Re: meminfo returns inaccurate NR_FILE_PAGES

[Christoph Lameter <clameter@sgi.com>]

On Tue, 17 Apr 2007, Ethan Solomita wrote:

> > Fix NR_FILE_PAGES and NR_ANON_PAGES accounting.
> >   
> 
>    I don't think that there's a problem with NR_ANON_PAGES. unmap_and_move(),
> the caller of move_to_new_page(), calls try_to_unmap() which calls
> try_to_unmap_anon() which calls try_to_unmap_one() which calls
> page_remove_rmap() which in turn makes the call to __dec_zone_page_state. i.e.
> the rmap() code is handling NR_ANON_PAGES and NR_FILE_MAPPED pages correctly.

Hmmmm...... Ok I see that NR_ANON_PAGES is decremented. But where does 
NR_ANON_PAGES get incremented for the new zone? Ahh in page_add_anon_rmap. 
So that is fine the same way as NR_FILE_MAPPED.

> It's just the NR_FILE_PAGES which are tied to the mapping's page tree, where
> the problem lies.

Ah. I see.

However, anonymous pages may also have a mapping (swap). So we need to 
check first that it is not an anonymous page and then eventually shift 
the count between zones.

Do you think this is right?


Index: linux-2.6.21-rc6/mm/migrate.c
===================================================================
--- linux-2.6.21-rc6.orig/mm/migrate.c	2007-04-17 17:01:58.000000000 -0700
+++ linux-2.6.21-rc6/mm/migrate.c	2007-04-17 22:08:22.000000000 -0700
@@ -333,6 +333,17 @@ static int migrate_page_move_mapping(str
 	 */
 	__put_page(page);
 
+	/*
+	 * If moved to a different zone then also account
+	 * the page for that zone. Other VM counters will be
+	 * taken care of when we establish references to the
+	 * new page and drop references to the old page.
+	 */
+	if (page_zone(newpage) != page_zone(page) && !PageAnon(page)) {
+		__dec_zone_page_state(page, NR_FILE_PAGES);
+		__inc_zone_page_state(newpage, NR_FILE_PAGES);
+	}
+
 	write_unlock_irq(&mapping->tree_lock);
 
 	return 0;

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