block device driver

[valip]

I'm trying to make a driver which block write operations directly on a block device file( /dev/hdxy).

I tried to start with this code:

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/blkdev.h>


MODULE_DESCRIPTION("test module");
MODULE_AUTHOR("vali");
MODULE_LICENSE("GPL");

struct block_device *bd;
struct file_operations *saved_fop;

ssize_t my_write(struct file *file, const char __user *data, size_t size, loff_t *offset)
{
	printk(KERN_DEBUG "intercept block write\n");

	return saved_fop->write(file, data, size, offset);
}

int test_init(void)
{
	struct file_operations *fop;

	bd = lookup_bdev("/dev/hdb1");
	
	saved_fop = (struct file_operations *)bd->bd_inode->i_fop;
	
	fop = (struct file_operations *)kmalloc(sizeof(*fop), GFP_KERNEL);
	memcpy(fop, saved_fop, sizeof(struct file_operations));
	fop->write = my_write;
	
	bd->bd_inode->i_fop = fop;
	
	printk(KERN_DEBUG "load module\n");
	
	return 0;
}

void test_exit(void)
{
        kfree(bd->bd_inode->i_fop);
	bd->bd_inode->i_fop = saved_fop;
	
	printk(KERN_DEBUG "exit module\n");
}

module_init(test_init);
module_exit(test_exit);

When I run "dd if=/dev/zero bs=512 count=10 of=/dev/hdb1" command I expect to get some "intercept block write" message(s).
But I only get "load module" when the module in loaded.

What is wrong ? Why my_write is not executed ?

Thanks! Any help is appreciated.