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

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 31, 2023 at 08:42:20PM +0900, Kyungsan Kim wrote:
> Given our experiences/design and industry's viewpoints/inquiries,
> I will prepare a few slides in the session to explain 
>   1. Usecase - user/kernespace memory tiering for near/far placement, memory virtualization between hypervisor/baremetal OS
>   2. Issue - movability(movable/unmovable), allocation(explicit/implicit), migration(intented/unintended)
>   3. HW - topology(direct, switch, fabric), feature(pluggability,error-handling,etc)

I think you'll find everybody else in the room understands these issues
rather better than you do.  This is hardly the first time that we've
talked about CXL, and CXL is not the first time that people have
proposed disaggregated memory, nor heterogenous latency/bandwidth
systems.  All the previous attempts have failed, and I expect this
one to fail too.  Maybe there's something novel that means this time
it really will work, so any slides you do should focus on that.

A more profitable discussion might be:

1. Should we have the page allocator return pages from CXL or should
   CXL memory be allocated another way?
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?
3. Do we continue with the current ZONE_DEVICE model, or do we come up
   with something new?