Mastering Memory Management with Policy Groups: A Practical Guide

Overview

Memory management in Linux has long relied on control groups (cgroups) to allocate and limit resources. While cgroups excel at resource accounting and enforcement, they fall short in flexibility—especially for complex workloads that require fine-grained memory policies. At the 2026 Linux Storage, Filesystem, Memory Management, and BPF Summit, Chris Li introduced policy groups as a proposed enhancement to address these gaps. This guide will walk you through the concept, prerequisites, a step-by-step implementation (using conceptual examples), common pitfalls, and a summary of where the project stands. By the end, you'll understand how policy groups can offer a more dynamic and policy-driven approach to memory management compared to traditional cgroups.

Mastering Memory Management with Policy Groups: A Practical Guide

Prerequisites

Before diving into policy groups, ensure you have the following:

Linux kernel development knowledge: Familiarity with memory management subsystems (slab, page allocator, etc.).
Understanding of cgroups (v1 and v2): Know how CPU, memory, and I/O controllers work.
System with a recent kernel: Policy groups are still experimental in upcoming kernels (e.g., 6.12+).
Access to kernel source and build tools: For patching and testing prototypes.
BPF basics: Some features may leverage BPF programs for policy attachment.

Step-by-Step Guide to Implementing Policy Groups

1. Understand the Motivation Behind Policy Groups

Traditional cgroups enforce hard limits and proportional weights. However, they lack the ability to define intent—for example, “prefer to keep database cache rather than filesystem metadata in memory.” Policy groups aim to fill this void by allowing administrators to apply high-level policies to groups of processes, such as:

Prioritizing certain page types (anonymous pages, page cache, slab)
Setting reclaim preferences (e.g., try to reclaim file-backed pages before anonymous pages)
Enforcing memory tiering (e.g., keep hot data in DRAM, cold data in persistent memory)

Chris Li’s presentation at the summit highlighted that these use cases are poorly served by cgroups alone. Policy groups act as a layer above cgroups, providing a more expressive way to control memory behavior.

2. Set Up Your Environment

Since policy groups are not yet merged into mainline, you'll need a development kernel with the patchset applied. As of 2026, patches are available from Chris Li’s Git tree. Clone and build:

git clone https://git.kernel.org/pub/scm/linux/kernel/git/li/policy-groups.git
cd policy-groups
git checkout v2026-summit
make defconfig
make -j$(nproc) modules_prepare
make -j$(nproc) bzImage modules
sudo make modules_install
sudo make install

Reboot into the new kernel.

3. Enable Policy Groups in Kernel Configuration

During make menuconfig, enable:

CONFIG_POLICY_GROUPS=y under “Memory Management” section.
Optionally CONFIG_POLICY_GROUPS_DEBUG=y for debugging.

4. Create a Policy Group

Policy groups are managed via /sys/fs/policy/ (similar to cgroup hierarchy). First mount the policy filesystem:

sudo mount -t policy none /sys/fs/policy

Then create a group for your database workload:

sudo mkdir /sys/fs/policy/db-group

5. Define Policy Rules

Each policy group exposes files to set rules. For example, to prioritize anonymous pages over page cache for reclaim:

echo "anon_first" > /sys/fs/policy/db-group/reclaim_order
echo "500" > /sys/fs/policy/db-group/swap_weight   # higher = more swap willingness

Other possible attributes:

memory.tier: set preferred memory tier (e.g., DRAM, PMEM)
page.cache.weight: influence page cache eviction
slab.priority: higher values protect slab cache from reclaim

6. Assign Processes to the Group

Similar to cgroups, write PID to the tasks file:

echo 1234 > /sys/fs/policy/db-group/tasks

Policies take effect immediately on the next memory allocation or reclaim for those processes.

7. Validate Behavior

Use cat /sys/fs/policy/db-group/stat to view current counters (e.g., pages reclaimed from each type). Also monitor via perf stat -e memory:policy_reclaim_anon if using the debug option.

8. Integrate with BPF (Advanced)

Policy groups can be extended with BPF programs attached to policy hooks. For example, to dynamically adjust reclaim order based on system load:

#include 
char LICENSE[] SEC("license") = "GPL";

SEC("policy/reclaim_order")
int adjust(struct reclaim_ctx *ctx) {
    if (ctx->nr_unexpected_reclaims > 1000)
        return POLICY_RECLAIM_FILE_FIRST;  // enforce file-first reclaim
    return POLICY_DECISION_DEFAULT;
}

Load the BPF program and attach it to the policy group:

bpftool prog load my_reclaim_order.o /sys/fs/bpf/reclaim
bpftool cgroup attach /sys/fs/policy/db-group policy/reclaim_order pinned /sys/fs/bpf/reclaim

Common Mistakes and Pitfalls

Confusing policy groups with cgroups: Policy groups are an extension, not a replacement. Make sure cgroups are already configured for resource limits; policy groups add behavioral policies.
Ignoring namespace visibility: Policy groups are currently global; processes can only belong to one policy group at a time. Overlapping assignments may cause undefined behavior.
Not testing on an isolated workload: Memory policies can have system-wide ripple effects. Always test in a controlled environment.
Missing kernel config: Forgetting to enable CONFIG_POLICY_GROUPS leads to missing sysfs entries. Double-check menuconfig.
Assuming immediate consensus: As of the 2026 summit, the community hasn't reached consensus on the design. Expect APIs to change; do not rely on the current interface in production.

Summary

Policy groups represent a promising evolution in Linux memory management, enabling intent-based policies beyond the hard limits of cgroups. This guide provided a practical walkthrough—from motivation and kernel setup to creating groups, defining rules, assigning processes, and even integrating BPF. The key takeaway: policy groups fill a critical gap for modern workloads that need fine-grained memory behaviors. However, as the feature is still under active discussion (per Chris Li's presentation), stay tuned to kernel mailing lists for updates. With careful application, you can already experiment with this technology and shape its future.

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