Protect critical WSL processes under heavy memory load with cgroup & isolate distro cgroups by chemwolf6922 · Pull Request #40519 · microsoft/WSL

chemwolf6922 · 2026-05-13T05:59:35Z

Summary of the Pull Request

Issue 1:

When under heavy memory load, the critical WSL processes can fail due to various OOM failures. And may end up in an error state after the memory storm ended. Causing "Catastrophic Failures" afterwards.

Issue 2:

All distro's systemd instances shared the same sets of cgroup. This will cause conflicts. For example, when booting multiple wsl distros at the same time. Only one can create the systemd user session successfully.

systemd[1023]: user@1000.service: Failed to attach to cgroup /user.slice/user-1000.slice/user@1000.service: Device or resource busy

Changes

This PR reorganize the wsl cgroup with the follow structure:

# When systemd is enabled
/                                                  # critical wsl processes live in root with no resource limit.
    --wsl-user                                     # resource limited cgroup
        -- non-distro                              # for processes not belonging to a distro, like plugins
        -- distro-<init pid>                       # per distro root
            -- non-systemd                         # for non systemd user processes
            -- systemd                             # for systemd and it's cgroup tree

# When systemd is not enabled
/                                                  # critical wsl processes live in root with no resource limit.
    --wsl-user                                     # resource limited cgroup
        -- non-distro                              # for processes not belonging to a distro, like plugins
        -- distro-<init pid>                       # per distro cgroup

The wsl-user cgroup has memory.max and cpu.max set so it can only take max - 32MiB of RAM and max - 0.05 CPU cores. Effectively reserving 32MiB RAM and 0.05 cores for processes not under this cgroup.

Note

Since this uses cgroupv2. Enabling this will not allow the use of cgroupv1. A .wslconfig option IsolateDistroCgroup (default true) is added so users can opt-out of this and keep using cgorupv1.

PR Checklist

Closes: WSL exits unexpectedly when compiling software. #40458 wsl: Failed to start the systemd user session for '...'. See journalctl for more details #40593
Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
Tests: Added/updated if needed and all pass
Localization: All end user facing strings can be localized
Dev docs: Added/updated if needed
Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Issue 1

Validated by the OP of this issue: #40458 (comment)
The original issue is not deterministic and is hard to repro. Thus, no new test is added.

Issue 2

The test_distro won't repro the issue. No new test is added.
Validated manually with launching Ubuntu and Debian at the same time.

General

Add tests to validate the cgroup isolation works:
UnitTests::UnitTests::IsolatedCgroupLayout
UnitTests::UnitTests::IsolatedCgroupLayoutSystemd
UnitTests::UnitTests::IsolatedCgroupLayoutDisabled

Co-authored-by: Copilot <copilot@github.com>

…es-with-memory-cgroup

chemwolf6922 · 2026-05-13T06:01:23Z

If we want this change. Please help determine the reserved memory size. 128M might be too much.

Copilot

Pull request overview

This PR aims to improve WSL2 reliability under heavy memory pressure by placing user/workload processes into a cgroup v2 with a memory cap, while keeping critical WSL system processes outside that cap to reduce the chance of catastrophic failures after OOM events.

Changes:

Add cgroup path constants for a wsl-user cgroup and its cgroup.procs / memory.max control files.
Create and configure the wsl-user cgroup at mini_init startup, setting memory.max to totalram - 128MB.
Move key workload processes (session leaders, boot command, systemd-spawned workload) into wsl-user by writing 0 to cgroup.procs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/linux/init/util.h	Adds constants for the `wsl-user` cgroup paths.
src/linux/init/util.cpp	Moves create-process children into `wsl-user` cgroup before exec.
src/linux/init/main.cpp	Adds cgroup setup routine and invokes it after mounting cgroup2.
src/linux/init/init.cpp	Moves session leaders and systemd into `wsl-user` cgroup.
src/linux/init/config.cpp	Moves boot command into `wsl-user` cgroup.

Comments suppressed due to low confidence (2)

src/linux/init/main.cpp:3944

Enabling the memory controller in cgroup v2 via /sys/fs/cgroup/cgroup.subtree_control generally requires the parent cgroup to have no internal processes ("no internal process" rule). At this point mini_init is in the root cgroup, and wsl-user is created before enabling the controller, so the write is likely to fail (e.g., EBUSY) and the feature becomes a no-op. Consider creating a dedicated top-level cgroup hierarchy (e.g., move system processes into a sibling cgroup and leave the root empty), and enable the controller before creating/using children so the memory controller is actually available.

    if (UtilMkdir(WSL_USER_CGROUP_PATH, 0755) < 0)
    {
        LOG_ERROR("Failed to create wsl-user cgroup directory {}", errno);
        return;
    }

    if (WriteToFile(CGROUP_MOUNTPOINT "/cgroup.subtree_control", "+memory") < 0)
    {
        LOG_ERROR("Failed to enable memory controller {}", errno);
        return;
    }

src/linux/init/init.cpp:1265

This cgroup move is relied on to ensure session leaders are subject to the memory cap, but the return value from WriteToFile is ignored. If it fails, the session leader will stay in the root cgroup and can still starve system processes. Consider checking the return and logging a warning/error (or propagating failure) so the protection isn't silently skipped.

            "SessionLeader", [ListenSocket = std::move(ListenSocket), &Channel, &Config, Mask = Config.Umask, SocketAddress]() {
                // Move session leader into the memory-limited user cgroup.
                WriteToFile(WSL_USER_CGROUP_PROCS, "0");

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

…es-with-memory-cgroup

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (2)

src/linux/init/init.cpp:1272

This cgroup move is treated as non-critical but logs failures with LOG_ERROR. To avoid noisy/false errors, consider downgrading to LOG_WARNING/LOG_INFO and stating the impact (session leader not placed in wsl-user cgroup).

