copy_to_user() returns the number of bytes left to be copied.
This was a typo from: 
d82ef020cf31 "proc: pagemap: Hold mmap_sem during
page walk".
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
 
                start_vaddr = end;
 
                len = min(count, PM_ENTRY_BYTES * pm.pos);
-               if (copy_to_user(buf, pm.buffer, len) < 0) {
+               if (copy_to_user(buf, pm.buffer, len)) {
                        ret = -EFAULT;
                        goto out_free;
                }