Re: [PATCH 0/3] mm: use memmap_on_memory semantics for dax/kmem

[David Hildenbrand]

On 16.06.23 00:00, Vishal Verma wrote:
> The dax/kmem driver can potentially hot-add large amounts of memory
> originating from CXL memory expanders, or NVDIMMs, or other 'device
> memories'. There is a chance there isn't enough regular system memory
> available to fit ythe memmap for this new memory. It's therefore
> desirable, if all other conditions are met, for the kmem managed memory
> to place its memmap on the newly added memory itself.
>
> Arrange for this by first allowing for a module parameter override for
> the mhp_supports_memmap_on_memory() test using a flag, adjusting the
> only other caller of this interface in dirvers/acpi/acpi_memoryhotplug.c,
> exporting the symbol so it can be called by kmem.c, and finally changing
> the kmem driver to add_memory() in chunks of memory_block_size_bytes().

1) Why is the override a requirement here? Just let the admin configure 
it then then add conditional support for kmem.

2) I recall that there are cases where we don't want the memmap to land 
on slow memory (which online_movable would achieve). Just imagine the 
slow PMEM case. So this might need another configuration knob on the 
kmem side.


I recall some discussions on doing that chunk handling internally (so 
kmem can just perform one add_memory() and we'd split that up internally).

--
Cheers,

David / dhildenb