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Subject[PATCH 4/9] readahead: data structure and routines
Extend struct file_ra_state to support the on-demand readahead logic.
Also define some helpers for it.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
include/linux/fs.h | 64 +++++++++++++++++++++++++++++++++++++++++++
mm/readahead.c | 19 ++++++++++++
2 files changed, 83 insertions(+)
--- linux-2.6.22-rc1-mm1.orig/include/linux/fs.h
+++ linux-2.6.22-rc1-mm1/include/linux/fs.h
@@ -702,6 +702,10 @@ struct fown_struct {

/*
* 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 */
@@ -711,6 +715,12 @@ struct file_ra_state {
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 */
@@ -719,6 +729,60 @@ struct file_ra_state {
#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
--- linux-2.6.22-rc1-mm1.orig/mm/readahead.c
+++ linux-2.6.22-rc1-mm1/mm/readahead.c
@@ -592,3 +592,22 @@ unsigned long max_sane_readahead(unsigne
return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE)
+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
}
+
+/*
+ * Submit IO for the read-ahead request in file_ra_state.
+ */
+unsigned long ra_submit(struct file_ra_state *ra,
+ struct address_space *mapping, struct file *filp)
+{
+ unsigned long ra_size;
+ unsigned long la_size;
+ int actual;
+
+ ra_size = ra_readahead_size(ra);
+ la_size = ra_lookahead_size(ra);
+ actual = __do_page_cache_readahead(mapping, filp,
+ ra->ra_index, ra_size, la_size);
+
+ return actual;
+}
+EXPORT_SYMBOL_GPL(ra_submit);
--
-
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