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Subject[PATCH 01/13] mm: delayed page activation
When a page is referenced the second time in inactive_list, mark it with
PG_activate instead of moving it into active_list immediately. The actual
moving work is delayed to vmscan time.

This means two essential changes:
- keeps the adjecency of pages in lru;
- lifts the page reference counter max from 1 to 3.

And leads to the following improvements:
- read-ahead for a leading reader will not be disturbed by a following reader;
- enables the thrashing protection logic to save pages for following readers;
- keeping relavant pages together helps improve I/O efficiency;
- and also helps decrease vm fragmantation;
- increased refcnt space might help page replacement algorithms.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
include/linux/page-flags.h | 31 +++++++++++++++++++++++++++++++
mm/page_alloc.c | 1 +
mm/swap.c | 9 ++++-----
mm/vmscan.c | 6 ++++++
4 files changed, 42 insertions(+), 5 deletions(-)
--- linux-2.6.14-rc5-mm1.orig/include/linux/page-flags.h
+++ linux-2.6.14-rc5-mm1/include/linux/page-flags.h
@@ -76,6 +76,7 @@
#define PG_reclaim 17 /* To be reclaimed asap */
#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 */

/*
* Global page accounting. One instance per CPU. Only unsigned longs are
@@ -308,6 +309,12 @@ extern void __mod_page_state(unsigned lo
#define SetPageUncached(page) set_bit(PG_uncached, &(page)->flags)
#define ClearPageUncached(page) clear_bit(PG_uncached, &(page)->flags)

+#define PageActivate(page) test_bit(PG_activate, &(page)->flags)
+#define SetPageActivate(page) set_bit(PG_activate, &(page)->flags)
+#define ClearPageActivate(page) clear_bit(PG_activate, &(page)->flags)
+#define TestClearPageActivate(page) test_and_clear_bit(PG_activate, &(page)->flags)
+#define TestSetPageActivate(page) test_and_set_bit(PG_activate, &(page)->flags)
+
struct page; /* forward declaration */

int test_clear_page_dirty(struct page *page);
@@ -333,4 +340,28 @@ static inline void set_page_writeback(st
#define ClearPageFsMisc(page) clear_bit(PG_fs_misc, &(page)->flags)
#define TestClearPageFsMisc(page) test_and_clear_bit(PG_fs_misc, &(page)->flags)

+#if PG_activate < PG_referenced
+#error unexpected page flags order
+#endif
+
+#define PAGE_REFCNT_0 0
+#define PAGE_REFCNT_1 (1 << PG_referenced)
+#define PAGE_REFCNT_2 (1 << PG_activate)
+#define PAGE_REFCNT_3 ((1 << PG_activate) | (1 << PG_referenced))
+#define PAGE_REFCNT_MASK PAGE_REFCNT_3
+
+/*
+ * STATUS REFERENCE COUNT
+ * __ 0
+ * _R PAGE_REFCNT_1
+ * A_ PAGE_REFCNT_2
+ * AR PAGE_REFCNT_3
+ *
+ * A/R: Active / Referenced
+ */
+static inline unsigned long page_refcnt(struct page *page)
+{
+ return page->flags & PAGE_REFCNT_MASK;
+}
+
#endif /* PAGE_FLAGS_H */
--- linux-2.6.14-rc5-mm1.orig/mm/page_alloc.c
+++ linux-2.6.14-rc5-mm1/mm/page_alloc.c
@@ -488,6 +488,7 @@ static void prep_new_page(struct page *p

page->flags &= ~(1 << PG_uptodate | 1 << PG_error |
1 << PG_referenced | 1 << PG_arch_1 |
+ 1 << PG_activate |
1 << PG_checked | 1 << PG_mappedtodisk);
set_page_private(page, 0);
set_page_refs(page, order);
--- linux-2.6.14-rc5-mm1.orig/mm/swap.c
+++ linux-2.6.14-rc5-mm1/mm/swap.c
@@ -29,7 +29,6 @@
#include <linux/percpu.h>
#include <linux/cpu.h>
#include <linux/notifier.h>
-#include <linux/init.h>

/* How many pages do we try to swap or page in/out together? */
int page_cluster;
@@ -117,13 +116,13 @@ void fastcall activate_page(struct page
* Mark a page as having seen activity.
*
* inactive,unreferenced -> inactive,referenced
- * inactive,referenced -> active,unreferenced
- * active,unreferenced -> active,referenced
+ * inactive,referenced -> activate,unreferenced
+ * activate,unreferenced -> activate,referenced
*/
void fastcall mark_page_accessed(struct page *page)
{
- if (!PageActive(page) && PageReferenced(page) && PageLRU(page)) {
- activate_page(page);
+ if (!PageActivate(page) && PageReferenced(page) && PageLRU(page)) {
+ SetPageActivate(page);
ClearPageReferenced(page);
} else if (!PageReferenced(page)) {
SetPageReferenced(page);
--- linux-2.6.14-rc5-mm1.orig/mm/vmscan.c
+++ linux-2.6.14-rc5-mm1/mm/vmscan.c
@@ -407,6 +407,12 @@ static int shrink_list(struct list_head
if (PageWriteback(page))
goto keep_locked;

+ if (PageActivate(page)) {
+ ClearPageActivate(page);
+ ClearPageReferenced(page);
+ goto activate_locked;
+ }
+
referenced = page_referenced(page, 1, sc->priority <= 0);
/* In active use or really unfreeable? Activate it. */
if (referenced && page_mapping_inuse(page))
--
-
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