[PATCH 06/13] readahead: call scheme

[Wu Fengguang]

An new page flag PG_readahead is introduced as a look-ahead mark.
The look-ahead mark corresponds to `ahead_start' of the current logic.

The read-ahead logic is called when
        - read reaches a look-ahead mark;
        - read on a non-present page.

And ra_access() is called on every page reference to maintain the cache_hit
counter.

This scheme has the following benefits:
        - makes all stateful/stateless methods happy;
        - eliminates the cache hit problem naturally;
        - lives in harmony with application managed read-aheads via
          fadvise/madvise.

Signed-off-by: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
---

 include/linux/mm.h         |    7 +
 include/linux/page-flags.h |    5 +
 mm/filemap.c               |   66 +++++++++++++++---
 mm/readahead.c             |  162 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 229 insertions(+), 11 deletions(-)

--- linux-2.6.14-rc5-mm1.orig/include/linux/page-flags.h
+++ linux-2.6.14-rc5-mm1/include/linux/page-flags.h
@@ -77,6 +77,7 @@
 #define PG_nosave_free		18	/* Free, should not be written */
 #define PG_uncached		19	/* Page has been mapped as uncached */
 #define PG_activate		20	/* delayed activate */
+#define PG_readahead		21	/* check readahead when reading this page */
 
 /*
  * Global page accounting.  One instance per CPU.  Only unsigned longs are
@@ -315,6 +316,10 @@ extern void __mod_page_state(unsigned lo
 #define TestClearPageActivate(page) test_and_clear_bit(PG_activate, &(page)->flags)
 #define TestSetPageActivate(page) test_and_set_bit(PG_activate, &(page)->flags)
 
+#define PageReadahead(page)	test_bit(PG_readahead, &(page)->flags)
+#define SetPageReadahead(page)	set_bit(PG_readahead, &(page)->flags)
+#define TestClearPageReadahead(page) test_and_clear_bit(PG_readahead, &(page)->flags)
+
 struct page;	/* forward declaration */
 
 int test_clear_page_dirty(struct page *page);
--- linux-2.6.14-rc5-mm1.orig/include/linux/mm.h
+++ linux-2.6.14-rc5-mm1/include/linux/mm.h
@@ -944,6 +944,13 @@ unsigned long  page_cache_readahead(stru
 void handle_ra_miss(struct address_space *mapping, 
 		    struct file_ra_state *ra, pgoff_t offset);
 unsigned long max_sane_readahead(unsigned long nr);
+unsigned long
+page_cache_readahead_adaptive(struct address_space *mapping,
+			struct file_ra_state *ra, struct file *filp,
+			struct page *prev_page, struct page *page,
+			unsigned long first_index,
+			unsigned long index, unsigned long last_index);
+void fastcall ra_access(struct file_ra_state *ra, struct page *page);
 
 /* Do stack extension */
 extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
--- linux-2.6.14-rc5-mm1.orig/mm/filemap.c
+++ linux-2.6.14-rc5-mm1/mm/filemap.c
@@ -724,6 +724,8 @@ grab_cache_page_nowait(struct address_sp
 
 EXPORT_SYMBOL(grab_cache_page_nowait);
 
+extern int readahead_ratio;
+
 /*
  * This is a generic file read routine, and uses the
  * mapping->a_ops->readpage() function for the actual low-level
@@ -751,10 +753,12 @@ void do_generic_mapping_read(struct addr
 	unsigned long prev_index;
 	loff_t isize;
 	struct page *cached_page;
+	struct page *prev_page;
 	int error;
 	struct file_ra_state ra = *_ra;
 
 	cached_page = NULL;
+	prev_page = NULL;
 	index = *ppos >> PAGE_CACHE_SHIFT;
 	next_index = index;
 	prev_index = ra.prev_page;
@@ -783,16 +787,36 @@ void do_generic_mapping_read(struct addr
 		nr = nr - offset;
 
