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 variable high_memory, max_pfn and max_low_pfn.

Memory hotplug still has critical issues for 32-bit kernel, and it is more 
important for 64-bit kernel, we fix it on 64-bit first. We add a function 
update_end_of_memory_vars in file arch/x86/mm/init.c to update these variables.

Signed-off-by: Shaohui Zheng <shaohui.zheng@intel.com>
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>
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index d406c52..b6a85cc 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -1,6 +1,7 @@
 #include <linux/initrd.h>
 #include <linux/ioport.h>
 #include <linux/swap.h>
+#include <linux/bootmem.h>
 
 #include <asm/cacheflush.h>
 #include <asm/e820.h>
@@ -386,3 +387,24 @@ void free_initrd_mem(unsigned long start, unsigned long end)
 	free_init_pages("initrd memory", start, end);
 }
 #endif
+
+/**
+ * After memory hotplug, the variable max_pfn, max_low_pfn and high_memory will
+ * be affected, it will be updated in this function. Memory hotplug still has
+ * critical issues on 32-bit kennel, it was more important on 64-bit kernel,
+ * so we update the variables for 64-bit kernel first, fix me in future for
+ * 32-bit kenrel.
+ */
+void __meminit __attribute__((weak)) update_end_of_memory_vars(u64 start,
+		u64 size)
+{
+#ifdef CONFIG_X86_64
+	unsigned long start_pfn = start >> PAGE_SHIFT;
+	unsigned long end_pfn = PFN_UP(start + size);
+
+	if (end_pfn > max_pfn) {
+		max_low_pfn = max_pfn = end_pfn;
+		high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;
+	}
+#endif /* CONFIG_X86_64 */
+}
diff --git a/include/linux/bootmem.h b/include/linux/bootmem.h
index b10ec49..84533a5 100644
--- a/include/linux/bootmem.h
+++ b/include/linux/bootmem.h
@@ -13,6 +13,7 @@
 
 extern unsigned long max_low_pfn;
 extern unsigned long min_low_pfn;
+extern void update_end_of_memory_vars(u64 start, u64 size);
 
 /*
  * highest page
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 030ce8a..3e94b23 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -523,6 +523,9 @@ int __ref add_memory(int nid, u64 start, u64 size)
 		BUG_ON(ret);
 	}
 
+	/* update max_pfn, max_low_pfn and high_memory */
+	update_end_of_memory_vars(start, size);
+
 	goto out;
 
 error: