Re: [PATCH v6 3/5] lazy tlb: shoot lazies, non-refcounting lazy tlb mm reference handling scheme

From: "Nicholas Piggin" <npiggin@gmail.com>
To: "Nadav Amit" <nadav.amit@gmail.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"linux-arch" <linux-arch@vger.kernel.org>,
	"linux-mm" <linux-mm@kvack.org>, <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v6 3/5] lazy tlb: shoot lazies, non-refcounting lazy tlb mm reference handling scheme
Date: Thu, 19 Jan 2023 10:53:08 +1000	[thread overview]
Message-ID: <CPVR8BY8XFWE.BF91Z0FOZWH8@bobo> (raw)
In-Reply-To: <5F3590B8-3F25-4EFB-BE3A-D32AAAC0B2F4@gmail.com>

On Thu Jan 19, 2023 at 8:22 AM AEST, Nadav Amit wrote:
>
>
> > On Jan 18, 2023, at 12:00 AM, Nicholas Piggin <npiggin@gmail.com> 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