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

[Jeff Moyer]

David Hildenbrand <david@xxxxxxxxxx> writes:

> 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.

>From my memory, the case where you don't want the memmap to land on
*persistent memory* is when the device is small (such as NVDIMM-N), and
you want to reserve as much space as possible for the application data.
This has nothing to do with the speed of access.

-Jeff