Re: [RFC] arm64: mm: update max_pfn after memory hotplug

[David Hildenbrand]

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@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > Signed-off-by: Chris Goldsworthy <quic_cgoldswo@xxxxxxxxxxx>
> > ---
> >   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.

> Will


--
Thanks,

David / dhildenb