unlock;
> -		}
> -
> -		dst->rfer = src->rfer - level_size;
> -		dst->rfer_cmpr = src->rfer_cmpr - level_size;
> -	}
> -	for (i = 0; i <  inherit->num_excl_copies; ++i, i_qgroups += 2) {
> -		struct btrfs_qgroup *src;
> -		struct btrfs_qgroup *dst;
> -
> -		if (!i_qgroups[0] || !i_qgroups[1])
> -			continue;
> -
> -		src = find_qgroup_rb(fs_info, i_qgroups[0]);
> -		dst = find_qgroup_rb(fs_info, i_qgroups[1]);
> -
> -		if (!src || !dst) {
> -			ret = -EINVAL;
> -			goto unlock;
> -		}
> -
> -		dst->excl = src->excl + level_size;
> -		dst->excl_cmpr = src->excl_cmpr + level_size;
> -	}
> -
>  unlock:
>  	spin_unlock(&fs_info->qgroup_lock);
>  out:
> diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
> index ce615b75e855..099e088414d6 100644
> --- a/include/uapi/linux/btrfs.h
> +++ b/include/uapi/linux/btrfs.h
> @@ -80,8 +80,8 @@ struct btrfs_qgroup_limit {
>  struct btrfs_qgroup_inherit {
>  	__u64	flags;
>  	__u64	num_qgroups;
> -	__u64	num_ref_copies;
> -	__u64	num_excl_copies;
> +	__u64	__num_ref_copies;	/* DEPRECATED */
> +	__u64	__num_excl_copies;	/* DEPRECATED */

I'd prefer we name them something even more generic i.e. :
pad1, pad2 or unused1, unused2 to really deter any efforts to use them.
I guess this could shouldn't have been merged in the first place ...

>  	struct btrfs_qgroup_limit lim;
>  	__u64	qgroups[0];
>  };
>