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

Matthew Wilcox
2025-02-10T15:56:03Z

On Mon, Feb 10, 2025 at 10:47:58AM -0500, Gregory Price wrote:
> I also think someone should actively ask whether `struct page` can be
> hosted on remote memory without performance loss. I may look into this.

Given that it contains a refcount and various flags, some of which
are quite hot, I would expect performance to suffer. It also suffers
contention between different CPUs, so depending on your cache protocol
(can it do cache-to-cche transfers or does it have to be written back
to memory first?) it may perform quite poorly. But this is something
that can be measured.

Of course, the question must be asked whetheer we care. Certainly Intel's
Apache Pass and similar Optane RAM products put the memmap on the 3DXP
because there wasn't enough DRAM to put it there. So the pages are
slower, but they were slower anyway!

What I always wondered was what effect it would have on wear. But
that's not a consideration for DRAM attached via CXL.