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From: Wu Fengguang on 1 Mar 2010 00:50
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(a)suse.de>
Acked-by: Rik van Riel <riel(a)redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu(a)intel.com>
---
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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