[PATCH 06/45] writeback, blkcg: associate each blkcg_gq with the corresponding bdi_writeback

[Tejun Heo]

A blkg (blkcg_gq) can be congested and decongested independently from
other blkgs on the same request_queue.  Accordingly, for cgroup
writeback support, the congestion status at bdi (backing_dev_info)
should be split to per-cgroup wb's (bdi_writeback's) and updated
separately from matching blkg's.

This patch prepares by adding blkg->wb and associating a blkg with its
matching per-cgroup wb on creation.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
---
 block/blk-cgroup.c         | 15 +++++++++++++++
 include/linux/blk-cgroup.h |  6 ++++++
 2 files changed, 21 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 8bebaa9..6fe085c 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -182,6 +182,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 				    struct blkcg_gq *new_blkg)
 {
 	struct blkcg_gq *blkg;
+	struct bdi_writeback *wb;
 	int i, ret;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
@@ -193,6 +194,19 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 		goto err_free_blkg;
 	}
 
+	/*
+	 * Once created, @wb will stay alive longer than @blkg.  @wb is
+	 * destroyed iff either its bdi or @blkcg is destroyed.  The bdi is
+	 * part of the request_queue and will outlive @blkg, and, while
+	 * @blkcg is being brought down, @wb will be destroyed the last in
+	 * ->css_released().
+	 */
+	wb = cgwb_lookup_create(&q->backing_dev_info, &blkcg->css);
+	if (!wb) {
+		ret = -ENOMEM;
+		goto err_free_blkg;
+	}
+
 	/* allocate */
 	if (!new_blkg) {
 		new_blkg = blkg_alloc(blkcg, q, GFP_ATOMIC);
@@ -202,6 +216,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 		}
 	}
 	blkg = new_blkg;
+	blkg->wb = wb;
 
 	/* link parent */
 	if (blkcg_parent(blkcg)) {
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 3033eb1..97ceee3 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -99,6 +99,12 @@ struct blkcg_gq {
 	struct hlist_node		blkcg_node;
 	struct blkcg			*blkcg;
 
+	/*
+	 * Each blkg gets congested separately and the congestion state is
+	 * propagated to the matching cgroup wb.
+	 */
+	struct bdi_writeback		*wb;
+
 	/* all non-root blkcg_gq's are guaranteed to have access to parent */
 	struct blkcg_gq			*parent;
 
-- 
2.1.0

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