[PATCH 15/15] readahead: pagecache context based mmap read-around

[Wu Fengguang]

Do mmap read-around when there are cached pages in the nearby 256KB
(covered by one radix tree node).

There is a failure case though: for a sequence of page faults at page
index 64*i+1, i=1,2,3,..., this heuristic will keep doing pointless
read-arounds.  Hopefully the pattern won't appear in real workloads.
Note that the readahead heuristic has similiar failure case.

CC: Nick Piggin <npiggin@xxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/filemap.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

--- linux.orig/mm/filemap.c	2010-02-23 13:20:39.000000000 +0800
+++ linux/mm/filemap.c	2010-02-23 13:22:36.000000000 +0800
@@ -1421,11 +1421,17 @@ static void do_sync_mmap_readahead(struc
 
 
 	/*
-	 * Do we miss much more than hit in this file? If so,
-	 * stop bothering with read-ahead. It will only hurt.
+	 * Do we miss much more than hit in this file? If so, stop bothering
+	 * with read-around, unless some nearby pages were accessed recently.
 	 */
-	if (ra_mmap_miss_inc(ra) > MMAP_LOTSAMISS)
-		return;
+	if (ra_mmap_miss_inc(ra) > MMAP_LOTSAMISS) {
+		struct radix_tree_node *node;
+		rcu_read_lock();
+		node = radix_tree_lookup_leaf_node(&mapping->page_tree, offset);
+		rcu_read_unlock();
+		if (!node)
+			return;
+	}
 
 	/*
 	 * mmap read-around


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