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

Matthew Wilcox
2025-02-01T14:04:26Z

On Sat, Feb 01, 2025 at 10:29:23PM +0900, Hyeonggon Yoo wrote:
> The Linux kernel supports hot-plugging CXL memory via dax/kmem functionality.
> The hot-plugged memory allows either unmovable kernel allocations
> (ZONE_NORMAL), or restricts them to movable allocations (ZONE_MOVABLE)
> depending on the hot-plug policy.

This all seems like a grand waste of time. Don't do that. Don't allow
kernel allocations from CXL at all. Don't build systems that have
vast quantities of CXL memory (or if you do, expose it as really fast
swap, not as memory).

All of the CXL topics I see this year are "It really hurts performance
when ..." and my reaction is "Yes, I told you it would hurt and you did
it anyway". Just stop doing it. CXL is this decade's Infiniband / ATM
/ (name your favourite misguided dead technology here). You can't stop
other people from doing foolish things, but you don't have to join in.
And we don't have to take stupid patches.