RFC mm/damon: Introduce node_target_mem_bp Quota Goal Metric

This series introduces a new DAMON quota goal metric, node_target_mem_bp, designed for controlling memory migration in heterogeneous memory systems (e.g., DRAM and CXL memory tiering).

v1: https://lore.kernel.org/linux-mm/[email protected]/T/#u

Changes since v1:
=================

- Renamed metric from node_sys_bp to node_target_mem_bp for consistency with existing node-related quota goal metrics (node_mem_used_bp, node_mem_free_bp) as suggested by SJ.

- Fixed the metric calculation:
* Numerator: Now correctly counts only scheme-eligible bytes (regions matching the scheme's access pattern criteria).
* Denominator: Now uses node capacity instead of total system memory.

- Removed the get_goal_metric() ops callback. The implementation now resides in core.c, following the existing pattern for other metrics that have ops-layer dependencies.

- Removed the early-exit optimization patch. As SJ noted, this would introduce a behavioral change for existing users and should be an opt-in feature with a properly designed interface. This can be addressed in a separate follow-up series.

- Removed capacity clamping logic (was tied to early-exit behavior).

Background and Motivation
=========================

A previous patch series [1] added weighted interleave support for DAMON migrate_{hot,cold} actions for vaddr schemes. That approach requires VMA offset information to determine target nodes, which for paddr schemes would require costly rmap walks.

This series takes a different approach for PA-based migration control using basis points (bp) target-state goals instead of weight-based action rates, avoiding the need for rmap walks entirely.

What This Metric Does
=====================

The node_target_mem_bp metric measures:

scheme_eligible_bytes_on_node / node_capacity

expressed in basis points (bp, 1/10000).

"Scheme-eligible bytes" are regions that match the scheme's access pattern criteria (size, nr_accesses, age). This allows users to specify goals like:

"Migrate hot pages until node N contains X% hot memory"

Unlike weight-based approaches that specify ACTION RATES, this metric specifies a TARGET STATE, which naturally prevents oscillation issues that would occur with weight-based PA migration without rmap.

Two-Context Setup for Hot Page Distribution
===========================================

For distributing hot pages between two NUMA nodes (e.g., DRAM node 0 and CXL node 1), two DAMON contexts work together:

    Context 0: monitors node 0, migrate_hot -> node 1
      goal: node_target_mem_bp, nid=0, target=6000
      "Migrate hot pages out when node 0 exceeds 60% hot"

    Context 1: monitors node 1, migrate_hot -> node 0
      goal: node_target_mem_bp, nid=1, target=4000
      "Migrate hot pages out when node 1 exceeds 40% hot"

Each context migrates excess hot pages to the other node. The system converges when both nodes reach their target hot memory ratios.

Complementary to Existing vaddr Migration
=========================================

This series complements rather than replaces the vaddr weighted interleave migration:

vaddr migration (weight-based):
- Per-process control
- Fine-grained interleave patterns via VMA offset
- Deterministic placement based on weights

paddr migration (bp-based, this series):
- System-wide control
- Target-state goals for node capacity management
- No rmap overhead

Patch Organization
==================

1. mm/damon/core: add DAMOS_QUOTA_NODE_TARGET_MEM_BP metric
- Adds new enum value and documentation

2. mm/damon/core: implement NODE_TARGET_MEM_BP metric calculation
- Adds damos_get_node_target_mem_bp() function
- Updates function signatures to pass ctx and scheme through call chain

3. mm/damon/sysfs-schemes: expose NODE_TARGET_MEM_BP metric
- Exposes metric as 'node_target_mem_bp' in sysfs

Status
======

These patches have been compile-tested but have NOT been tested on actual hardware. Feedback on the design and approach is appreciated.

References
==========

[1] mm/damon/vaddr: Allow interleaving in migrate_{hot,cold} actions

    https://lore.kernel.org/linux-mm/[email protected]/

Ravi Jonnalagadda (3):
mm/damon/core: add DAMOS_QUOTA_NODE_TARGET_MEM_BP metric mm/damon/core: implement NODE_TARGET_MEM_BP metric calculation mm/damon/sysfs-schemes: expose NODE_TARGET_MEM_BP metric

 include/linux/damon.h    |  5 +++
 mm/damon/core.c          | 66 +++++++++++++++++++++++++++++++++++-----
 mm/damon/sysfs-schemes.c |  5 +++
 3 files changed, 69 insertions(+), 7 deletions(-)

2.43.0