Re: [PATCH RFC 00/11] mm: ASI integration for the page allocator

[Brendan Jackman]

On Thu Mar 13, 2025 at 6:11 PM UTC, Brendan Jackman wrote:
> .:: Patchset overview

Hey all, I have been down the pagetable mines lately trying to figure
out a solution to the page cache issue (the 70% FIO degradatation [0]).
I've got a prototype based on the idea I discussed at LSF/MM/BPF
that's slowly coming together. My hope is that as soon as I can
convincingly claim with a straight face that I know how to solve that
problem, I can transition from <post an RFC every N months then
disappear> mode into being a bit more visible with development
iterations...

[0] https://lore.kernel.org/linux-mm/20250129144320.2675822-1-jackmanb@xxxxxxxxxx/

In the meantime, I am still provisionally planning to make the topic
of this RFC the first [PATCH] series for ASI. Obviously before I can
seriously ask Andrew to merge I'll also need to establish some
consensus on the x86 side, but in the meantime I think we're getting
close enough to start discussing the mm code.

So.. does anyone have a bit of time to look over this and see if the
implementation makes sense? Is the basic idea on the right lines?
Also if there's anything I can do to make that easier (is it worth
rebasing?) let me know.

Also, I guess I should also note my aspirational plan for the next few
months, it goes...

1. Get a convincing PoC working that improves the FIO degradation.

2. Gather it into a fairly messy but at least surveyable branch and push
   that to Github or whatever.

3. Show that to x86 folks and hopefully (!!) get some maintainers to
   give a nod like "yep we want ASI and we're more or less sold that
   the developers know how to make it performant".

4. Turn this [RFC] into a [PATCH]. So start by trying to merge the stuff
   that manages the restricted address space, leaving the logic of actually
   _using_ it for a later series.

5. [Maybe this can be partially paralellised with 4] start a new [PATCH]
   series that starts adding in the x86 stuff to actually switch address
   spaces etc. Basically this means respinning the patches that Boris
   has reviewed in [1]. Since we already have the page_alloc stuff, it
   should be possible to start testing this code end-to-end quickly.

[1] https://lore.kernel.org/linux-mm/20250110-asi-rfc-v2-v2-0-8419288bc805@xxxxxxxxxx/

Anyone have any thoughts on that overall strategy?

Cheers,
Brendan