linux內存頁面分配問題 page_alloc, page_address, pa

驅動代碼

dma.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/gfp.h>
#include <linux/mm.h>

MODULE_LICENSE("DUAL BSD/GPL");

static	int __init alloc_pages_init(void);
static	void __exit alloc_pages_exit(void);

struct page *pages = NULL;
int	__init alloc_pages_init(void)
{
	pages = alloc_pages(GFP_KERNEL, 3);
	if(!pages)
		return -ENOMEM;
	else
	{
		printk(KERN_ALERT "alloc_pages Successfully!\n");
		printk(KERN_ALERT "pages=0x%lx\n", (unsigned long)pages);
		printk(KERN_ALERT "size of 'pages'=%ld\n", sizeof pages);
		printk(KERN_ALERT "pages=0x%lx\n", pages);
		printk(KERN_ALERT "mem_map = 0x%lx\n",(unsigned long)mem_map);
		printk(KERN_ALERT "size of 'mem_map'=%ld\n", sizeof mem_map);
		printk(KERN_ALERT "mem_map = 0x%lx\n",mem_map);
		printk(KERN_ALERT "pages - mem_map = 0x%lx\n",(unsigned long)pages - (unsigned long)mem_map);
		printk(KERN_ALERT "pages - mem_map = 0x%lx\n",(unsigned long)(pages - mem_map));
		//the physical address of the head of the pages
		printk(KERN_ALERT "(pages - mem_map) << 12 = 0x%lx\n",((unsigned long)pages - (unsigned long) mem_map) * 4096);
		printk(KERN_ALERT "(pages - mem_map) << 12 = 0x%lx\n",(unsigned long)(pages - mem_map) * 4096);
		//the kernel logic address of the head of the pages
		printk(KERN_ALERT "page_address(pages) = 0x%lx\n", (unsigned long)page_address(pages));
	}
	return 0;
}

void	__exit alloc_pages_exit(void)
{
	if(pages)
	{
		__free_pages(pages, 3);
		printk(KERN_ALERT "__free_pages ok!\n");
	}
	printk(KERN_ALERT "exit\n");
}

module_init(alloc_pages_init);
module_exit(alloc_pages_exit);

Makefile

KERNELDIR=/lib/modules/$(shell uname -r)/build

obj-m = dma.o

modules:
	$(MAKE) -C $(KERNELDIR) M=`pwd` modules
clean:
	rm -rf *~ *.o *.ko *.mod* *.order *.mar* *.sym*

測試腳本

make clean
make


sudo rmmod dma
sudo insmod dma.ko


dmesg

我想學習linux內存頁面分配相關的知識，用到alloc_pages和page_address函數，其中在alloc_pages函數中我遇到了一個問題。
參考地址代碼地址爲：http://hi.baidu.com/armlinuxqt/item/2c1f9e049f9106c775cd3c66
寫一個內核模塊，這個內核模塊在初始化時分配8個頁面大小的內存塊，然後打印出分配到的內存頁面的邏輯地址，進而得到頁框號pfn，最後使用page_address得到物理內存地址。

輸出結果爲：
alloc_pages Successfully!
pages=0xc1192700
size of 'pages'=4
pages=0xc1192700
mem_map = 0xc1000000
size of 'mem_map'=4
mem_map = 0xc1000000
pages - mem_map = 0x192700
pages - mem_map = 0xc938
(pages - mem_map) << 12 = 0x92700000
(pages - mem_map) << 12 = 0xc938000
page_address(pages) = 0xcc938000

我的問題是，究竟應該怎樣才能得到內存頁面的確切地址。