[PATCH v2 3/3] um: Stop tracking stub's PID via userspace_pid[]

[Johannes Berg]

On Fri, 2025-07-11 at 14:50 +0800, Tiwei Bie wrote:
> From: Tiwei Bie <tiwei.btw at antgroup.com>
> 
> The PID of the stub process can be obtained from current_mm_id().
> There is no need to track it via userspace_pid[]. Stop doing that
> to simplify the code.

So that is really obvious cleanups, and I can go apply them on that
grounds, but I started wondering if we're not separately being
inconsistent here, which perhaps didn't matter due to non-SMP:

>  #define activate_mm activate_mm
>  static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
>  {
> -	/*
> -	 * This is called by fs/exec.c and sys_unshare()
> -	 * when the new ->mm is used for the first time.
> -	 */
> -	__switch_mm(&new->context.id);
>  }

This is now empty, so I wondered if we can just remove it _entirely_.

But the generic version calls switch_mm():
 
>  static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, 
> @@ -28,11 +23,9 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  {
>  	unsigned cpu = smp_processor_id();
>  
> -	if(prev != next){
> +	if (prev != next) {
>  		cpumask_clear_cpu(cpu, mm_cpumask(prev));
>  		cpumask_set_cpu(cpu, mm_cpumask(next));
> -		if(next != &init_mm)
> -			__switch_mm(&next->context.id);
>  	}
>  }

which plays with the CPU mask, but realistically being non-SMP the CPU
mask is never really used?

Certainly removing activate_mm() entirely seems to _work_, but of course
it never does anything since smp_processor_id() eventually is just
macros that expand to "0" (unless preempt debug is enabled). Any
thoughts?

johannes