linux-kernel - Re: [RFC] arm64: mm: update max

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Date:   Wed, 29 Sep 2021 11:42:42 +0100
From:   Will Deacon <will@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Chris Goldsworthy <quic_cgoldswo@...cinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug

On Wed, Sep 29, 2021 at 12:29:32PM +0200, David Hildenbrand wrote:
> On 29.09.21 12:10, Will Deacon wrote:
> > On Thu, Sep 23, 2021 at 03:54:48PM -0700, Chris Goldsworthy wrote:
> > > From: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
> > > 
> > > After new memory blocks have been hotplugged, max_pfn and max_low_pfn
> > > needs updating to reflect on new PFNs being hot added to system.
> > > 
> > > Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
> > > Signed-off-by: Chris Goldsworthy <quic_cgoldswo@...cinc.com>
> > > ---
> > >   arch/arm64/mm/mmu.c | 5 +++++
> > >   1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > index cfd9deb..fd85b51 100644
> > > --- a/arch/arm64/mm/mmu.c
> > > +++ b/arch/arm64/mm/mmu.c
> > > @@ -1499,6 +1499,11 @@ int arch_add_memory(int nid, u64 start, u64 size,
> > >   	if (ret)
> > >   		__remove_pgd_mapping(swapper_pg_dir,
> > >   				     __phys_to_virt(start), size);
> > > +	else {
> > > +		max_pfn = PFN_UP(start + size);
> > > +		max_low_pfn = max_pfn;
> > > +	}
> > 
> > We use 'max_pfn' as part of the argument to set_max_mapnr(). Does that need
> > updating as well?
> > 
> > Do we have sufficient locking to ensure nobody is looking at max_pfn or
> > max_low_pfn while we update them?
> 
> Only the write side is protected by memory hotplug locking. The read side is
> lockless -- just like all of the other pfn_to_online_page() machinery.

Hmm. So the readers can see one of the variables updated but the other one
stale?

Will