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

[Byungchul Park <byungchul@sk.com>]

On Mon, Feb 10, 2025 at 01:00:02AM -0500, Gregory Price wrote:
> 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?

Then we can hold this one until it turns out it's harmful or give up.

>    ex) Are page tables on slow mem actually harmful? - known, yes.

Defenitly yes.  What can be the next?

> 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

I'm really courious about this.  Is there any reason that we should work
these in a serialized manner?

> 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

Same.  Should it be serialized?

> 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.

Ah.  You seem to mean those works should be serialized.  Right?  If it
should be for some reason, then it could be sensible.

	Byungchul

> > 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