Re: [PATCH 2/2] mm,memory_hotplug: {READ,WRITE}_ONCE unsynchronized zone data

[Brendan Jackman <jackmanb@google.com>]

On Wed, May 22, 2024 at 05:20:08PM +0800, Lance Yang wrote:
> On Wed, May 22, 2024 at 4:38 PM Brendan Jackman <jackmanb@google.com> wrote:
> >
> > Hi Lance, thanks for taking a look.
> >
> > On Wed, May 22, 2024 at 12:25:30PM +0800, Lance Yang wrote:
> > > Hi Brendan,
> > >
> > > On Tue, May 21, 2024 at 8:57 PM Brendan Jackman <jackmanb@google.com> wrote:
> > > > @@ -1077,7 +1081,7 @@ void adjust_present_page_count(struct page *page, struct memory_group *group,
> > > >          */
> > > >         if (early_section(__pfn_to_section(page_to_pfn(page))))
> > > >                 zone->present_early_pages += nr_pages;
> > > > -       zone->present_pages += nr_pages;
> > > > +       WRITE_ONCE(zone->present_pages, zone->present_pages + nr_pages);
> > >
> > > I'm not sure that using the WRITE_ONCE() wrapper would prevent load tearing
> > > on 'zone->present_pages', but it's probably just me overthinking it :)
> >
> > Hmm.. this isn't for load-tearing, it's for store-tearing. I have a
> > feeling I might be missing your pont here though, can you elaborate?
> 
> Sorry, my explanation wasn't clear :(
> 
> I'm a bit confused about whether 'WRITE_ONCE(zone->present_pages,
> zone->present_pages + nr_pages);'
> is equivalent to the following:
> 
> 1 a = zone->present_pages + nr_pages;
> 2 WRITE_ONCE(zone->present_pages, a);
> 
> If so, is there any possibility of load tearing on
> 'zone->present_pages' in line 1?

Ah gotcha, thanks for clarifying. Loads are protected by
mem_hotplug_lock here, so it's fine for them to get split up (because
the value can't change between loads). This is what I was referring to
in the bit of the commit message about not needing READ_ONCE.