读文件时，出现invalid length提示，求高手指导下。

[chen_q07]

static ssize_t reg_read_file(struct file* file, char __user *user_buf, size_t count, loff_t *ppos)
{    
  ssize_t ret = 0;
  int i;
  /*buf allocation*/
  char *buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
  if (NULL == buf)
    return -ENOMEM;

  /*copy reg to buf*/
  ret += sprintf(buf, "scull registers\n");
  for (i=0; i<ARRAY_SIZE(reg); i++)
    {
      ret += sprintf(buf+ret, "%4d = %8d\n", i, reg[i]);
    }

  if (ret >= PAGE_SIZE - 1)
    {
      ret = PAGE_SIZE - 1;
    }
    /*copy buf to user buf*/
  loff_t pos = *ppos;
  if (pos > ret || count==0)
    {
      kfree(buf);
      return 0;
    }
  if (count > ret - pos)
    count = ret - pos;

  ret = copy_to_user(user_buf, buf+pos, count);/*ret = left count*/
  if (ret == count)
    {
      kfree(buf);
      return -EFAULT;
    }
  count -= ret;
  *ppos = pos + count;
  kfree(buf);
  return count;
  /*free buf*/
}

以上这段是我读文件操作。
当我用cat命令读一个文件内容时，出现以下结果
# cat reg
scull registers
0 = 100
1 = 0
2 = 0
3 = 0
4 = 0
5 = 0
6 = 0
7 = 0
8 = 0
9 = 0
10 = 200
11 = 0
12 = 0
13 = 0
14 = 0
15 = 0
16 = 0
17 = 0
18 = 0
19 = 0
20 = 300
reg: invalid length
为什么会有invalid length呢？