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Subject[PATCH 17/19] readahead: disable look-ahead for loopback file
Loopback files normally contain filesystem, 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 ++++++
include/linux/fs.h | 1 +
mm/readahead.c | 8 ++++++++
3 files changed, 15 insertions(+)

--- linux-2.6.15-rc2-mm1.orig/include/linux/fs.h
+++ linux-2.6.15-rc2-mm1/include/linux/fs.h
@@ -620,6 +620,7 @@ struct file_ra_state {
};
#define RA_FLAG_MISS 0x01 /* a cache miss occured against this file */
#define RA_FLAG_INCACHE 0x02 /* file is already in cache */
+#define RA_FLAG_NO_LOOKAHEAD (1UL<<31) /* disable look-ahead */

struct file {
/*
--- linux-2.6.15-rc2-mm1.orig/drivers/block/loop.c
+++ linux-2.6.15-rc2-mm1/drivers/block/loop.c
@@ -782,6 +782,12 @@ 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;

--- linux-2.6.15-rc2-mm1.orig/mm/readahead.c
+++ linux-2.6.15-rc2-mm1/mm/readahead.c
@@ -1312,6 +1312,11 @@ static inline void ra_state_update(struc
if (ra_size < old_ra && ra_cache_hit(ra, 0))
ra_account(ra, RA_EVENT_READAHEAD_SHRINK, old_ra - ra_size);
#endif
+
+ /* Disable look-ahead for loopback file. */
+ if (unlikely(ra->flags & RA_FLAG_NO_LOOKAHEAD))
+ la_size = 0;
+
ra_addup_cache_hit(ra);
ra->ra_index = ra->readahead_index;
ra->la_index = ra->lookahead_index;
@@ -1672,6 +1677,9 @@ static int query_page_cache(struct addre
if (count < ra_max)
goto out;

+ if (unlikely(ra->flags & RA_FLAG_NO_LOOKAHEAD))
+ goto out;
+
/*
* Check the far pages coarsely.
* The big count here helps increase la_size.
--
-
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