Re: [RFC v2 1/1] mm: add source selector to memory.reclaim

["Gregory Price (Meta)" <gourry@gourry.net>]

On 2026-11-02 15:04 +0800, Li Zhe wrote:
> +	Specifying a source value constrains where proactive reclaim applies
> +	pressure. Currently only source=nodes:<nodelist> is supported. For
> +	example::
> +
> +	  echo "1G source=nodes:0-1" > memory.reclaim
> +
> +	This reclaims memory of the target cgroup resident on nodes 0-1.

you buried the fact that you want to allow source=zswap in your cover
letter. I was confused by nodes: was needed here until i re-read.

As Joshua mentioned, i don't think you need the separate cover letter,
and I agree this is far too long for the proposed change.

---

When I asked for testing, I wasn't concerned with the functionality,
I was asking what problem this actually solves. performance? fairness?

Shaping the distribution of memory is neat and clearly works, but why
does userspace want this?

The interface "lacking flexibility" is not a problem in and of itself.

I CAN see why you might want to proactively shape what data is swapped.
Let me give you an example:

---
The global node reclaim interfaces allow us to put pressure on distinct
nodes - which lets us shape which node is swapped from first.

Example: node0-1=DRAM, node2-3:CXL

  echo 1G > node2/reclaim   # swap from CXL
  echo 1G > node3/reclaim   # swap from CXL
  echo 1G > node0/reclaim   # demote to CXL
  echo 1G > node1/reclaim   # demote to CXL

But the memcg.reclaim interface only allows us to pressure the complete
set of cpuset.mems nodes for that memcg

  cat cpuset.mems           # 0-3
  echo 1G > memory.reclaim  # makes progress from any of 0-3

Presently, proactive reclaim iterates nodes in node or fallback order,
meaning it attempts to make progress from the fastest node possible
first - which attempts to demote by default.  If the lower nodes are
full, this demotion will fail and we'll swap directly from the top tier,
which is (presumably) warmer than the bottom tiers.

Enable source-bound reclaim in the memory.reclaim interface so that user
space can have the same control over memcg memory tiers that global
proactive reclaim has.

  echo 2G source=nodes:2-3 > memory.reclaim  # [z]swap from CXL
  echo 2G source=nodes:0-1 > memory.reclaim  # demote to CXL

This lets user proactive reclaim to target the coldest memory first
rather than preferring to reclaim from node0 first and potentially
swap out memory warmer than what is present on a CXL tier.
---

With this, you might be able to demonstrate fewer refaults and less
DRAM (node0) memory consumed by zswap. These are clear benefits.

I would argue that memory.reclaim should operate this way anyway,
and that would could make proactive reclaim do this by default, but
I could see the policy bits in userland being useful as well

Performance may not be the only reason you want this, but you need to
help me understand what you're trying to accomplish.

-- 
~Gregory