1 files changed, 101 insertions, 44 deletions

diff --git a/mm/readahead.c b/mm/readahead.c
index 133b6d52551..aa1aa234523 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -133,15 +133,12 @@ out:
 }
 
 /*
- * do_page_cache_readahead actually reads a chunk of disk.  It allocates all
+ * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
  * the pages first, then submits them all for I/O. This avoids the very bad
  * behaviour which would occur if page allocations are causing VM writeback.
  * We really don't want to intermingle reads and writes like that.
  *
  * Returns the number of pages requested, or the maximum amount of I/O allowed.
- *
- * do_page_cache_readahead() returns -1 if it encountered request queue
- * congestion.
  */
 static int
 __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
@@ -210,6 +207,7 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 		return -EINVAL;
 
+	nr_to_read = max_sane_readahead(nr_to_read);
 	while (nr_to_read) {
 		int err;
 
@@ -231,22 +229,6 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 }
 
 /*
- * This version skips the IO if the queue is read-congested, and will tell the
- * block layer to abandon the readahead if request allocation would block.
- *
- * force_page_cache_readahead() will ignore queue congestion and will block on
- * request queues.
- */
-int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
-			pgoff_t offset, unsigned long nr_to_read)
-{
-	if (bdi_read_congested(mapping->backing_dev_info))
-		return -1;
-
-	return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
-}
-
-/*
  * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
  * sensible upper limit.
  */
@@ -259,7 +241,7 @@ unsigned long max_sane_readahead(unsigned long nr)
 /*
  * Submit IO for the read-ahead request in file_ra_state.
  */
-static unsigned long ra_submit(struct file_ra_state *ra,
+unsigned long ra_submit(struct file_ra_state *ra,
 		       struct address_space *mapping, struct file *filp)
 {
 	int actual;
@@ -348,6 +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
@@ -356,34 +391,26 @@ ondemand_readahead(struct address_space *mapping,
 		   bool hit_readahead_marker, pgoff_t offset,
 		   unsigned long req_size)
 {
-	int	max = ra->ra_pages;	/* max readahead pages */
-	pgoff_t prev_offset;
-	int	sequential;
+	unsigned long max = max_sane_readahead(ra->ra_pages);
+
+	/*
+	 * start of file
+	 */
+	if (!offset)
+		goto initial_readahead;
 
 	/*
 	 * It's the expected callback offset, assume sequential access.
 	 * Ramp up sizes, and push forward the readahead window.
 	 */
-	if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
-			offset == (ra->start + ra->size))) {
+	if ((offset == (ra->start + ra->size - ra->async_size) ||
+	     offset == (ra->start + ra->size))) {
 		ra->start += ra->size;
 		ra->size = get_next_ra_size(ra, max);
 		ra->async_size = ra->size;
 		goto readit;
 	}
 
-	prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;
-	sequential = offset - prev_offset <= 1UL || req_size > max;
-
-	/*
-	 * Standalone, small read.
-	 * Read as is, and do not pollute the readahead state.
-	 */
-	if (!hit_readahead_marker && !sequential) {
-		return __do_page_cache_readahead(mapping, filp,
-						offset, req_size, 0);
-	}
-
 	/*
 	 * Hit a marked page without valid readahead state.
 	 * E.g. interleaved reads.
@@ -394,7 +421,7 @@ ondemand_readahead(struct address_space *mapping,
 		pgoff_t start;
 
 		rcu_read_lock();
-		start = radix_tree_next_hole(&mapping->page_tree, offset,max+1);
+		start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
 		rcu_read_unlock();
 
 		if (!start || start - offset > max)
@@ -402,23 +429,53 @@ ondemand_readahead(struct address_space *mapping,
 
 		ra->start = start;
 		ra->size = start - offset;	/* old async_size */
+		ra->size += req_size;
 		ra->size = get_next_ra_size(ra, max);
 		ra->async_size = ra->size;
 		goto readit;
 	}
 
 	/*
-	 * It may be one of
-	 * 	- first read on start of file
-	 * 	- sequential cache miss
-	 * 	- oversize random read
-	 * Start readahead for it.
+	 * oversize read
+	 */
+	if (req_size > max)
+		goto initial_readahead;
+
+	/*
+	 * sequential cache miss
+	 */
+	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
+		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.
 	 */
+	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
+
+initial_readahead:
 	ra->start = offset;
 	ra->size = get_init_ra_size(req_size, max);
 	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 
 readit:
+	/*
+	 * Will this read hit the readahead marker made by itself?
+	 * If so, trigger the readahead marker hit now, and merge
+	 * the resulted next readahead window into the current one.
+	 */
+	if (offset == ra->start && ra->size == ra->async_size) {
+		ra->async_size = get_next_ra_size(ra, max);
+		ra->size += ra->async_size;
+	}
+
 	return ra_submit(ra, mapping, filp);
 }