Re: Re: RE: FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 05, 2023 at 11:01:21AM +0900, Kyungsan Kim wrote:
> >1. Should we have the page allocator return pages from CXL or should
> >   CXL memory be allocated another way?
> I think yes. Using CXL DRAM as System RAM interface would be the primary use case in real-world application in regards to compatibility.
> So, on the System RAM interface, we think it should be managed by Linux MM subsystem. (Node - Zonelist - buddy page allocator)

I don't think this is the right approach.

> >2. Should there be a way for userspace to indicate that it prefers CXL
> >   memory when it calls mmap(), or should it always be at the discretion
> >   of the kernel?
> I think yes. Both implcit and explict ways are meaningful for users on a different purpose.
> The dynamic performance variation of CXL DRAM is likely bigger than other memory types due to the topology expansion and link negotiation.
> I think it strengthens the needs.

I also disagree with your answer here.

> >3. Do we continue with the current ZONE_DEVICE model, or do we come up
> >   with something new?
> In fact, ZONE_DEVICE was the our first candidate for CXL DRAM.
> But because ZONE_DEVICE is not managed by buddy, we thought it does not fit to provide System RAM interface.

But what you're proposing (separate GFP_EXMEM, ZONE_EXMEM, etc) doesn't
let the buddy allocator satisfy GFP_KERNEL allocations from CXL.  So
what's the point?