 		cond_resched();
-		if (index == next_index)
+
+		if (readahead_ratio <= 9 && index == next_index)
 			next_index = page_cache_readahead(mapping, &ra, filp,
 					index, last_index - index);
 
 find_page:
 		page = find_get_page(mapping, index);
+		if (readahead_ratio > 9) {
+			if (unlikely(page == NULL)) {
+				page_cache_readahead_adaptive(mapping, &ra,
+						filp, prev_page, NULL,
+						*ppos >> PAGE_CACHE_SHIFT,
+						index, last_index);
+				page = find_get_page(mapping, index);
+			} else if (PageReadahead(page)) {
+				page_cache_readahead_adaptive(mapping, &ra,
+						filp, prev_page, page,
+						*ppos >> PAGE_CACHE_SHIFT,
+						index, last_index);
+			}
+		}
 		if (unlikely(page == NULL)) {
-			handle_ra_miss(mapping, &ra, index);
+			if (readahead_ratio <= 9)
+				handle_ra_miss(mapping, &ra, index);
 			goto no_cached_page;
 		}
+		if (prev_page)
+			page_cache_release(prev_page);
+		prev_page = page;
+		ra_access(&ra, page);
 		if (!PageUptodate(page))
 			goto page_not_up_to_date;
 page_ok:
@@ -808,8 +832,9 @@ page_ok:
 		 * When (part of) the same page is read multiple times
 		 * in succession, only mark it as accessed the first time.
 		 */
-		if (prev_index != index)
+		if (prev_index != index) {
 			mark_page_accessed(page);
+		}
 		prev_index = index;
 
 		/*
@@ -827,7 +852,6 @@ page_ok:
 		index += offset >> PAGE_CACHE_SHIFT;
 		offset &= ~PAGE_CACHE_MASK;
 
-		page_cache_release(page);
 		if (ret == nr && desc->count)
 			continue;
 		goto out;
@@ -839,7 +863,6 @@ page_not_up_to_date:
 		/* Did it get unhashed before we got the lock? */
 		if (!page->mapping) {
 			unlock_page(page);
-			page_cache_release(page);
 			continue;
 		}
 
@@ -864,7 +887,6 @@ readpage:
 					 * invalidate_inode_pages got it
 					 */
 					unlock_page(page);
-					page_cache_release(page);
 					goto find_page;
 				}
 				unlock_page(page);
@@ -885,7 +907,6 @@ readpage:
 		isize = i_size_read(inode);
 		end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
 		if (unlikely(!isize || index > end_index)) {
-			page_cache_release(page);
 			goto out;
 		}
 
@@ -894,7 +915,6 @@ readpage:
 		if (index == end_index) {
 			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
 			if (nr <= offset) {
-				page_cache_release(page);
 				goto out;
 			}
 		}
@@ -904,7 +924,6 @@ readpage:
 readpage_error:
 		/* UHHUH! A synchronous read error occurred. Report it */
 		desc->error = error;
-		page_cache_release(page);
 		goto out;
 
 no_cached_page:
@@ -929,15 +948,22 @@ no_cached_page:
 		}
 		page = cached_page;
 		cached_page = NULL;
+		if (prev_page)
+			page_cache_release(prev_page);
+		prev_page = page;
 		goto readpage;
 	}
 
 out:
 	*_ra = ra;
+	if (readahead_ratio > 9)
+		_ra->prev_page = prev_index;
 
 	*ppos = ((loff_t) index << PAGE_CACHE_SHIFT) + offset;
 	if (cached_page)
 		page_cache_release(cached_page);
+	if (prev_page)
+		page_cache_release(prev_page);
 	if (filp)
 		file_accessed(filp);
 }
@@ -1235,19 +1261,33 @@ retry_all:
 	 *
 	 * For sequential accesses, we use the generic readahead logic.
 	 */
-	if (VM_SequentialReadHint(area))
+	if (readahead_ratio <= 9 && VM_SequentialReadHint(area))
 		page_cache_readahead(mapping, ra, file, pgoff, 1);
 
