Re: [PATCH] mm/huge_memory: fix huge zero folio publication race

[Lorenzo Stoakes (ARM)]

On Wed, Jul 29, 2026 at 06:02:41PM -0700, Andrew Morton wrote:
> On Wed, 29 Jul 2026 08:44:12 +0200 "David Hildenbrand (Arm)" <david@xxxxxxxxxx> wrote:
>
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -245,8 +245,11 @@ static bool get_huge_zero_folio(void)
> > >  	}
> > >  	WRITE_ONCE(huge_zero_pfn, folio_pfn(zero_folio));
> > >
> > > -	/* We take additional reference here. It will be put back by shrinker */
> > > -	atomic_set(&huge_zero_refcount, 2);
> > > +	/*
> > > +	 * Publish the folio and PFN before making them available to lockless
> > > +	 * users. Pairs with a successful atomic_inc_not_zero() above.
> > > +	 */
> > > +	atomic_set_release(&huge_zero_refcount, 2);
> > >  	preempt_enable();
> > >  	count_vm_event(THP_ZERO_PAGE_ALLOC);
> > >  	return true;
> >
> > https://lore.kernel.org/r/20260728-fix-refcounted-huge-zero-v1-0-3f261f5447b4@xxxxxxxxxx
>
> Does that actually fix the same thing?

???

This patch was sent after mine and tries to change code that doesn't exist?

If you look at [0] you will see:

+	/* Paired with atomic_inc_not_zero(). +1 for shrinker pin. */
+	atomic_set_release(&huge_zero_refcount, 2);

Which is... doing what this does anyway?

(I don't actually think this patch is correct anyway since the cmpxchg() is
fully-ordered with a try loop and the issue with PFN vs folio publication still
exists.)

BUT it doesn't matter anyway.

The race is fixed and I went into great detail in [0] and [1] explaining why and
how. If somebody can poke holes in that then go ahead, otherwise I'm not sure
why David's response didn't suffice here.

>
> Is so, Xueyuan's Fixes: is 97ae17497e99 ("thp: implement refcounting
> for huge zero page").  Which to believe?
>

Again ???

3b77e8c8cde5 is where the PFN actually started to matter, before that the huge
zero page was compared directly.

The patch proposed here says the PFN is the issue so it already had an incorrect
fixes tag.

Cheers, Lorenzo

[0]:https://lore.kernel.org/all/20260728-fix-refcounted-huge-zero-v1-2-3f261f5447b4@xxxxxxxxxx/
[1]:https://lore.kernel.org/all/ameC4IWCOVBGlXZu@lucifer/