Name: per-cpu only for possible CPUs
Author: Rusty Russell
Status: Tested on 2.5.34 2-way i386

D: This allocates per-cpu areas only for those CPUs which may actually
D: exist, before each one comes online.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .3418-linux-2.5.38/init/main.c .3418-linux-2.5.38.updated/init/main.c
--- .3418-linux-2.5.38/init/main.c	2002-09-21 13:55:19.000000000 +1000
+++ .3418-linux-2.5.38.updated/init/main.c	2002-09-23 10:53:02.000000000 +1000
@@ -306,32 +306,36 @@ static void __init smp_init(void)
 #define smp_init()	do { } while (0)
 #endif
 
-static inline void setup_per_cpu_areas(void) { }
+static inline void setup_per_cpu_area(unsigned int cpu) { }
 static inline void smp_prepare_cpus(unsigned int maxcpus) { }
 
 #else
 
 #ifdef __GENERIC_PER_CPU
+/* Created by linker magic */
+extern char __per_cpu_start[], __per_cpu_end[];
+
 unsigned long __per_cpu_offset[NR_CPUS];
 
-static void __init setup_per_cpu_areas(void)
+/* Sets up per-cpu area for boot CPU. */
+static void __init setup_per_cpu_area(unsigned int cpu)
 {
-	unsigned long size, i;
+	unsigned long size;
 	char *ptr;
-	/* Created by linker magic */
-	extern char __per_cpu_start[], __per_cpu_end[];
 
 	/* Copy section for each CPU (we discard the original) */
 	size = ALIGN(__per_cpu_end - __per_cpu_start, SMP_CACHE_BYTES);
 	if (!size)
 		return;
 
-	ptr = alloc_bootmem(size * NR_CPUS);
+	/* First CPU happens really early... */
+	if (cpu == smp_processor_id())
+		ptr = alloc_bootmem(size);
+	else
+		ptr = kmalloc(size, GFP_ATOMIC);
 
-	for (i = 0; i < NR_CPUS; i++, ptr += size) {
-		__per_cpu_offset[i] = ptr - __per_cpu_start;
-		memcpy(ptr, __per_cpu_start, size);
-	}
+	__per_cpu_offset[cpu] = ptr - __per_cpu_start;
+	memcpy(ptr, __per_cpu_start, size);
 }
 #endif /* !__GENERIC_PER_CPU */
 
@@ -340,7 +344,16 @@ static void __init smp_init(void)
 {
 	unsigned int i;
 
-	/* FIXME: This should be done in userspace --RR */
+	for (i = 0; i < NR_CPUS; i++) {
+		if (cpu_possible(i)) {
+			if (i != smp_processor_id())
+				setup_per_cpu_area(i);
+		} else {
+			/* Force a NULL deref on use */
+			__per_cpu_offset[i] = (char *)0 - __per_cpu_start;
+		}
+	}
+
 	for (i = 0; i < NR_CPUS; i++) {
 		if (num_online_cpus() >= max_cpus)
 			break;
@@ -392,7 +405,7 @@ asmlinkage void __init start_kernel(void
 	lock_kernel();
 	printk(linux_banner);
 	setup_arch(&command_line);
-	setup_per_cpu_areas();
+	setup_per_cpu_area(smp_processor_id());
 	build_all_zonelists();
 	printk("Kernel command line: %s\n", saved_command_line);
 	parse_options(command_line);