[PATCH 21/28] readahead: thrashing recovery method

[Wu Fengguang <wfg@mail.ustc.edu.cn>]

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Readahead policy after thrashing.

It tries to recover gracefully from the thrashing.

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
@@ -1522,6 +1522,50 @@ try_backward_prefetching(struct file_ra_
 }
 
 /*
+ * Readahead thrashing recovery.
+ */
+static unsigned long
+thrashing_recovery_readahead(struct address_space *mapping,
+				struct file *filp, struct file_ra_state *ra,
+				pgoff_t offset, unsigned long ra_max)
+{
+	unsigned long ra_size;
+
+#ifdef CONFIG_DEBUG_READAHEAD
+	if (probe_page(mapping, offset - 1))
+		ra_account(ra, RA_EVENT_READAHEAD_MUTILATE,
+						ra->readahead_index - offset);
+	ra_account(ra, RA_EVENT_READAHEAD_THRASHING,
+						ra->readahead_index - offset);
+#endif
+
+	/*
+	 * Some thrashing occur in (ra_index, la_index], in which case the
+	 * old read-ahead chunk is lost soon after the new one is allocated.
+	 * Ensure that we recover all needed pages in the old chunk.
+	 */
+	if (offset < ra->ra_index)
+		ra_size = ra->ra_index - offset;
+	else {
+		/* After thrashing, we know the exact thrashing-threshold. */
+		ra_size = offset - ra->ra_index;
+		update_ra_thrash_bytes(mapping->backing_dev_info, ra_size);
+
+		/* And we'd better be a bit conservative. */
+		ra_size = ra_size * 3 / 4;
+	}
+
+	if (ra_size > ra_max)
+		ra_size = ra_max;
+
+	ra_set_class(ra, RA_CLASS_THRASHING);
+	ra_set_index(ra, offset, offset);
+	ra_set_size(ra, ra_size, ra_size / LOOKAHEAD_RATIO);
+
+	return ra_submit(ra, mapping, filp);
+}
+
+/*
  * ra_min is mainly determined by the size of cache memory. Reasonable?
  *
  * Table of concrete numbers for 4KB page size:

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