diff -urN 2.4.10pre12/arch/i386/config.in highmem/arch/i386/config.in
--- 2.4.10pre12/arch/i386/config.in	Thu Sep 20 01:43:26 2001
+++ highmem/arch/i386/config.in	Thu Sep 20 06:38:48 2001
@@ -398,6 +398,9 @@
    bool '  Magic SysRq key' CONFIG_MAGIC_SYSRQ
    bool '  Spinlock debugging' CONFIG_DEBUG_SPINLOCK
    bool '  Verbose BUG() reporting (adds 70K)' CONFIG_DEBUG_BUGVERBOSE
+   if [ "$CONFIG_HIGHMEM" = "y" ]; then
+      bool 'Debug HIGHMEM on lowmem machines' CONFIG_HIGHMEM_DEBUG
+   fi
 fi
 
 endmenu
diff -urN 2.4.10pre12/arch/i386/kernel/setup.c highmem/arch/i386/kernel/setup.c
--- 2.4.10pre12/arch/i386/kernel/setup.c	Thu Sep 20 01:43:27 2001
+++ highmem/arch/i386/kernel/setup.c	Thu Sep 20 06:38:07 2001
@@ -821,7 +821,19 @@
  */
 #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)
 
 	/*