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Subject[PATCH 20/28] readahead: backward prefetching method
Readahead policy for reading backward.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
DESC
readahead: backward prefetching method - add use case comment
EDESC
From: Wu Fengguang <wfg@mail.ustc.edu.cn>

Backward prefetching is vital to structural analysis and some other
scientific applications. Comment this use case.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@osdl.org>
--- linux-2.6.19-rc5-mm2.orig/mm/readahead.c
+++ linux-2.6.19-rc5-mm2/mm/readahead.c
@@ -1476,6 +1476,52 @@ initial_readahead(struct address_space *
}

/*
+ * 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;
+}
+
+/*
* ra_min is mainly determined by the size of cache memory. Reasonable?
*
* Table of concrete numbers for 4KB page size:
--
-
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