[PATCH] fix page->count discrepancy for zero page

[Dave Hansen]

While writing some analysis tools for memory hot-remove, we came across
a single page which had a ->count that always increased, without bound. 
It ended up always being the zero page, and it was caused by a leaked
reference in some do_wp_page() code that ends up avoiding PG_reserved
pages.

Basically what happens is that page_cache_release()/put_page() ignore
PG_reserved pages, while page_cache_get()/get_page() go ahead and take
the reference.  So, each time there's a COW fault on the zero-page, you
get a leaked page->count increment.

It's pretty rare to have a COW fault on anything that's PG_reserved, in
fact, I can't think of anything else that this applies to other than the
zero page.

In any case, it the bug doesn't cause any real problems, but it is a bit
of an annoyance and is obviously incorrect.  We've been running with
this patch for about 3 months now, and haven't run into any problems
with it.

Attached patch is against 2.6.7 and applies to -mm3 properly.

-- Dave
diff -urp linux-2.6.7/mm/memory.c linux-2.6.7-zpage/mm/memory.c
--- linux-2.6.7/mm/memory.c	Tue Jun 15 22:19:22 2004
+++ linux-2.6.7-zpage/mm/memory.c	Thu Jun 24 12:08:42 2004
@@ -1064,7 +1064,8 @@ static int do_wp_page(struct mm_struct *
 	/*
 	 * Ok, we need to copy. Oh, well..
 	 */
-	page_cache_get(old_page);
+	if (!PageReserved(old_page))
+		page_cache_get(old_page);
 	spin_unlock(&mm->page_table_lock);
 
 	if (unlikely(anon_vma_prepare(vma)))