X-Git-Url: http://ftp.safe.ca/?p=safe%2Fjmp%2Flinux-2.6;a=blobdiff_plain;f=mm%2Freadahead.c;h=033bc135a41f3885cae863ca0c9f5185d6c5d665;hp=ceed7e4790bd99c4aa1e67fe3760a2c8ad8099d3;hb=e071041be037eca208b62b84469a06bdfc692bea;hpb=045a2529a3513faed2d45bd82f9013b124309d94

diff --git a/mm/readahead.c b/mm/readahead.c
index ceed7e4..033bc13 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -330,6 +330,59 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra,
  */
 
 /*
+ * 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.
  */
 static unsigned long
@@ -395,6 +448,13 @@ ondemand_readahead(struct address_space *mapping,
 		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.
 	 */
@@ -487,5 +547,17 @@ page_cache_async_readahead(struct address_space *mapping,
 
 	/* do read-ahead */
 	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+
+#ifdef CONFIG_BLOCK
+	/*
+	 * Normally the current page is !uptodate and lock_page() will be
+	 * immediately called to implicitly unplug the device. However this
+	 * is not always true for RAID conifgurations, where data arrives
+	 * not strictly in their submission order. In this case we need to
+	 * explicitly kick off the IO.
+	 */
+	if (PageUptodate(page))
+		blk_run_backing_dev(mapping->backing_dev_info, NULL);
+#endif
 }
 EXPORT_SYMBOL_GPL(page_cache_async_readahead);