linux-kernel - Re: [PATCH 2/2] mm,memory_hotplug: {READ,WRITE}

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Date: Wed, 22 May 2024 19:23:20 +0800
From: Lance Yang <ioworker0@...il.com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: David Hildenbrand <david@...hat.com>, Oscar Salvador <osalvador@...e.de>, 
	Andrew Morton <akpm@...ux-foundation.org>, Mike Rapoport <rppt@...nel.org>, 
	Michal Hocko <mhocko@...e.com>, Anshuman Khandual <anshuman.khandual@....com>, 
	Vlastimil Babka <vbabka@...e.cz>, Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm,memory_hotplug: {READ,WRITE}_ONCE unsynchronized
 zone data

On Wed, May 22, 2024 at 6:10 PM Brendan Jackman <jackmanb@...gle.com> wrote:
>
> 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@...gle.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@...gle.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.

I see, thanks again for clarifying!
Lance