On 29.09.21 12:42, Will Deacon wrote:

On Wed, Sep 29, 2021 at 12:29:32PM +0200, David Hildenbrand wrote:

quoted

On 29.09.21 12:10, Will Deacon wrote:

quoted

On Thu, Sep 23, 2021 at 03:54:48PM -0700, Chris Goldsworthy wrote:

quoted

From: Sudarshan Rajagopalan <redacted>

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 <redacted>
Signed-off-by: Chris Goldsworthy <redacted>
---
   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,

David / dhildenb


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