Re: [PATCH 1/2] Page migration via Swap V2: Page Eviction

[Christoph Lameter <clameter@engr.sgi.com>]

On Wed, 19 Oct 2005, Magnus Damm wrote:

> I'm trying to figure out if this code works in all cases:
> 
> +               spin_lock_irq(&zone->lru_lock);
> +               list_del(&page->lru);
> +               if (!TestSetPageLRU(page)) {
> +                       if (PageActive(page))
> +                               add_page_to_active_list(zone, page);
> +                       else
> +                               add_page_to_inactive_list(zone, page);
> +                       count++;
> +               }
> +               spin_unlock_irq(&zone->lru_lock);
> 
> Why not use if (TestSetPageLRU(page)) BUG()?

The memory hotplug project has a BUG() there and I cannot find a way that
something else could legitimately set the LRU bit. You are right. Thus 
this fix which also includes the addition of put_page() already included 
in an earlier patch.

Index: linux-2.6.14-rc4-mm1/mm/vmscan.c
===================================================================
--- linux-2.6.14-rc4-mm1.orig/mm/vmscan.c	2005-10-17 16:19:21.000000000 -0700
+++ linux-2.6.14-rc4-mm1/mm/vmscan.c	2005-10-19 13:30:00.000000000 -0700
@@ -886,14 +886,16 @@ int putback_lru_pages(struct list_head *
 
 		spin_lock_irq(&zone->lru_lock);
 		list_del(&page->lru);
-		if (!TestSetPageLRU(page)) {
-			if (PageActive(page))
-				add_page_to_active_list(zone, page);
-			else
-				add_page_to_inactive_list(zone, page);
-			count++;
-		}
+		if (TestSetPageLRU(page))
+			BUG();
+		if (PageActive(page))
+			add_page_to_active_list(zone, page);
+		else
+			add_page_to_inactive_list(zone, page);
+		count++;
 		spin_unlock_irq(&zone->lru_lock);
+		/* Undo the get from isolate_lru_page */
+		put_page(page);
 	}
 	return count;
 }

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