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

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Message-ID: <Zk3EoLOvRhSO9ScG@google.com>
Date: Wed, 22 May 2024 10:10:40 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Lance Yang <ioworker0@...il.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 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.