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

[Will Deacon]

On Wed, Sep 29, 2021 at 12:49:58PM +0200, David Hildenbrand wrote:
> On 29.09.21 12:42, Will Deacon wrote:
> > 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@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.
> > 
> > Hmm. So the readers can see one of the variables updated but the other one
> > stale?
> 
> Yes, just like it has been on x86-64 for a long time:
> 
> arch/x86/mm/init_64.c:update_end_of_memory_vars()
> 
> Not sure if anyone really cares about slightly delayed updates while memory
> is getting hotplugged. The users that I am aware of don't care.

Thanks, I'd missed that x86 also updates max_low_pfn. So at least we're not
worse off in that respect.

Looking at set_max_mapnr(), I'm wondering why we need to call that at all
on arm64 as 'max_mapnr' only seems to be used for nommu.

Will