btrfs: qgroup: preallocate memory before adding a relation

There's a transaction joined in the qgroup relation add/remove ioctl and
any error will lead to abort/error. We could lift the allocation from
btrfs_add_qgroup_relation() and move it outside of the transaction
context. The relation deletion does not need that.

The ownership of the structure is moved to the add relation handler.

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

fs/btrfs/ioctl.c

@@ -3829,6 +3829,7 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
 	struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct btrfs_ioctl_qgroup_assign_args *sa;
+	struct btrfs_qgroup_list *prealloc = NULL;
 	struct btrfs_trans_handle *trans;
 	int ret;
 	int err;
@@ -3849,14 +3850,27 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
 		goto drop_write;
 	}
 
+	if (sa->assign) {
+		prealloc = kzalloc(sizeof(*prealloc), GFP_KERNEL);
+		if (!prealloc) {
+			ret = -ENOMEM;
+			goto drop_write;
+		}
+	}
+
 	trans = btrfs_join_transaction(root);
 	if (IS_ERR(trans)) {
 		ret = PTR_ERR(trans);
 		goto out;
 	}
 
+	/*
+	 * Prealloc ownership is moved to the relation handler, there it's used
+	 * or freed on error.
+	 */
 	if (sa->assign) {
-		ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst);
+		ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst, prealloc);
+		prealloc = NULL;
 	} else {
 		ret = btrfs_del_qgroup_relation(trans, sa->src, sa->dst);
 	}
@@ -3873,6 +3887,7 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
 		ret = err;
 
 out:
+	kfree(prealloc);
 	kfree(sa);
 drop_write:
 	mnt_drop_write_file(file);

fs/btrfs/qgroup.c

@@ -155,16 +155,6 @@ static inline u64 btrfs_qgroup_get_new_refcnt(const struct btrfs_qgroup *qg, u64
 	return qg->new_refcnt - seq;
 }
 
-/*
- * glue structure to represent the relations between qgroups.
- */
-struct btrfs_qgroup_list {
-	struct list_head next_group;
-	struct list_head next_member;
-	struct btrfs_qgroup *group;
-	struct btrfs_qgroup *member;
-};
-
 static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
@@ -1569,15 +1559,21 @@ 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)
+/*
+ * Add relation between @src and @dst qgroup. The @prealloc is allocated by the
+ * callers and transferred here (either used or freed on error).
+ */
+int btrfs_add_qgroup_relation(struct btrfs_trans_handle *trans, u64 src, u64 dst,
+			      struct btrfs_qgroup_list *prealloc)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_qgroup *parent;
 	struct btrfs_qgroup *member;
 	struct btrfs_qgroup_list *list;
-	struct btrfs_qgroup_list *prealloc = NULL;
 	int ret = 0;
 
+	ASSERT(prealloc);
+
 	/* Check the level of src and dst first */
 	if (btrfs_qgroup_level(src) >= btrfs_qgroup_level(dst))
 		return -EINVAL;
@@ -1602,11 +1598,6 @@ int btrfs_add_qgroup_relation(struct btrfs_trans_handle *trans, u64 src, u64 dst
 		}
 	}
 
-	prealloc = kzalloc(sizeof(*list), GFP_NOFS);
-	if (!prealloc) {
-		ret = -ENOMEM;
-		goto out;
-	}
 	ret = add_qgroup_relation_item(trans, src, dst);
 	if (ret)
 		goto out;

fs/btrfs/qgroup.h

@@ -278,6 +278,14 @@ struct btrfs_qgroup {
 	struct kobject kobj;
 };
 
+/* Glue structure to represent the relations between qgroups. */
+struct btrfs_qgroup_list {
+	struct list_head next_group;
+	struct list_head next_member;
+	struct btrfs_qgroup *group;
+	struct btrfs_qgroup *member;
+};
+
 struct btrfs_squota_delta {
 	/* The fstree root this delta counts against. */
 	u64 root;
@@ -321,7 +329,8 @@ int btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info);
 void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info);
 int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 				     bool interruptible);
-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_qgroup_list *prealloc);
 int btrfs_del_qgroup_relation(struct btrfs_trans_handle *trans, u64 src,
 			      u64 dst);
 int btrfs_create_qgroup(struct btrfs_trans_handle *trans, u64 qgroupid);