Memory-Hotplug: Fix the bug on interface /dev/mem for 64-bit kernel

The new added memory can not be access by interface /dev/mem, because we do not
 update the variables high_memory, max_pfn and max_low_pfn.

Add a function update_end_of_memory_vars to update these variables for 64-bit 
kernel.

CC: Andi Kleen <ak@linux.intel.com>
CC: Li Haicheng <haicheng.li@intel.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Shaohui Zheng <shaohui.zheng@intel.com>
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 5198b9b..e1c9202 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -49,6 +49,7 @@
 #include <asm/numa.h>
 #include <asm/cacheflush.h>
 #include <asm/init.h>
+#include <linux/bootmem.h>
 
 static unsigned long dma_reserve __initdata;
 
@@ -616,6 +617,21 @@ void __init paging_init(void)
  */
 #ifdef CONFIG_MEMORY_HOTPLUG
 /*
+ * After memory hotplug, the variables max_pfn, max_low_pfn and high_memory will
+ * be affected, they will be updated in this function.
+ */
+static void  update_end_of_memory_vars(u64 start, u64 size)
+{
+	unsigned long end_pfn = PFN_UP(start + size);
+
+	if (end_pfn > max_pfn) {
+		max_pfn = end_pfn;
+		max_low_pfn = end_pfn;
+		high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
+	}
+}
+
+/*
  * Memory is added always to NORMAL zone. This means you will never get
  * additional DMA/DMA32 memory.
  */
@@ -634,6 +650,9 @@ int arch_add_memory(int nid, u64 start, u64 size)
 	ret = __add_pages(nid, zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
 
+	/* update max_pfn, max_low_pfn and high_memory */
+	update_end_of_memory_vars(start, size);
+
 	return ret;
 }
 EXPORT_SYMBOL_GPL(arch_add_memory);