+
 	/*
 	 * Do we have something in the page cache already?
 	 */
 retry_find:
 	page = find_get_page(mapping, pgoff);
+	if (VM_SequentialReadHint(area) && readahead_ratio > 9) {
+		if (!page) {
+			page_cache_readahead_adaptive(mapping, ra,
+						file, NULL, NULL,
+						pgoff, pgoff, pgoff + 1);
+			page = find_get_page(mapping, pgoff);
+		} else if (PageReadahead(page)) {
+			page_cache_readahead_adaptive(mapping, ra,
+						file, NULL, page,
+						pgoff, pgoff, pgoff + 1);
+		}
+	}
 	if (!page) {
 		unsigned long ra_pages;
 
 		if (VM_SequentialReadHint(area)) {
-			handle_ra_miss(mapping, ra, pgoff);
+			if (readahead_ratio <= 9)
+				handle_ra_miss(mapping, ra, pgoff);
 			goto no_cached_page;
 		}
 		ra->mmap_miss++;
@@ -1284,6 +1324,8 @@ retry_find:
 	if (!did_readaround)
 		ra->mmap_hit++;
 
+	ra_access(ra, page);
+
 	/*
 	 * Ok, found a page in the page cache, now we need to check
 	 * that it's up-to-date.
@@ -1298,6 +1340,8 @@ success:
 	mark_page_accessed(page);
 	if (type)
 		*type = majmin;
+	if (readahead_ratio > 9)
+		ra->prev_page = page->index;
 	return page;
 
 outside_data_content:
--- linux-2.6.14-rc5-mm1.orig/mm/readahead.c
+++ linux-2.6.14-rc5-mm1/mm/readahead.c
@@ -15,6 +15,38 @@
 #include <linux/backing-dev.h>
 #include <linux/pagevec.h>
 
+/* Detailed classification of read-ahead behaviors. */
+#define RA_CLASS_SHIFT 3
+#define RA_CLASS_MASK  ((1 << RA_CLASS_SHIFT) - 1)
+enum ra_class {
+	RA_CLASS_ALL,
+	RA_CLASS_NEWFILE,
+	RA_CLASS_STATE,
+	RA_CLASS_CONTEXT,
+	RA_CLASS_CONTEXT_ACCELERATED,
+	RA_CLASS_BACKWARD,
+	RA_CLASS_RANDOM_THRASHING,
+	RA_CLASS_RANDOM_SEEK,
+	RA_CLASS_END,
+};
+
+/* Read-ahead events to be accounted. */
+enum ra_event {
+	RA_EVENT_CACHE_MISS,		/* read cache misses */
+	RA_EVENT_READRANDOM,		/* random reads */
+
+	RA_EVENT_READAHEAD,		/* read-ahead issued */
+	RA_EVENT_READAHEAD_HIT,		/* read-ahead page hit */
+	RA_EVENT_LOOKAHEAD,		/* look-ahead issued */
+	RA_EVENT_LOOKAHEAD_HIT,		/* look-ahead mark hit */
+	RA_EVENT_READAHEAD_EOF,		/* read-ahead reaches EOF */
+	RA_EVENT_READAHEAD_SHRINK,	/* ra_size decreased, reflects var. */
+	RA_EVENT_READAHEAD_THRASHING,	/* read-ahead thrashing happened */
+	RA_EVENT_READAHEAD_RESCUE,	/* read-ahead rescued */
+
+	RA_EVENT_END
+};
+
 /*
  * Debug facilities.
  */
@@ -708,3 +740,133 @@ out:
 
