diff --git a/include/linux/fs.h b/include/linux/fs.h
index 75dd16e..9a5f562 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -695,6 +695,10 @@
 
 /*
  * Track a single file's readahead state
+ *
+ *  ================#============|==================#==================|
+ *                  ^            ^                  ^                  ^
+ *  file_ra_state.la_index    .ra_index   .lookahead_index   .readahead_index
  */
 struct file_ra_state {
 	unsigned long start;		/* Current window */
@@ -704,6 +708,12 @@
 	unsigned long prev_index;	/* Cache last read() position */
 	unsigned long ahead_start;	/* Ahead window */
 	unsigned long ahead_size;
+
+	pgoff_t la_index;               /* enqueue time */
+	pgoff_t ra_index;               /* begin offset */
+	pgoff_t lookahead_index;        /* time to do next readahead */
+	pgoff_t readahead_index;        /* end offset */
+
 	unsigned long ra_pages;		/* Maximum readahead window */
 	unsigned long mmap_hit;		/* Cache hit stat for mmap accesses */
 	unsigned long mmap_miss;	/* Cache miss stat for mmap accesses */
@@ -712,6 +722,60 @@
 #define RA_FLAG_MISS 0x01	/* a cache miss occured against this file */
 #define RA_FLAG_INCACHE 0x02	/* file is already in cache */
 
+/*
+ * Measuring read-ahead sizes.
+ *
+ *                  |----------- readahead size ------------>|
+ *  ===#============|==================#=====================|
+ *     |------- invoke interval ------>|-- lookahead size -->|
+ */
+static inline unsigned long ra_readahead_size(struct file_ra_state *ra)
+{
+	return ra->readahead_index - ra->ra_index;
+}
+
+static inline unsigned long ra_lookahead_size(struct file_ra_state *ra)
+{
+	return ra->readahead_index - ra->lookahead_index;
+}
+
+static inline unsigned long ra_invoke_interval(struct file_ra_state *ra)
+{
+	return ra->lookahead_index - ra->la_index;
+}
+
+/*
+ * Check if @index falls in the readahead windows.
+ */
+static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
+{
+	return (index >= ra->la_index &&
+		index <  ra->readahead_index);
+}
+
+/*
+ * Where is the old read-ahead and look-ahead?
+ */
+static inline void ra_set_index(struct file_ra_state *ra,
+				pgoff_t la_index, pgoff_t ra_index)
+{
+	ra->la_index = la_index;
+	ra->ra_index = ra_index;
+}
+
+/*
+ * Where is the new read-ahead and look-ahead?
+ */
+static inline void ra_set_size(struct file_ra_state *ra,
+				unsigned long ra_size, unsigned long la_size)
+{
+	ra->readahead_index = ra->ra_index + ra_size;
+	ra->lookahead_index = ra->ra_index + ra_size - la_size;
+}
+
+unsigned long ra_submit(struct file_ra_state *ra,
+		       struct address_space *mapping, struct file *filp);
+
 struct file {
 	/*
 	 * fu_list becomes invalid after file_free is called and queued via