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

From: David Hildenbrand
Date: Wed Sep 29 2021 - 08:09:43 EST

On 29.09.21 13:03, Will Deacon wrote:
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.

I think max_mapnr is only helpful without SPARSE, I can spot the most prominent consumer being simplistic pfn_valid() implementation.

MEMORY_HOTPLUG on arm64 implies SPARSE. ... and I recall that FLATMEM is no longer possible on arm64. So most probably the arm64 call of set_max_mapnr() can just be dropped.

--
Thanks,

David / dhildenb