bdi: separate out congested state into a separate struct

Currently, a wb's (bdi_writeback) congestion state is carried in its
->state field; however, cgroup writeback support will require multiple
wb's sharing the same congestion state.  This patch separates out
congestion state into its own struct - struct bdi_writeback_congested.
A new field wb field, wb_congested, points to its associated congested
struct.  The default wb, bdi->wb, always points to bdi->wb_congested.

While this patch adds a layer of indirection, it doesn't introduce any
behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/backing-dev-defs.h

@@ -16,12 +16,15 @@ struct dentry;
  * Bits in bdi_writeback.state
  */
 enum wb_state {
-	WB_async_congested,	/* The async (write) queue is getting full */
-	WB_sync_congested,	/* The sync queue is getting full */
 	WB_registered,		/* bdi_register() was done */
 	WB_writeback_running,	/* Writeback is in progress */
 };
 
+enum wb_congested_state {
+	WB_async_congested,	/* The async (write) queue is getting full */
+	WB_sync_congested,	/* The sync queue is getting full */
+};
+
 typedef int (congested_fn)(void *, int);
 
 enum wb_stat_item {
@@ -34,6 +37,10 @@ enum wb_stat_item {
 
 #define WB_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))
 
+struct bdi_writeback_congested {
+	unsigned long state;		/* WB_[a]sync_congested flags */
+};
+
 struct bdi_writeback {
 	struct backing_dev_info *bdi;	/* our parent bdi */
 
@@ -48,6 +55,8 @@ struct bdi_writeback {
 
 	struct percpu_counter stat[NR_WB_STAT_ITEMS];
 
+	struct bdi_writeback_congested *congested;
+
 	unsigned long bw_time_stamp;	/* last time write bw is updated */
 	unsigned long dirtied_stamp;
 	unsigned long written_stamp;	/* pages written at bw_time_stamp */
@@ -84,6 +93,7 @@ struct backing_dev_info {
 	unsigned int max_ratio, max_prop_frac;
 
 	struct bdi_writeback wb;  /* default writeback info for this bdi */
+	struct bdi_writeback_congested wb_congested;
 
 	struct device *dev;
 

include/linux/backing-dev.h

@@ -167,7 +167,7 @@ static inline int bdi_congested(struct backing_dev_info *bdi, int bdi_bits)
 {
 	if (bdi->congested_fn)
 		return bdi->congested_fn(bdi->congested_data, bdi_bits);
-	return (bdi->wb.state & bdi_bits);
+	return (bdi->wb.congested->state & bdi_bits);
 }
 
 static inline int bdi_read_congested(struct backing_dev_info *bdi)

mm/backing-dev.c

@@ -383,6 +383,9 @@ int bdi_init(struct backing_dev_info *bdi)
 	if (err)
 		return err;
 
+	bdi->wb_congested.state = 0;
+	bdi->wb.congested = &bdi->wb_congested;
+
 	return 0;
 }
 EXPORT_SYMBOL(bdi_init);
@@ -504,7 +507,7 @@ void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
 	wait_queue_head_t *wqh = &congestion_wqh[sync];
 
 	bit = sync ? WB_sync_congested : WB_async_congested;
-	if (test_and_clear_bit(bit, &bdi->wb.state))
+	if (test_and_clear_bit(bit, &bdi->wb.congested->state))
 		atomic_dec(&nr_bdi_congested[sync]);
 	smp_mb__after_atomic();
 	if (waitqueue_active(wqh))
@@ -517,7 +520,7 @@ void set_bdi_congested(struct backing_dev_info *bdi, int sync)
 	enum wb_state bit;
 
 	bit = sync ? WB_sync_congested : WB_async_congested;
-	if (!test_and_set_bit(bit, &bdi->wb.state))
+	if (!test_and_set_bit(bit, &bdi->wb.congested->state))
 		atomic_inc(&nr_bdi_congested[sync]);
 }
 EXPORT_SYMBOL(set_bdi_congested);