Read data of a page frame (linux) make freeze the system

[hahai]

Hello,

I'm writing a linux driver that reading the data of a page frame of an process. But when I use it, it make immediately freeze the system. Can you help me? Thank for reading my question!

system: Ubuntu 9.04, kernel 2.6.28.15, Intel Duo

static int read_addr(int pid, unsigned long linear_addr, unsigned int n_bytes, char* buff){
	/* 
	pid: id of process
	linear_addr: linear address of memory region to be read
	n_bytes: nombre bytes to be read
	buff: the buffer containing the result
	*/
	
	struct task_struct *task;
   	struct mm_struct *mm = NULL;

    pgd_t *pgd;
    pmd_t *pmd;

    pte_t *pte;

    unsigned long pteval;

	int ret = 0;    

	for_each_process(task) {
		if ( task->pid == pid) {
		    mm = task->mm;
		}
	}
	if(mm == NULL)
		return 1;


    spin_lock(&mm->page_table_lock);

    
    pgd = pgd_offset(mm, linear_addr);

    if (pgd_none(*pgd)) { ret = 2; goto out; }
    
    pmd = pmd_offset(pgd, linear_addr);

    if (pmd_none(*pmd)) { ret = 3; goto out; }
    
    pte = pte_offset_map(pmd, linear_addr);
	
    if (pte_present(*pte)){
		unsigned long pteid = pte_index(linear_addr);
		pteval = pte_val(*pte);
		memcpy(buff, pteval + pteid, n_bytes);     
    } else {

		ret = 4;
		goto out;
    }

    pte_unmap(pte);
    spin_unlock(&mm->page_table_lock);
    return 0;


 out:
 	printk("error: %d\n", ret);

    spin_unlock(&mm->page_table_lock);
	return ret;		
}

[itCbitC]

Narrow down the snippet of code where it is hanging and go from there.