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

[Chris Goldsworthy]

On Tue, Sep 28, 2021 at 02:22:59AM +0300, Georgi Djakov wrote:
> On 9/27/2021 6:51 PM, David Hildenbrand wrote:
> >On 24.09.21 00:54, Chris Goldsworthy wrote:
> >>From: Sudarshan Rajagopalan <quic_sudaraja at quicinc.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 at quicinc.com>
> >>Signed-off-by: Chris Goldsworthy <quic_cgoldswo at quicinc.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;
> >>+    }
> >>+
> >>      return ret;
> >
> >Note: didn't verify if updating max_low_pfn is correct here.
> 
> My understanding is that max_low_pfn defines the low/high memory
> boundary and it should be also updated.
> 
> Thanks,
> Georgi

To build more on Georgi's response, our assumption here after an offline
discussion is that max_low_pfn would not be equal to max_pfn only if there is
high memory - another assumption is that arm64 kernels will not need high memory
due to their large logical mappings. Under these two assumptions, the patch is
correct. Perhaps Catalin can ack or critique this, as he initially set max_pfn =
max_low_pfn in the first arm64 mm initialization code:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c1cc1552616d0f354d040823151e61634e7ad01f