Re: [PATCH 5/5] btrfs: qgroup: use qgroup_iterator facility to replace @tmp ulist of qgroup_update_refcnt()

[Boris Burkov <boris@bur.io>]

On Wed, Aug 30, 2023 at 06:37:27PM +0800, Qu Wenruo wrote:
> For function qgroup_update_refcnt(), we use @tmp list to iterate all the
> involved qgroups of a subvolume.
> 
> It's a perfect match for qgroup_iterator facility, as that @tmp ulist
> has a very limited lifespan (just inside the while() loop).
> 
> By migrating to qgroup_iterator, we can get rid of the GFP_ATOMIC memory
> allocation and no more error handling needed.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/qgroup.c | 37 +++++++++++--------------------------
>  1 file changed, 11 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 08f4fc622180..6fcf302744e2 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -2463,13 +2463,11 @@ int btrfs_qgroup_trace_subtree(struct btrfs_trans_handle *trans,
>   * Walk all of the roots that points to the bytenr and adjust their refcnts.
>   */
>  static int qgroup_update_refcnt(struct btrfs_fs_info *fs_info,
> -				struct ulist *roots, struct ulist *tmp,
> -				struct ulist *qgroups, u64 seq, int update_old)
> +				struct ulist *roots, struct ulist *qgroups,
> +				u64 seq, int update_old)
>  {
>  	struct ulist_node *unode;
>  	struct ulist_iterator uiter;
> -	struct ulist_node *tmp_unode;
> -	struct ulist_iterator tmp_uiter;
>  	struct btrfs_qgroup *qg;
>  	int ret = 0;
>  
> @@ -2477,40 +2475,35 @@ static int qgroup_update_refcnt(struct btrfs_fs_info *fs_info,
>  		return 0;
>  	ULIST_ITER_INIT(&uiter);
>  	while ((unode = ulist_next(roots, &uiter))) {
> +		LIST_HEAD(tmp);
> +
>  		qg = find_qgroup_rb(fs_info, unode->val);
>  		if (!qg)
>  			continue;
>  
> -		ulist_reinit(tmp);
>  		ret = ulist_add(qgroups, qg->qgroupid, qgroup_to_aux(qg),
>  				GFP_ATOMIC);
>  		if (ret < 0)
>  			return ret;
> -		ret = ulist_add(tmp, qg->qgroupid, qgroup_to_aux(qg), GFP_ATOMIC);
> -		if (ret < 0)
> -			return ret;
> -		ULIST_ITER_INIT(&tmp_uiter);
> -		while ((tmp_unode = ulist_next(tmp, &tmp_uiter))) {
> +		qgroup_iterator_add(&tmp, qg);
> +		list_for_each_entry(qg, &tmp, iterator) {
>  			struct btrfs_qgroup_list *glist;
>  
> -			qg = unode_aux_to_qgroup(tmp_unode);
>  			if (update_old)
>  				btrfs_qgroup_update_old_refcnt(qg, seq, 1);
>  			else
>  				btrfs_qgroup_update_new_refcnt(qg, seq, 1);
> +
>  			list_for_each_entry(glist, &qg->groups, next_group) {
>  				ret = ulist_add(qgroups, glist->group->qgroupid,
>  						qgroup_to_aux(glist->group),
>  						GFP_ATOMIC);

Thinking out loud, could we use the same trick and put another global
list on the qgroups to handle this one? Or some other "single global
allocation up to #qgroups" trick.

>  				if (ret < 0)
>  					return ret;
> -				ret = ulist_add(tmp, glist->group->qgroupid,
> -						qgroup_to_aux(glist->group),
> -						GFP_ATOMIC);
> -				if (ret < 0)
> -					return ret;
> +				qgroup_iterator_add(&tmp, glist->group);
>  			}
>  		}
> +		qgroup_iterator_clean(&tmp);
>  	}
>  	return 0;
>  }
> @@ -2675,7 +2668,6 @@ int btrfs_qgroup_account_extent(struct btrfs_trans_handle *trans, u64 bytenr,
>  {
>  	struct btrfs_fs_info *fs_info = trans->fs_info;
>  	struct ulist *qgroups = NULL;
> -	struct ulist *tmp = NULL;
>  	u64 seq;
>  	u64 nr_new_roots = 0;
>  	u64 nr_old_roots = 0;
> @@ -2714,12 +2706,6 @@ int btrfs_qgroup_account_extent(struct btrfs_trans_handle *trans, u64 bytenr,
>  		ret = -ENOMEM;
>  		goto out_free;
>  	}
> -	tmp = ulist_alloc(GFP_NOFS);
> -	if (!tmp) {
> -		ret = -ENOMEM;
> -		goto out_free;
> -	}
> -
>  	mutex_lock(&fs_info->qgroup_rescan_lock);
>  	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) {
>  		if (fs_info->qgroup_rescan_progress.objectid <= bytenr) {
> @@ -2734,13 +2720,13 @@ int btrfs_qgroup_account_extent(struct btrfs_trans_handle *trans, u64 bytenr,
>  	seq = fs_info->qgroup_seq;
>  
>  	/* Update old refcnts using old_roots */
> -	ret = qgroup_update_refcnt(fs_info, old_roots, tmp, qgroups, seq,
> +	ret = qgroup_update_refcnt(fs_info, old_roots, qgroups, seq,
>  				   UPDATE_OLD);
>  	if (ret < 0)
>  		goto out;
>  
>  	/* Update new refcnts using new_roots */
> -	ret = qgroup_update_refcnt(fs_info, new_roots, tmp, qgroups, seq,
> +	ret = qgroup_update_refcnt(fs_info, new_roots, qgroups, seq,
>  				   UPDATE_NEW);
>  	if (ret < 0)
>  		goto out;
> @@ -2755,7 +2741,6 @@ int btrfs_qgroup_account_extent(struct btrfs_trans_handle *trans, u64 bytenr,
>  out:
>  	spin_unlock(&fs_info->qgroup_lock);
>  out_free:
> -	ulist_free(tmp);
>  	ulist_free(qgroups);
>  	ulist_free(old_roots);
>  	ulist_free(new_roots);
> -- 
> 2.41.0
>