[RESEND][PATCH] readahead:add blk_run_backing_dev

[Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>]

Hi Andrew.
Following patch improves sequential read performance and does not harm
other performance.
Please merge my patch.
Comments?
Thanks.

#dd if=testdir/testfile of=/dev/null bs=16384
-2.6.30-rc6
1048576+0 records in
1048576+0 records out
17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s

-2.6.30-rc6-patched
1048576+0 records in
1048576+0 records out
17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s

Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c linux-2.6.30-rc6.unplug/mm/readahead.c
--- linux-2.6.30-rc6.org/mm/readahead.c	2009-05-18 10:46:15.000000000 +0900
+++ linux-2.6.30-rc6.unplug/mm/readahead.c	2009-05-18 13:00:42.000000000 +0900
@@ -490,5 +490,7 @@ page_cache_async_readahead(struct addres
 
 	/* do read-ahead */
 	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+
+	blk_run_backing_dev(mapping->backing_dev_info, NULL);
 }
 EXPORT_SYMBOL_GPL(page_cache_async_readahead);