Re: [LSF/MM/BPF TOPIC] Restricting or migrating unmovable kernel allocations from slow tier

[Gregory Price <gourry@gourry.net>]

On Mon, Feb 10, 2025 at 11:33:47AM +0900, Harry (Hyeonggon) Yoo wrote:
> 
> Premise: Some ZONE_NORMAL capacity exists on CXL memory
>          due to its large capacity.
>
What you actually need to show to justify increasing the complexity is
(at least - but not limited to)

1) structures you want to migrate are harmful when placed on slow memory

   ex) Is `struct page` on slow mem actually harmful? - no data?
   ex) Are page tables on slow mem actually harmful? - known, yes.

2) The structures cannot be made to take up less space on local tier

   ex) struct page can be shrunk - do that first
   ex) huge-pages can be deployed - do that first

3) the structures take up sufficient space that it matters

   ex) struct page after shrunk might not - do that first
   ex) page tables with multi-sized huge pages may not - do that first

4) Making the structures migratable actually does something useful

   are `struct page` or page tables after #2 and #3 both:

   a) going through hot/cold phases enough to warrant being tiered

   b) hot enough for long enough that migration matters?

   You can probably actually (maybe?) collect data on this today - but
   you still have to contend with #2 and #3.

>
> I don't understand why we shouldn't introduce more kernel movable memory
> if that turns out to be beneficial?
> 

No one is going to stop research you want to do. I'm simply expressing
that I think it's an ill-advised path to take.

~Gregory