[PATCH 19/23] readahead: loop case

[Wu Fengguang]

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@xxxxxxxxxxxxxxxx>
---

I'd like to thank Tero Grundstr?m for uncovering the loopback problem.

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

--- linux-2.6.16-rc6-mm2.orig/drivers/block/loop.c
+++ linux-2.6.16-rc6-mm2/drivers/block/loop.c
@@ -779,6 +779,12 @@ 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;
 

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