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>

1 files changed, 5 insertions, 2 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 805b287..5ec7658 100644
--- a/mm/backing-dev.c
+++ b/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);