put_page() tries to handle hugepages but fails

[David Gibson <david@gibson.dropbear.id.au>]

Andrew, please apply.

The code of put_page() is misleading, in that it appears to have code
handling PageCompound pages (i.e. hugepages).  However it won't
actually handle them correctly - __page_cache_release() will not work
properly on a compound.  Instead, hugepages should be and are released
with huge_page_release() from mm/hugetlb.c.  This patch removes the
broken PageCompound path from put_page(), replacing it with a
BUG_ON().  This also removes the initialization of page[1].mapping
from compoound pages, which was only ever used in this broken code
path.

Index: working-2.6/mm/swap.c
===================================================================
--- working-2.6.orig/mm/swap.c	2004-04-14 12:22:49.000000000 +1000
+++ working-2.6/mm/swap.c	2004-04-23 18:13:53.932263936 +1000
@@ -38,17 +38,7 @@
 
 void put_page(struct page *page)
 {
-	if (unlikely(PageCompound(page))) {
-		page = (struct page *)page->private;
-		if (put_page_testzero(page)) {
-			if (page[1].mapping) {	/* destructor? */
-				(*(void (*)(struct page *))page[1].mapping)(page);
-			} else {
-				__page_cache_release(page);
-			}
-		}
-		return;
-	}
+	BUG_ON(PageCompound(page));
 	if (!PageReserved(page) && put_page_testzero(page))
 		__page_cache_release(page);
 }
Index: working-2.6/mm/page_alloc.c
===================================================================
--- working-2.6.orig/mm/page_alloc.c	2004-04-13 11:42:42.000000000 +1000
+++ working-2.6/mm/page_alloc.c	2004-04-23 18:18:06.799295248 +1000
@@ -118,7 +118,6 @@
 	int i;
 	int nr_pages = 1 << order;
 
-	page[1].mapping = 0;
 	page[1].index = order;
 	for (i = 0; i < nr_pages; i++) {
 		struct page *p = page + i;




-- 
David Gibson			| For every complex problem there is a
david AT gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson