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

Readahead policy for reading backward.

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

 mm/readahead.c |   46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff -puN mm/readahead.c~readahead-backward-prefetching-method mm/readahead.c
--- a/mm/readahead.c~readahead-backward-prefetching-method
+++ a/mm/readahead.c
@@ -1491,6 +1491,52 @@ initial_readahead(struct address_space *
 	return ra_submit(ra, mapping, filp);
 }
 
+/*
+ * Backward prefetching.
+ *
+ * No look-ahead and thrashing safety guard: should be unnecessary.
+ *
+ * Important for certain scientific arenas(i.e. structural analysis).
+ */
+static int
+try_backward_prefetching(struct file_ra_state *ra, pgoff_t offset,
+			 unsigned long size, unsigned long ra_max)
+{
+	pgoff_t prev = ra->prev_page;
+
+	/* Reading backward? */
+	if (offset >= prev)
+		return 0;
+
+	/* Close enough? */
+	size += readahead_hit_rate;
+	if (offset + 2 * size <= prev)
+		return 0;
+
+	if (ra_class_new(ra) == RA_CLASS_BACKWARD && ra_has_index(ra, prev)) {
+		prev = ra->la_index;
+		size += 2 * ra_readahead_size(ra);
+	} else
+		size *= 2;
+
+	if (size > ra_max)
+		size = ra_max;
+	if (size > prev)
+		size = prev;
+
+	/* The readahead-request covers the read-request? */
+	if (offset < prev - size)
+		return 0;
+
+	offset = prev - size;
+
+	ra_set_class(ra, RA_CLASS_BACKWARD);
+	ra_set_index(ra, offset, offset);
+	ra_set_size(ra, size, 0);
+
+	return 1;
+}
+
 #endif /* CONFIG_ADAPTIVE_READAHEAD */
 
 /*
_