btrfs: qgroup: iterate qgroups without memory allocation for qgroup_reserve()

Qgroup heavily relies on ulist to go through all the involved
qgroups, but since we're using ulist inside fs_info->qgroup_lock
spinlock, this means we're doing a lot of GFP_ATOMIC allocations.

This patch reduces the GFP_ATOMIC usage for qgroup_reserve() by
eliminating the memory allocation completely.

This is done by moving the needed memory to btrfs_qgroup::iterator
list_head, so that we can put all the involved qgroup into a on-stack
list, thus eliminating the need to allocate memory while holding
spinlock.

The only cost is the slightly higher memory usage, but considering the
reduce GFP_ATOMIC during a hot path, it should still be acceptable.

Function qgroup_reserve() is the perfect start point for this
conversion.
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -208,6 +208,7 @@ static struct btrfs_qgroup *add_qgroup_rb(struct btrfs_fs_info *fs_info,
 	INIT_LIST_HEAD(&qgroup->groups);
 	INIT_LIST_HEAD(&qgroup->members);
 	INIT_LIST_HEAD(&qgroup->dirty);
+	INIT_LIST_HEAD(&qgroup->iterator);
 
 	rb_link_node(&qgroup->node, parent, p);
 	rb_insert_color(&qgroup->node, &fs_info->qgroup_tree);
@@ -1342,6 +1343,24 @@ static void qgroup_dirty(struct btrfs_fs_info *fs_info,
 		list_add(&qgroup->dirty, &fs_info->dirty_qgroups);
 }
 
+static void qgroup_iterator_add(struct list_head *head, struct btrfs_qgroup *qgroup)
+{
+	if (!list_empty(&qgroup->iterator))
+		return;
+
+	list_add_tail(&qgroup->iterator, head);
+}
+
+static void qgroup_iterator_clean(struct list_head *head)
+{
+	while (!list_empty(head)) {
+		struct btrfs_qgroup *qgroup;
+
+		qgroup = list_first_entry(head, struct btrfs_qgroup, iterator);
+		list_del_init(&qgroup->iterator);
+	}
+}
+
 /*
  * The easy accounting, we're updating qgroup relationship whose child qgroup
  * only has exclusive extents.
@@ -3125,8 +3144,7 @@ static int qgroup_reserve(struct btrfs_root *root, u64 num_bytes, bool enforce,
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	u64 ref_root = root->root_key.objectid;
 	int ret = 0;
-	struct ulist_node *unode;
-	struct ulist_iterator uiter;
+	LIST_HEAD(qgroup_list);
 
 	if (!is_fstree(ref_root))
 		return 0;
@@ -3146,49 +3164,28 @@ static int qgroup_reserve(struct btrfs_root *root, u64 num_bytes, bool enforce,
 	if (!qgroup)
 		goto out;
 
-	/*
-	 * in a first step, we check all affected qgroups if any limits would
-	 * be exceeded
-	 */
-	ulist_reinit(fs_info->qgroup_ulist);
-	ret = ulist_add(fs_info->qgroup_ulist, qgroup->qgroupid,
-			qgroup_to_aux(qgroup), GFP_ATOMIC);
-	if (ret < 0)
-		goto out;
-	ULIST_ITER_INIT(&uiter);
-	while ((unode = ulist_next(fs_info->qgroup_ulist, &uiter))) {
-		struct btrfs_qgroup *qg;
+	qgroup_iterator_add(&qgroup_list, qgroup);
+	list_for_each_entry(qgroup, &qgroup_list, iterator) {
 		struct btrfs_qgroup_list *glist;
 
-		qg = unode_aux_to_qgroup(unode);
-
-		if (enforce && !qgroup_check_limits(qg, num_bytes)) {
+		if (enforce && !qgroup_check_limits(qgroup, num_bytes)) {
 			ret = -EDQUOT;
 			goto out;
 		}
 
-		list_for_each_entry(glist, &qg->groups, next_group) {
-			ret = ulist_add(fs_info->qgroup_ulist,
-					glist->group->qgroupid,
-					qgroup_to_aux(glist->group), GFP_ATOMIC);
-			if (ret < 0)
-				goto out;
-		}
+		list_for_each_entry(glist, &qgroup->groups, next_group)
+			qgroup_iterator_add(&qgroup_list, glist->group);
 	}
+
 	ret = 0;
 	/*
 	 * no limits exceeded, now record the reservation into all qgroups
 	 */
-	ULIST_ITER_INIT(&uiter);
-	while ((unode = ulist_next(fs_info->qgroup_ulist, &uiter))) {
-		struct btrfs_qgroup *qg;
-
-		qg = unode_aux_to_qgroup(unode);
-
-		qgroup_rsv_add(fs_info, qg, num_bytes, type);
-	}
+	list_for_each_entry(qgroup, &qgroup_list, iterator)
+		qgroup_rsv_add(fs_info, qgroup, num_bytes, type);
 
 out:
+	qgroup_iterator_clean(&qgroup_list);
 	spin_unlock(&fs_info->qgroup_lock);
 	return ret;
 }

fs/btrfs/qgroup.h

@@ -220,6 +220,15 @@ struct btrfs_qgroup {
 	struct list_head groups;  /* groups this group is member of */
 	struct list_head members; /* groups that are members of this group */
 	struct list_head dirty;   /* dirty groups */
+
+	/*
+	 * For qgroup iteration usage.
+	 *
+	 * The iteration list should always be empty until qgroup_iterator_add()
+	 * is called.  And should be reset to empty after the iteration is
+	 * finished.
+	 */
+	struct list_head iterator;
 	struct rb_node node;	  /* tree of qgroups */
 
 	/*