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

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

Code:

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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;               
}

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