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

[David Hildenbrand]

On 01.10.21 08:38, Anshuman Khandual wrote:
> On 9/29/21 12:21 AM, 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.
> > Without this patch, debug-related functions that use max_pfn such as
> > get_max_dump_pfn() or read_page_owner() will not work with any page in
> > memory that is hot-added after boot.
> >
> > Fixes: 4ab215061554 ("arm64: Add memory hotplug support")
> > Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@xxxxxxxxxxx>
> > Signed-off-by: Chris Goldsworthy <quic_cgoldswo@xxxxxxxxxxx>
> > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Florian Fainelli <f.fainelli@xxxxxxxxx>
> > Cc: Georgi Djakov <quic_c_gdjako@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;
> > +	}
>
> Do these variables get updated on *all* platforms that support memory
> hotplug via an arch_add_memory() ? If not, dont they all face similar
> issues as well ? Why not just update these in generic hotplug instead
> , after looking into arch_add_memory() return code.

s390x sets them to the possible maximum (based on the direct mapping 
size) in init code.

Other archs like x86-64 have to update other parameters. So I guess it 
just made sense to let the archs handle updating these 2 variables.

--
Thanks,

David / dhildenb