writeback: implement bdi_for_each_wb()

This will be used to implement bdi-wide operations which should be
distributed across all its cgroup bdi_writebacks.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/backing-dev.h

@@ -383,6 +383,61 @@ static inline struct bdi_writeback *inode_to_wb(struct inode *inode)
 	return inode->i_wb;
 }
 
+struct wb_iter {
+	int			start_blkcg_id;
+	struct radix_tree_iter	tree_iter;
+	void			**slot;
+};
+
+static inline struct bdi_writeback *__wb_iter_next(struct wb_iter *iter,
+						   struct backing_dev_info *bdi)
+{
+	struct radix_tree_iter *titer = &iter->tree_iter;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	if (iter->start_blkcg_id >= 0) {
+		iter->slot = radix_tree_iter_init(titer, iter->start_blkcg_id);
+		iter->start_blkcg_id = -1;
+	} else {
+		iter->slot = radix_tree_next_slot(iter->slot, titer, 0);
+	}
+
+	if (!iter->slot)
+		iter->slot = radix_tree_next_chunk(&bdi->cgwb_tree, titer, 0);
+	if (iter->slot)
+		return *iter->slot;
+	return NULL;
+}
+
+static inline struct bdi_writeback *__wb_iter_init(struct wb_iter *iter,
+						   struct backing_dev_info *bdi,
+						   int start_blkcg_id)
+{
+	iter->start_blkcg_id = start_blkcg_id;
+
+	if (start_blkcg_id)
+		return __wb_iter_next(iter, bdi);
+	else
+		return &bdi->wb;
+}
+
+/**
+ * bdi_for_each_wb - walk all wb's of a bdi in ascending blkcg ID order
+ * @wb_cur: cursor struct bdi_writeback pointer
+ * @bdi: bdi to walk wb's of
+ * @iter: pointer to struct wb_iter to be used as iteration buffer
+ * @start_blkcg_id: blkcg ID to start iteration from
+ *
+ * Iterate @wb_cur through the wb's (bdi_writeback's) of @bdi in ascending
+ * blkcg ID order starting from @start_blkcg_id.  @iter is struct wb_iter
+ * to be used as temp storage during iteration.  rcu_read_lock() must be
+ * held throughout iteration.
+ */
+#define bdi_for_each_wb(wb_cur, bdi, iter, start_blkcg_id)		\
+	for ((wb_cur) = __wb_iter_init(iter, bdi, start_blkcg_id);	\
+	     (wb_cur); (wb_cur) = __wb_iter_next(iter, bdi))
+
 #else	/* CONFIG_CGROUP_WRITEBACK */
 
 static inline bool inode_cgwb_enabled(struct inode *inode)
@@ -445,6 +500,14 @@ static inline void wb_blkcg_offline(struct blkcg *blkcg)
 {
 }
 
+struct wb_iter {
+	int		next_id;
+};
+
+#define bdi_for_each_wb(wb_cur, bdi, iter, start_blkcg_id)		\
+	for ((iter)->next_id = (start_blkcg_id);			\
+	     ({	(wb_cur) = !(iter)->next_id++ ? &(bdi)->wb : NULL; }); )
+
 static inline int inode_congested(struct inode *inode, int cong_bits)
 {
 	return wb_congested(&inode_to_bdi(inode)->wb, cong_bits);