Page LRU State as Scalar

RFC New Daniel Phillips 1 participant 1 message Jul 6, 2001

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Memory management MM core Page allocator MM swap

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Daniel Phillips

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2001-07-06 17:20 UTC #0 lore raw

RFC 1/1 Page LRU State as Scalar

This patch combines the three lru state bits in page->flags into a single scalar field of two bits. In this case, setting the scalar value never costs more than setting a single bit does because each change of state goes through an intermediate step of clearing the state bits to zero, so a single or instruction suffices.

There is in fact a slight improvement in efficiency because switch statements can be substituted for chained conditionals, and if there is room for more if gcc is improved to generate better code for switches (it overlooks the fact that the switch expression is two bits wide and generates a redundant test). Testing the state costs an extra masking instruction, but this is done only on debug paths. The long series of debug checks in free_pages_ok is shortened by two. All in all, this should be a wash in terms of efficiency.

The state is always tested and set under protection of the pagemap_lru_lock, so there should not be any new races.

The motivation for this patch is to provide some infrastructure for memory management experiments using more than three lru lists, along the lines of what we have been discussing recently in the VM Requirement Document thread.

To apply:

cd /your/2.4.6-sourcetree
patch <thispatch -p0

include/linux/mm.h renamed from uml.2.4.6.clean/include/linux/mm.h +21 ~~-15~~

@@ -274,15 +274,14 @@
 #define PG_uptodate		 3
 #define PG_dirty		 4
 #define PG_decr_after		 5
-#define PG_active		 6
-#define PG_inactive_dirty	 7
 #define PG_slab			 8
 #define PG_swap_cache		 9
 #define PG_skip			10
-#define PG_inactive_clean	11
 #define PG_highmem		12
 #define PG_checked		13	/* kill me in 2.5.<early>. */
-				/* bits 21-29 unused */
+				/* bits 6, 7, 11 and 21-29 unused */
+#define PG_state_shift		14
+#define PG_state_mask		(3 << PG_state_shift)
 #define PG_arch_1		30
 #define PG_reserved		31

@@ -344,17 +343,24 @@
 
 #define PageTestandClearSwapCache(page)	test_and_clear_bit(PG_swap_cache, &(page)->flags)
 
-#define PageActive(page)	test_bit(PG_active, &(page)->flags)
-#define SetPageActive(page)	set_bit(PG_active, &(page)->flags)
-#define ClearPageActive(page)	clear_bit(PG_active, &(page)->flags)
-
-#define PageInactiveDirty(page)	test_bit(PG_inactive_dirty, &(page)->flags)
-#define SetPageInactiveDirty(page)	set_bit(PG_inactive_dirty, &(page)->flags)
-#define ClearPageInactiveDirty(page)	clear_bit(PG_inactive_dirty, &(page)->flags)
-
-#define PageInactiveClean(page)	test_bit(PG_inactive_clean, &(page)->flags)
-#define SetPageInactiveClean(page)	set_bit(PG_inactive_clean, &(page)->flags)
-#define ClearPageInactiveClean(page)	clear_bit(PG_inactive_clean, &(page)->flags)
+#define PG_Nolist 0
+#define PG_Active 1
+#define PG_InactiveClean 2
+#define PG_InactiveDirty 3
+
+#define PageNolist(page)	(((page)->flags & PG_state_mask) == PG_Nolist << PG_state_shift)
+#define PageActive(page)	(((page)->flags & PG_state_mask) == PG_Active << PG_state_shift)
+#define PageInactiveClean(page)	(((page)->flags & PG_state_mask) == PG_InactiveClean << PG_state_shift)
+#define PageInactiveDirty(page)	(((page)->flags & PG_state_mask) == PG_InactiveDirty << PG_state_shift)
+
+#define SetPageNolist(page)	   ((page)->flags &= ~PG_state_mask)
+#define SetPageActive(page)	   ((page)->flags |= PG_Active << PG_state_shift)
+#define SetPageInactiveClean(page) ((page)->flags |= PG_InactiveClean << PG_state_shift)
+#define SetPageInactiveDirty(page) ((page)->flags |= PG_InactiveDirty << PG_state_shift)
+
+#define ClearPageActive SetPageNolist
+#define ClearPageInactiveClean SetPageNolist
+#define ClearPageInactiveDirty SetPageNolist
 
 #ifdef CONFIG_HIGHMEM
 #define PageHighMem(page)		test_bit(PG_highmem, &(page)->flags)

include/linux/swap.h renamed from uml.2.4.6.clean/include/linux/swap.h +1 -2

@@ -182,8 +182,7 @@
  * with the pagemap_lru_lock held!
  */
 #define DEBUG_ADD_PAGE \
-	if (PageActive(page) || PageInactiveDirty(page) || \
-					PageInactiveClean(page)) BUG();
+	if (!PageNolist(page)) BUG();
 
 #define ZERO_PAGE_BUG \
 	if (page_count(page) == 0) BUG();

mm/page_alloc.c renamed from uml.2.4.6.clean/mm/page_alloc.c +1 -5

@@ -81,11 +81,7 @@
 		BUG();
 	if (PageDecrAfter(page))
 		BUG();
-	if (PageActive(page))
-		BUG();
-	if (PageInactiveDirty(page))
-		BUG();
-	if (PageInactiveClean(page))
+	if (!PageNolist(page))
 		BUG();
 
 	page->flags &= ~((1<<PG_referenced) | (1<<PG_dirty));

mm/swap.c renamed from uml.2.4.6.clean/mm/swap.c +18 -9

@@ -197,13 +197,17 @@
  */
 void activate_page_nolock(struct page * page)
 {
-	if (PageInactiveDirty(page)) {
+	switch ((page->flags & PG_state_mask) >> PG_state_shift)
+	{
+	case PG_InactiveDirty:
 		del_page_from_inactive_dirty_list(page);
 		add_page_to_active_list(page);
-	} else if (PageInactiveClean(page)) {
+		break;
+	case PG_InactiveClean:
 		del_page_from_inactive_clean_list(page);
 		add_page_to_active_list(page);
-	} else {
+		break;
+	default:
 		/*
 		 * The page was not on any list, so we take care
 		 * not to do anything.

@@ -248,16 +252,21 @@
  */
 void __lru_cache_del(struct page * page)
 {
-	if (PageActive(page)) {
+	switch ((page->flags & PG_state_mask) >> PG_state_shift)
+	{
+	case PG_Active:
 		del_page_from_active_list(page);
-	} else if (PageInactiveDirty(page)) {
-		del_page_from_inactive_dirty_list(page);
-	} else if (PageInactiveClean(page)) {
+		break;
+	case PG_InactiveClean:
 		del_page_from_inactive_clean_list(page);
-	} else {
+		break;
+	case PG_InactiveDirty:
+		del_page_from_inactive_dirty_list(page);
+		break;
+	default:
 		printk("VM: __lru_cache_del, found unknown page ?!\n");
+		DEBUG_ADD_PAGE
 	}
-	DEBUG_ADD_PAGE
 }
 
 /**