1 files changed, 60 insertions, 0 deletions

diff --git a/mm/readahead.c b/mm/readahead.c
index ceed7e4790bd..aa1aa2345235 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.
          */