[PATCH] stop null ptr deference in __alloc_pages

["Martin J. Bligh" <Martin.Bligh@us.ibm.com>]

Summary: Avoid null ptr defererence in __alloc_pages
This exists in 2.4. and 2.5

Configuration: a NUMA (ia32) system which only has highmem 
on one or more nodes.

Action to create: Try to allocate ZONE_NORMAL memory 
from a node which only has highmem. What we should do
is fall back to another node, looking for ZONE_NORMAL
memory.

In looking at the specified zonelist, we panic because that zonelist
is NULL. The simple patch below avoids the null deference, and 
returns failure. alloc_pages will continue looking through the nodes
until it finds one with some ZONE_NORMAL memory. We actually
panic at the moment a few lines later when we do,
classzone->need_balance = 1; thus dereferencing the pointer.

--- linux-2.4.18-memalloc/mm/page_alloc.c.old	Fri Mar  8 18:21:41 2002
+++ linux-2.4.18-memalloc/mm/page_alloc.c	Fri Mar  8 18:23:27 2002
@@ -317,6 +317,8 @@
 
 	zone = zonelist->zones;
 	classzone = *zone;
+	if (classzone == NULL)
+		return NULL;
 	min = 1UL << order;
 	for (;;) {
 		zone_t *z = *(zone++);