diff -urN 2.4.3aa/arch/i386/config.in 2.4.3aa-highmemdebug/arch/i386/config.in
--- 2.4.3aa/arch/i386/config.in	Sun Apr  1 11:59:37 2001
+++ 2.4.3aa-highmemdebug/arch/i386/config.in	Sun Apr  1 13:00:01 2001
@@ -369,4 +369,7 @@
 
 #bool 'Debug kmalloc/kfree' CONFIG_DEBUG_MALLOC
 bool 'Magic SysRq key' CONFIG_MAGIC_SYSRQ
+if [ "$CONFIG_HIGHMEM" = "y" ]; then
+   bool 'Debug HIGHMEM on lowmem machines' CONFIG_HIGHMEM_DEBUG
+fi
 endmenu
diff -urN 2.4.3aa/arch/i386/kernel/setup.c 2.4.3aa-highmemdebug/arch/i386/kernel/setup.c
--- 2.4.3aa/arch/i386/kernel/setup.c	Sat Mar 31 15:17:07 2001
+++ 2.4.3aa-highmemdebug/arch/i386/kernel/setup.c	Sun Apr  1 13:00:01 2001
@@ -649,7 +649,19 @@
  */
 #define VMALLOC_RESERVE	(unsigned long)(128 << 20)
 #define MAXMEM		(unsigned long)(-PAGE_OFFSET-VMALLOC_RESERVE)
+#ifdef CONFIG_HIGHMEM_DEBUG
+#define MAXMEM_PFN				\
+({						\
+	int __max_pfn;				\
+	if (max_pfn > PFN_DOWN(MAXMEM))		\
+	     __max_pfn = PFN_DOWN(MAXMEM);	\
+	else					\
+	     __max_pfn = max_pfn / 2;		\
+	__max_pfn;				\
+})
+#else
 #define MAXMEM_PFN	PFN_DOWN(MAXMEM)
+#endif
 #define MAX_NONPAE_PFN	(1 << 20)
 
 	/*