linux/kernel/sched.c: context_switch().

[fabio de francesco <fabio@metanix.org>]

There must be something I am missing in sched.c at context_switch() function 
definition. 

There I can read the following code:

static inline void
context_switch(struct rq *rq, struct task_struct *prev,
               struct task_struct *next)
{
	...
	struct mm_struct *mm;
	mm = next->mm;
	if (likely(!mm)) {
                next->active_mm = oldmm;
                atomic_inc(&oldmm->mm_count);
                enter_lazy_tlb(oldmm, next);
        } else
                switch_mm(oldmm, mm, next);
	...
}

May someone please explain why the mm pointer is likely supposed to be NULL?

Thanks in advance.

fabio