                // Move session leader into the memory-limited user cgroup.
                if (WriteToFile(WSL_USER_CGROUP_PROCS, "0") != 0)
                {
                    // Non-critical.
                    LOG_ERROR("Failed to move session leader into user cgroup, {}", errno);
                }

src/linux/init/init.cpp:2413

This cgroup move is non-critical but uses LOG_ERROR on failure. Consider logging at warning/info level instead and clarifying that memory protection for systemd (and its subtree) is disabled if the move fails.

            // Move systemd into the memory-limited user cgroup.
            if (WriteToFile(WSL_USER_CGROUP_PROCS, "0") != 0)
            {
                // Non-critical.
                LOG_ERROR("Failed to move systemd to user cgroup {}", errno);
            }

…o user/chemwolf6922/protect-wsl-core-processes-with-memory-cgroup

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Copilot

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 4 comments.

…es-with-memory-cgroup

Copilot

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

+        auto MiniInitDirectChildPidPath = std::filesystem::read_symlink(PROCFS_PATH "/self");
+        pid_t MiniInitDirectChildPid = std::stoul(MiniInitDirectChildPidPath.string());
+


chemwolf6922 and others added 5 commits May 8, 2026 17:06

reserve memory for critical wsl processes with cgroup

da600f6

Co-authored-by: Copilot <copilot@github.com>

Merge branch 'master' into user/chemwolf6922/protect-wsl-core-process…

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…es-with-memory-cgroup

Merge branch 'master' into user/chemwolf6922/protect-wsl-core-process…

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…es-with-memory-cgroup

Merge branch 'master' into user/chemwolf6922/protect-wsl-core-process…

8fca7eb

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move macros together

5c24bca

chemwolf6922 requested a review from a team as a code owner May 13, 2026 05:59

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Comment thread src/linux/init/main.cpp Outdated

Comment thread src/linux/init/util.cpp Outdated

Comment thread src/linux/init/init.cpp Outdated

Comment thread src/linux/init/init.cpp Outdated

Comment thread src/linux/init/config.cpp Outdated

resolve comments

bdaf63a

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…es-with-memory-cgroup

benhillis reviewed May 19, 2026

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Comment thread src/linux/init/main.cpp Outdated

chemwolf6922 added 3 commits May 20, 2026 15:02

isolate distro systemd cgroup namespaces

c474a30

add cpu reservation

230e242

reduce memory reservation to 32MiB

94f22d1

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Merge branch 'master' into user/chemwolf6922/protect-wsl-core-process…

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…es-with-memory-cgroup

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Comment thread src/linux/init/main.cpp Outdated

chemwolf6922 added 4 commits May 21, 2026 10:40

change non critical errors to warnings

8297d18

avoid no internal process constraint

75bcde2

Merge branch 'user/chemwolf6922/isolate-distro-cgroup-namespaces' int…

52ce2db

…o user/chemwolf6922/protect-wsl-core-processes-with-memory-cgroup

Setup per distro cgroup

b69b33d

Copilot AI review requested due to automatic review settings May 21, 2026 06:58

chemwolf6922 changed the title ~~Protect critical WSL processes under heavy memory load with cgroup~~ Protect critical WSL processes under heavy memory load with cgroup & isolate distro cgroups May 21, 2026

Copilot started reviewing on behalf of chemwolf6922 May 21, 2026 07:01 View session

Copilot started reviewing on behalf of chemwolf6922 May 21, 2026 07:17 View session

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Comment thread src/linux/init/util.cpp

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Comment thread src/linux/init/main.cpp Outdated

Comment thread src/linux/init/init.cpp

chemwolf6922 added 2 commits May 21, 2026 16:55

resolve comments

7be1244

also limit non-distro non-critical processes

2a83b25

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add unit tests

8ecee9b

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chemwolf6922 added 2 commits May 21, 2026 17:38

resolve comments

82777da

fix race condition in test & remove redundant check

cd2cfe2

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chemwolf6922 added 2 commits May 22, 2026 12:48

add option to opt-out this feature

4e369d5

format code

4d3c8e8

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resolve comments

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Comment thread test/windows/UnitTests.cpp Outdated

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chemwolf6922 added 2 commits May 22, 2026 13:04

Merge branch 'master' into user/chemwolf6922/protect-wsl-core-process…

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…es-with-memory-cgroup

resolve comments

cf62be4

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resolve comments

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chemwolf6922 added 2 commits May 22, 2026 13:21

revert null change

7b9850c

resolve comments

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Comment thread src/linux/init/main.cpp

Comment on lines +2245 to +2247

auto MiniInitDirectChildPidPath = std::filesystem::read_symlink(PROCFS_PATH "/self");

pid_t MiniInitDirectChildPid = std::stoul(MiniInitDirectChildPidPath.string());

		auto MiniInitDirectChildPidPath = std::filesystem::read_symlink(PROCFS_PATH "/self");
		pid_t MiniInitDirectChildPid = std::stoul(MiniInitDirectChildPidPath.string());

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chemwolf6922 commented May 13, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary of the Pull Request

Issue 1:

Issue 2:

Changes

Note

PR Checklist

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Issue 1

Issue 2

General

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chemwolf6922 commented May 13, 2026

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chemwolf6922 commented May 13, 2026 •

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