From: Dave Hansen <haveblue@us.ibm.com>

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.

Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 25-akpm/mm/memory.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletion(-)

diff -puN mm/memory.c~fix-page-count-discrepancy-for-zero-page mm/memory.c
--- 25/mm/memory.c~fix-page-count-discrepancy-for-zero-page	2004-06-28 17:42:54.109988488 -0700
+++ 25-akpm/mm/memory.c	2004-06-28 17:42:54.114987728 -0700
@@ -1089,7 +1089,8 @@ 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)))
_