io-controller: Add a new interface "weight_device" for IO-Controller

Currently, IO Controller makes use of blkio.weight to assign weight for
all devices. Here a new user interface "blkio.weight_device" is introduced to
assign different weights for different devices. blkio.weight becomes the
default value for devices which are not configured by "blkio.weight_device"

You can use the following format to assigned specific weight for a given
device:
#echo "major:minor weight" > blkio.weight_device

major:minor represents device number.

And you can remove weight for a given device as following:
#echo "major:minor 0" > blkio.weight_device

V1->V2 changes:
- use user interface "weight_device" instead of "policy" suggested by Vivek
- rename some struct suggested by Vivek
- rebase to 2.6-block "for-linus" branch
- remove an useless list_empty check pointed out by Li Zefan
- some trivial typo fix

V2->V3 changes:
- Move policy_*_node() functions up to get rid of forward declarations
- rename related functions by adding prefix "blkio_"

Signed-off-by: Gui Jianfeng <[email protected]>
Acked-by: Vivek Goyal <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/blk-cgroup.h

@@ -70,6 +70,7 @@ struct blkio_cgroup {
 	unsigned int weight;
 	spinlock_t lock;
 	struct hlist_head blkg_list;
+	struct list_head policy_list; /* list of blkio_policy_node */
 };
 
 struct blkio_group_stats {
@@ -119,6 +120,15 @@ struct blkio_group {
 	struct blkio_group_stats stats;
 };
 
+struct blkio_policy_node {
+	struct list_head node;
+	dev_t dev;
+	unsigned int weight;
+};
+
+extern unsigned int blkcg_get_weight(struct blkio_cgroup *blkcg,
+				     dev_t dev);
+
 typedef void (blkio_unlink_group_fn) (void *key, struct blkio_group *blkg);
 typedef void (blkio_update_group_weight_fn) (struct blkio_group *blkg,
 						unsigned int weight);