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Subject[PATCH 27/32] readahead: loop case
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>
---
I'd like to thank Tero Grundstr?m for uncovering the loopback problem.

drivers/block/loop.c | 6 ++++++
1 files changed, 6 insertions(+)
--- linux-2.6.17-rc4-mm3.orig/drivers/block/loop.c
+++ linux-2.6.17-rc4-mm3/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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