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

Matthew Wilcox
2025-02-01T18:49:02Z

On Sat, Feb 01, 2025 at 11:30:24AM -0500, Gregory Price wrote:
> Rather than ask whether we can make portions of the kernel more ammenable
> to movable allocations, I think it's more beneficial to focus on whether
> we can reduce the ZONE_NORMAL cost of ZONE_MOVABLE capacity. That seems
> (to me) like the actual crux of this particular issue.

We can! This is actually the topic of the talk I'm giving at FOSDEM in
about 15 hours time.

https://fosdem.org/2025/schedule/event/fosdem-2025-4860-shrinking-memmap/

I'm just going to run through my slides and upload them in an hour or so.

My motivation isn't CXL related, but it's the sign of a good project
that it solves some unrelated problems. Short version: we can halve the
cost this year, halve it again in 2026 with a fairly managable amount
of work, and maybe halve it a third time (for a total reduction of 7/8)
with a lot more work in 2027.

Further reductions beyond that are possible, but will need a lot more
work. Some of that work we want to do anyway, regardless of whether
the reduction in overhead from 16MB/GB to 2MB/GB is sufficient.

... or we'll discover the performance effect is negative and shelve the
reduction in memmap size, having only accomplished a massive cleanup of
kernel data structures. Which would be sad.