On 1/19/23 6:22 AM, Nicholas Piggin wrote:

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

quoted

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 ]

Now I come to think of it, Rik had done this for x86 a while back.

https://lore.kernel.org/all/20180728215357.3249-10-riel@surriel.com/ (local)

I didn't know about it when I wrote this, so I never dug into why it
didn't get merged. It might have missed the final __mmdrop races but
I'm not not sure, x86 lazy tlb mode is too complicated to know at a
glance. I would check with him though.

My point was that naturally (i.e., as done today), when exit_mmap() is 
done, you release the page tables (not just the pages). On x86 it means 
that you also send shootdown IPI to all the *lazy* CPUs to perform a 
flush, so they would exit the lazy mode.

[ this should be true for 99% of the cases, excluding cases where there
   were not page-tables, for instance ]

So the patch of Rik, I think, does not help in the common cases, 
although it may perhaps make implicit actions more explicit in the code.