[patch]block: add plug for blkdev_issue_discard

[Shaohua Li]

Now discard merge works, add plug for blkdev_issue_discard. This will help
discard request merge especially for raid0 case. In raid0, a big discard
request is split to small requests, and if correct plug is added, such small
requests can be merged in low layer.

Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
---
 block/blk-lib.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/block/blk-lib.c
===================================================================
--- linux.orig/block/blk-lib.c	2012-09-28 15:09:38.655327319 +0800
+++ linux/block/blk-lib.c	2012-10-09 08:45:04.358598888 +0800
@@ -48,6 +48,7 @@ int blkdev_issue_discard(struct block_de
 	struct bio_batch bb;
 	struct bio *bio;
 	int ret = 0;
+	struct blk_plug plug;
 
 	if (!q)
 		return -ENXIO;
@@ -81,6 +82,7 @@ int blkdev_issue_discard(struct block_de
 	bb.flags = 1 << BIO_UPTODATE;
 	bb.wait = &wait;
 
+	blk_start_plug(&plug);
 	while (nr_sects) {
 		unsigned int req_sects;
 		sector_t end_sect;
@@ -117,6 +119,7 @@ int blkdev_issue_discard(struct block_de
 		atomic_inc(&bb.done);
 		submit_bio(type, bio);
 	}
+	blk_finish_plug(&plug);
 
 	/* Wait for bios in-flight */
 	if (!atomic_dec_and_test(&bb.done))
--
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