On Thu Jan 19, 2023 at 8:22 AM AEST, Nadav Amit wrote:

quoted

On Jan 18, 2023, at 12:00 AM, Nicholas Piggin [off-list ref] wrote:

+static void do_shoot_lazy_tlb(void *arg)
+{
+	struct mm_struct *mm = arg;
+
+ 	if (current->active_mm == mm) {
+ 		WARN_ON_ONCE(current->mm);
+ 		current->active_mm = &init_mm;
+ 		switch_mm(mm, &init_mm, current);
+ 	}
+}

I might be out of touch - doesn’t a flush already take place when we free
the page-tables, at least on common cases on x86?

IIUC exit_mmap() would free page-tables, and whenever page-tables are
freed, on x86, we do shootdown regardless to whether the target CPU TLB state
marks is_lazy. Then, flush_tlb_func() should call switch_mm_irqs_off() and
everything should be fine, no?

[ I understand you care about powerpc, just wondering on the effect on x86 ]

If you can easily piggyback on IPI work you already do in exit_mmap then
that's likely to be preferable. I don't know the details of x86 these
days but there is some discussion about it in last year's thread, it
sounded quite feasible.

This is stil required at final __mmdrop() time because it's still
possible that lazy mm refs will need to be cleaned. exit_mmap() itself
explicitly creates one, so if the __mmdrop() runs on a different CPU,
then there's one. kthreads using the mm could create others. If that
part of it is unclear or under-commented, I can try improve it.

Thanks,
Nick