 	return nr_pages ? index : 0;
 }
+
+/*
+ * This is the entry point of the adaptive read-ahead logic.
+ *
+ * It is only called on two conditions:
+ * 1. page == NULL
+ *    A cache miss happened, it can be either a random read or a sequential one.
+ * 2. page != NULL
+ *    There is a look-ahead mark(PG_readahead) from a previous sequential read.
+ *    It's time to do some checking and submit the next read-ahead IO.
+ *
+ * That has the merits of:
+ * - makes all stateful/stateless methods happy;
+ * - eliminates the cache hit problem naturally;
+ * - lives in harmony with application managed read-aheads via fadvise/madvise.
+ */
+unsigned long
+page_cache_readahead_adaptive(struct address_space *mapping,
+			struct file_ra_state *ra, struct file *filp,
+			struct page *prev_page, struct page *page,
+			unsigned long begin_index,
+			unsigned long index, unsigned long end_index)
+{
+	unsigned long size;
+	unsigned long ra_min;
+	unsigned long ra_max;
+	int ret;
+
+	if (page) {
+		if(!TestClearPageReadahead(page))
+			return 0;
+		if (bdi_read_congested(mapping->backing_dev_info))
+			return 0;
+	}
+
+	if (page)
+		ra_account(ra, RA_EVENT_LOOKAHEAD_HIT,
+				ra->readahead_index - ra->lookahead_index);
+	else if (index)
+		ra_account(ra, RA_EVENT_CACHE_MISS, end_index - begin_index);
+
+	size = end_index - index;
+	get_readahead_bounds(ra, &ra_min, &ra_max);
+
+	/* readahead disabled? */
+	if (unlikely(!ra_min || !readahead_ratio)) {
+		size = max_sane_readahead(size);
+		goto readit;
+	}
+
+	/*
+	 * Start of file.
+	 */
+	if (index == 0)
+		return newfile_readahead(mapping, filp, ra, end_index, ra_min);
+
+	/*
+	 * State based sequential read-ahead.
+	 */
+	if ((readahead_ratio % 5) == 0 &&
+		index == ra->lookahead_index &&
+		(page || index == ra->readahead_index) &&
+		(ra_cache_hit_ok(ra) ||
+		 end_index - begin_index >= ra_max))
+		return state_based_readahead(mapping, filp, ra, page, ra_max);
+
+	/*
+	 * Backward read-ahead.
+	 */
+	if (try_read_backward(ra, begin_index, end_index, size, ra_min, ra_max))
+		return ra_dispatch(ra, mapping, filp);
+
+	/*
+	 * Context based sequential read-ahead.
+	 */
+	ret = try_context_based_readahead(mapping, ra, prev_page, page,
+						index, size, ra_min, ra_max);
+	if (ret > 0)
+		return ra_dispatch(ra, mapping, filp);
+	if (ret < 0)
+		return 0;
+
+	/* No action on look ahead time? */
+	if (page)
+		return 0;
+
+	/*
+	 * Random read that follows a sequential one.
+	 */
+	if (try_random_readahead(ra, index, size, ra_max))
+		return ra_dispatch(ra, mapping, filp);
+
+	/*
+	 * Random read.
+	 */
+	if (size > ra_max)
+		size = ra_max;
+
+readit:
+	size = __do_page_cache_readahead(mapping, filp, index, size, 0);
+
+	ra_account(ra, RA_EVENT_READRANDOM, size);
+	dprintk("readrandom(ino=%lu, pages=%lu, index=%lu-%lu-%lu) = %lu\n",
+			mapping->host->i_ino, mapping->nrpages,
+			begin_index, index, end_index, size);
+
+	return size;
+}
+
+/*
+ * Call me!
+ */
+void fastcall ra_access(struct file_ra_state *ra, struct page *page)
+{
+	if (page->flags & ((1 << PG_active)   |
+			   (1 << PG_activate) |
+			   (1 << PG_referenced)))
+		return;
+
+	if (!ra_has_index(ra, page->index))
+		return;
+
+	ra->cache_hit++;
+
+	if (page->index >= ra->ra_index)
+		ra_account(ra, RA_EVENT_READAHEAD_HIT, 1);
+	else
+		ra_account(ra, RA_EVENT_READAHEAD_HIT, -1);
+}
+

--
-
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