Oleg Nesterov wrote:

Yes, yes, and I already tried to comment this part. We probably need a
dedicated kernel thread, but I still think (although I am not sure) that
initial change can use workueue. In the likely case system_unbound_wq pool
should have an idle thread, if not - OK, this change won't help in this
case. This is minor.

I imagined a dedicated kernel thread doing something like shown below.
(I don't know about mm->mmap management.)
mm->mmap_zapped corresponds to MMF_MEMDIE.
I think this kernel thread can be used for normal kill(pid, SIGKILL) cases.

----------
bool has_sigkill_task;
wait_queue_head_t kick_mm_zapper;

static void mm_zapper(void *unused)
{
	struct task_struct *g, *p;
	struct mm_struct *mm;

sleep:
	wait_event(kick_remover, has_sigkill_task);
	has_sigkill_task = false;
restart:
	rcu_read_lock();
	for_each_process_thread(g, p) {
		if (likely(!fatal_signal_pending(p)))
			continue;
		task_lock(p);
		mm = p->mm;
		if (mm && mm->mmap && !mm->mmap_zapped && down_read_trylock(&mm->mmap_sem)) {
			atomic_inc(&mm->mm_users);
			task_unlock(p);
			rcu_read_unlock();
			if (mm->mmap && !mm->mmap_zapped)
				zap_page_range(mm->mmap, 0, TASK_SIZE, NULL);
			mm->mmap_zapped = 1;
			up_read(&mm->mmap_sem);
			mmput(mm);
			cond_resched();
			goto restart;
		}
		task_unlock(p);
	}
	rcu_read_unlock();
	goto sleep;
}

kthread_run(mm_zapper, NULL, "mm_zapper");
----------

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