Excerpts from Andrew Morton's message of June 8, 2021 9:49 am:

On Sat,  5 Jun 2021 11:42:13 +1000 Nicholas Piggin [off-list ref] wrote:

quoted

Add explicit _lazy_tlb annotated functions for lazy mm refcounting.
This makes lazy mm references more obvious, and allows explicit
refcounting to be removed if it is not used.

...

--- a/kernel/kthread.c
+++ b/kernel/kthread.c

@@ -1314,14 +1314,14 @@ void kthread_use_mm(struct mm_struct *mm)
 	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
 	WARN_ON_ONCE(tsk->mm);
 
+	mmgrab(mm);
+
 	task_lock(tsk);
 	/* Hold off tlb flush IPIs while switching mm's */
 	local_irq_disable();
 	active_mm = tsk->active_mm;
-	if (active_mm != mm) {
-		mmgrab(mm);
+	if (active_mm != mm)
 		tsk->active_mm = mm;
-	}

Looks like a functional change.  What's happening here?

That's kthread_use_mm being clever about the lazy tlb mm. If it happened 
that the kthread had inherited a the lazy tlb mm that happens to be the 
one we want to use here, then we already have a refcount to it via the 
lazy tlb ref.

So then it doesn't have to touch the refcount, but rather just converts
it from the lazy tlb ref to the returned reference. If the lazy tlb mm
doesn't get a reference, we can't do that.

Thanks,
Nick