From: Wu Fengguang <wfg@mail.ustc.edu.cn>

Disable look-ahead for loop file.

Loopback files normally contain filesystems, in which case there are already
proper look-aheads in the upper layer, more look-aheads on the loopback file
only ruins the read-ahead hit rate.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 drivers/block/loop.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff -puN drivers/block/loop.c~readahead-loop-case drivers/block/loop.c
--- a/drivers/block/loop.c~readahead-loop-case
+++ a/drivers/block/loop.c
@@ -755,6 +755,12 @@ static int loop_set_fd(struct loop_devic
 	mapping = file->f_mapping;
 	inode = mapping->host;
 
+	/*
+	 * The upper layer should already do proper look-ahead,
+	 * one more look-ahead here only ruins the cache hit rate.
+	 */
+	file->f_ra.flags |= RA_FLAG_NO_LOOKAHEAD;
+
 	if (!(file->f_mode & FMODE_WRITE))
 		lo_flags |= LO_FLAGS_READ_ONLY;
 
_