init and zap low address mappings on demand for cpu hotplug

[Ashok Raj <ashok.raj@intel.com>]

Hi,

to simplyfy cpu hotplug we didnt zap low mem address since we would require
them post boot to bringup a new cpu. This caused bad effects when 
Suresh was testing some new code. More below.

Andrew: Please consider for -mm

-- 
Cheers,
Ashok Raj
- Open Source Technology Center


We need to dynamically map and unmap the low address mappings on demand.
Originally we left these low address mappings since they were required for cpu
hotplug. Due to the following sigthting, we are now doing this 
dynamically on demand just before we bringup a cpu and then zap when the 
secondary boot is complete. We defer the zap during first boot until smpboot 
process is complete.

>From Suresh: 
Identify mapped low address mappings cause corruption when udev process is 
spawned early in boot. Since these low mappings are mapped global, cr3 writes
during context switches to udev process was corrupting memory.

Thanks to suresh for identifying and help comming up with the fix.

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Suresh Siddha <suresh.siddha@intel.com>
---------------------------------------------------
 arch/x86_64/kernel/smpboot.c |   11 ++++++++---
 arch/x86_64/mm/init.c        |   24 +++++++++++++++++++++---
 include/asm-x86_64/smp.h     |    3 ++-
 3 files changed, 31 insertions(+), 7 deletions(-)

Index: linux-2.6.14-rc1-mm1/arch/x86_64/kernel/smpboot.c
===================================================================
--- linux-2.6.14-rc1-mm1.orig/arch/x86_64/kernel/smpboot.c
+++ linux-2.6.14-rc1-mm1/arch/x86_64/kernel/smpboot.c
@@ -748,6 +748,12 @@ do_rest:
 
 	cpu_pda[cpu].pcurrent = c_idle.idle;
 
+	/*
+	 * Re-establish low mappings to facilitate boot.
+	 * zap it back when boot process is complete.
+	 */
+	init_low_mappings();
+
 	start_rip = setup_trampoline();
 
 	init_rsp = c_idle.idle->thread.rsp;
@@ -829,6 +835,7 @@ do_rest:
 #endif
 		}
 	}
+	zap_low_mappings(0);
 	if (boot_error) {
 		cpu_clear(cpu, cpu_callout_map); /* was set here (do_boot_cpu()) */
 		clear_bit(cpu, &cpu_initialized); /* was set by cpu_init() */
@@ -1067,9 +1074,7 @@ int __cpuinit __cpu_up(unsigned int cpu)
  */
 void __init smp_cpus_done(unsigned int max_cpus)
 {
-#ifndef CONFIG_HOTPLUG_CPU
-	zap_low_mappings();
-#endif
+	zap_low_mappings(1);
 	smp_cleanup_boot();
 
 #ifdef CONFIG_X86_IO_APIC
Index: linux-2.6.14-rc1-mm1/arch/x86_64/mm/init.c
===================================================================
--- linux-2.6.14-rc1-mm1.orig/arch/x86_64/mm/init.c
+++ linux-2.6.14-rc1-mm1/arch/x86_64/mm/init.c
@@ -311,11 +311,29 @@ void __init init_memory_mapping(unsigned
 }
 
 extern struct x8664_pda cpu_pda[NR_CPUS];
+__cpuinitdata static int zap_low_first_time = 1;
 
 /* Assumes all CPUs still execute in init_mm */
-void zap_low_mappings(void)
+__cpuinit void init_low_mappings(void)
 {
-	pgd_t *pgd = pgd_offset_k(0UL);
+	if (!zap_low_first_time)
+		set_pgd(pgd_offset_k(0UL), *pgd_offset_k(PAGE_OFFSET));
+}
+
+/*
+ * mode
+ * 	0 indicates its for __cpu_up to kick an AP into boot sequence.
+ *  1 indicates completion os smp boot process, so we can zap the low
+ *    until there is need to bring a cpu up again.
+ */
+__cpuinit void zap_low_mappings(int mode)
+{
+	pgd_t *pgd;
+	if (!mode && zap_low_first_time)
+		return;
+
+	pgd = pgd_offset_k(0UL);
+	zap_low_first_time = 0;
 	pgd_clear(pgd);
 	flush_tlb_all();
 }
@@ -481,7 +499,7 @@ void __init mem_init(void)
 	 * the WP-bit has been tested.
 	 */
 #ifndef CONFIG_SMP
-	zap_low_mappings();
+	zap_low_mappings(1);
 #endif
 }
 
Index: linux-2.6.14-rc1-mm1/include/asm-x86_64/smp.h
===================================================================
--- linux-2.6.14-rc1-mm1.orig/include/asm-x86_64/smp.h
+++ linux-2.6.14-rc1-mm1/include/asm-x86_64/smp.h
@@ -47,7 +47,8 @@ extern void lock_ipi_call_lock(void);
 extern void unlock_ipi_call_lock(void);
 extern int smp_num_siblings;
 extern void smp_send_reschedule(int cpu);
-extern void zap_low_mappings(void);
+extern void zap_low_mappings(int mode);
+extern void init_low_mappings(void);
 void smp_stop_cpu(void);
 extern int smp_call_function_single(int cpuid, void (*func) (void *info),
 				void *info, int retry, int wait);
-
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