---

yaml
---
r: 151514
b: refs/heads/master
c: 10be0b3
h: refs/heads/master
v: v3

[refs]

@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 045a2529a3513faed2d45bd82f9013b124309d94
+refs/heads/master: 10be0b372cac50e2e7a477852f98bf069a97a3fa

trunk/mm/readahead.c

@@ -329,6 +329,59 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra,
  * it approaches max_readhead.
  */
 
+/*
+ * Count contiguously cached pages from @offset-1 to @offset-@max,
+ * this count is a conservative estimation of
+ * 	- length of the sequential read sequence, or
+ * 	- thrashing threshold in memory tight systems
+ */
+static pgoff_t count_history_pages(struct address_space *mapping,
+				   struct file_ra_state *ra,
+				   pgoff_t offset, unsigned long max)
+{
+	pgoff_t head;
+
+	rcu_read_lock();
+	head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
+	rcu_read_unlock();
+
+	return offset - 1 - head;
+}
+
+/*
+ * page cache context based read-ahead
+ */
+static int try_context_readahead(struct address_space *mapping,
+				 struct file_ra_state *ra,
+				 pgoff_t offset,
+				 unsigned long req_size,
+				 unsigned long max)
+{
+	pgoff_t size;
+
+	size = count_history_pages(mapping, ra, offset, max);
+
+	/*
+	 * no history pages:
+	 * it could be a random read
+	 */
+	if (!size)
+		return 0;
+
+	/*
+	 * starts from beginning of file:
+	 * it is a strong indication of long-run stream (or whole-file-read)
+	 */
+	if (size >= offset)
+		size *= 2;
+
+	ra->start = offset;
+	ra->size = get_init_ra_size(size + req_size, max);
+	ra->async_size = ra->size;
+
+	return 1;
+}
+
 /*
  * A minimal readahead algorithm for trivial sequential/random reads.
  */
@@ -394,6 +447,13 @@ ondemand_readahead(struct address_space *mapping,
 	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
 		goto initial_readahead;
 
+	/*
+	 * Query the page cache and look for the traces(cached history pages)
+	 * that a sequential stream would leave behind.
+	 */
+	if (try_context_readahead(mapping, ra, offset, req_size, max))
+		goto readit;
+
 	/*
 	 * standalone, small random read
 	 * Read as is, and do not pollute the readahead state.