Can mm_struct of a process be null?

Hi, I happen to find this code snippet in the kernel source : **/*struct mm_struct *mm=current->mm**;** ** /* some other code */ ** if(mm){ /*<==Why is **this here? */** ** /*read the contents by locking*/** ** } */The/if /checks whether the mm_struct of the current process is NULL or not. That means the mm_struct of a process can become NULL. How can that happen?