[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>

diff --git a/mm/readahead.c b/mm/readahead.c
index 97239562622d41822af8cda3979426e4f155df35..b7f8cd8ea9ca586b4e9f82b07e4806dbeba41230 100644 (file)
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -470,7 +470,11 @@ do_io:
                          * 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;