[PATCH] fix readahead breakage for sequential after random reads

Current readahead logic is broken when a random read pattern is followed by
a long sequential read.  The cause is that on a window miss ra->next_size
is set to ra->average, but ra->average is only updated at the end of a
sequence, so window size will remain 1 until the end of the sequential
read.

This patch fixes this by taking the current sequence length into account
(code taken from towards end of page_cache_readahead()), and also setting
ra->average to a decent value in handle_ra_miss() when sequential access is
detected.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/readahead.c

@@ -470,7 +470,11 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
 			  * pages shall be accessed in the next
 			  * current window.
 			  */
-			ra->next_size = min(ra->average , (unsigned long)max);
+			average = ra->average;
+			if (ra->serial_cnt > average)
+				average = (ra->serial_cnt + ra->average + 1) / 2;
+
+			ra->next_size = min(average , (unsigned long)max);
 		}
 		ra->start = offset;
 		ra->size = ra->next_size;
@@ -552,6 +556,7 @@ void handle_ra_miss(struct address_space *mapping,
 				ra->size = max;
 				ra->ahead_start = 0;
 				ra->ahead_size = 0;
+				ra->average = max / 2;
 			}
 		}
 		ra->prev_page = offset;