Understanding Policy Groups: A New Approach to Memory Management in Linux

In a session at the 2026 Linux Storage, Filesystem, Memory Management, and BPF Summit, kernel developer Chris Li introduced a proposed enhancement called policy groups. While the existing control-group (cgroup) subsystem effectively handles resource management, Li argued it falls short for other use cases, such as memory access policies or isolation. Policy groups aim to fill these gaps by providing a more flexible framework. However, a consensus on the design remains elusive. Below, we explore the key questions surrounding this proposal.

What Are Policy Groups and How Do They Differ from Control Groups?

Policy groups are a proposed kernel feature designed to manage memory access policies rather than purely resource allocation. Unlike control groups (cgroups), which primarily enforce limits on CPU, memory, and I/O usage, policy groups focus on defining rules for memory access patterns—such as which processes can access certain memory regions or how memory should be shared. The core distinction is that cgroups are resource-centric (e.g., limiting memory consumption), while policy groups are behavior-centric (e.g., controlling memory access permissions or caching policies). This allows for finer-grained control in scenarios like trusted execution environments or multi-tenant systems where isolation and policy enforcement are critical beyond simple resource caps.

Understanding Policy Groups: A New Approach to Memory Management in Linux

Why Are Policy Groups Needed if Control Groups Already Exist?

Control groups were designed primarily for accounting and limiting resources, but they lack flexibility for more nuanced policies. For example, in a system hosting multiple virtual machines or containers, an administrator may want to ensure that memory from one VM is not inadvertently accessed by another, even if both are within the same cgroup hierarchy. Cgroups cannot enforce such access controls—they only limit how much memory a group can use. Policy groups address this by allowing explicit policy definitions for memory access, sharing, and caching. Additionally, cgroups require significant overhead for tree traversal and rollback, making them less suitable for fine-grained, per-process policy enforcement. Policy groups aim to reduce this complexity while providing targeted controls for memory management.

How Would Policy Groups Work Technically in the Kernel?

Li’s proposal introduces a new policy group object that can be attached to processes or memory regions. Each policy group defines a set of rules—for instance, which memory pages are accessible, whether writing is allowed, or how cache coherence is handled. These rules are enforced at the memory management unit (MMU) level or through page-table modifications, ensuring minimal performance impact. The kernel would maintain a policy group hierarchy similar to cgroups but with a focus on access policies rather than resource limits. For instance, a parent policy group could grant its child groups read-only access to a shared memory segment, while revoking it for others. This differs from cgroup’s resource controllers, which only track usage and enforce limits via throttling or OOM killing.

What Challenges Did Li Highlight in Implementing Policy Groups?

During the summit, Li noted several hurdles. First, integrating policy groups with existing memory management subsystems (like the virtual file system or page cache) without causing regressions is complex. Second, ensuring backward compatibility with cgroups is crucial, as many users rely on them. Third, the interaction between policy groups and security mechanisms (e.g., LSMs) must be carefully designed. For instance, a policy group should not override mandatory access controls. Li also mentioned that achieving consensus on a unified API among kernel developers has been difficult—some prefer extending cgroups, while others advocate a separate, lightweight mechanism. Performance overhead from policy lookup in hot paths was another concern, though Li’s design attempts to mitigate this with caching.

What Were the Reactions from the Community at the Summit?

The session sparked lively debate. Some developers appreciated the flexibility, noting potential use cases in confidential computing (e.g., Intel SGX) and multi-tenant databases. Others worried about fragmentation—two similar but different systems (cgroups and policy groups) could confuse users. A few proposed merging policy groups into cgroups as a new controller type, but Li argued that would bloat cgroups. There was no consensus, but attendees agreed that the problem is real. The discussion concluded with a call for a more detailed design document before the next summit. Li encouraged volunteers to review his prototype patch set and provide feedback, especially on corner cases like memory hotplug or NUMA handling.

What Use Cases Would Benefit Most from Policy Groups?

Policy groups shine in environments requiring strict memory isolation and tailored access rules. For example, in containerized deployments, they could prevent a compromise in one container from reading another’s memory—going beyond cgroup’s memory limit enforcement. In embedded systems, policy groups could allow different firmware components to share memory with fine-grained permissions, reducing attack surface. Another use case is in high-performance computing (HPC), where workloads may require exclusive access to large memory regions with custom caching policies. Additionally, cloud providers could use policy groups to enforce tenant-specific memory encryption policies without heavy virtualization overhead. Li emphasized that policy groups are not a replacement for cgroups but a complementary tool for scenarios where resource control alone is insufficient.

What Are the Next Steps for Policy Groups Development?

Li plans to release an updated patch series based on feedback from the summit. Key areas of focus include: simplifying the API to avoid overlap with cgroups, adding comprehensive test cases, and documenting expected behaviors with NUMA and memory compaction. He also aims to demonstrate performance benchmarks comparing policy groups with existing cgroup-based memory access controls (which are currently nonexistent or ad-hoc). The kernel community expects a revised proposal by the next merge window, with a focus on clarity and minimal invasiveness. If adopted, policy groups could see inclusion in a future kernel version, potentially as an experimental feature behind a config option. Developers interested in contributing can follow the Linux kernel mailing list discussion under the subject “Policy groups v2.”