@samitouri / QOSAMI-WSL-2 / commits / d9f6ff8e

Add swap support for WSLC virtual machines (separate ephemeral VHD) (#40375)

* Implement swap VHDX for WSLC virtual machines Create a swap VHD sized to the VM's memory during WSLCVirtualMachine::Initialize(). The VHD is created in the user's temp directory under wslc/, attached to the VM, formatted with mkswap, and activated with swapon. On failure, the VHD is detached and deleted via scope_exit. On success, the file is cleaned up in the destructor after the VM exits. Changes: - Add MemoryMb to WSLCSessionInitSettings IDL and plumb through WSLCSessionManager - Add swap VHD creation/attach/format/enable in WSLCVirtualMachine::Initialize() - Add swap VHD cleanup in WSLCVirtualMachine destructor - Consolidate temp folder under wslc/ (crashes subfolder) - Add SwapConfigured test to verify swap is active in the VM * Use dynamic VHD for swap and make test retry-tolerant - Use dynamic VHD instead of fixed for swap (reduces IO on boot) - Make SwapConfigured test use RetryWithTimeout to handle async swapon Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Ben Hillis committed May 1, 2026 at 16:32 UTC d9f6ff8e7a695297fddf6c76ab0ea573b0ec0246

5 files changed +42

src/windows/service/exe/WSLCSessionManager.cpp

@@ -352,6 +352,7 @@ WSLCSessionInitSettings WSLCSessionManagerImpl::CreateSessionSettings(

352 sessionSettings.FeatureFlags = Settings->FeatureFlags;

353 sessionSettings.RootVhdTypeOverride = Settings->RootVhdTypeOverride;

354 sessionSettings.StorageFlags = Settings->StorageFlags;

355 + sessionSettings.SwapSizeMb = Settings->MemoryMb;

356 return sessionSettings;

357 }

358

src/windows/service/inc/wslc.idl

@@ -674,6 +674,7 @@ typedef struct _WSLCSessionInitSettings

674 LPCWSTR StoragePath;

675 WSLCSessionStorageFlags StorageFlags;

676 ULONGLONG MaximumStorageSizeMb;

677 + ULONG SwapSizeMb;

678 ULONG BootTimeoutMs;

679 WSLCNetworkingMode NetworkingMode;

680 WSLCFeatureFlags FeatureFlags;

src/windows/wslcsession/WSLCSession.cpp

+26

@@ -357,6 +357,25 @@ void WSLCSession::ConfigureStorage(const WSLCSessionInitSettings& Settings, PSID

357 // Mount the device to /root.

358 m_virtualMachine->Mount(diskDevice.c_str(), c_containerdStorage, "ext4", "", 0);

359

360 + // Configure swap on a separate ephemeral VHD.

361 + if (Settings.SwapSizeMb > 0)

362 + {

363 + try

364 + {

365 + m_swapVhdPath = storagePath / "swap.vhdx";

366 + DeleteFileW(m_swapVhdPath.c_str()); // Remove stale swap from prior run

367 + wsl::core::filesystem::CreateVhd(m_swapVhdPath.c_str(), static_cast<ULONGLONG>(Settings.SwapSizeMb) * _1MB, UserSid, false, false);

368 +

369 + auto [_, swapDevice] = m_virtualMachine->AttachDisk(m_swapVhdPath.c_str(), false);

370 +

371 + // Fire-and-forget: mkswap + swapon runs asynchronously since swap is best-effort.

372 + auto cmd = std::format("/usr/sbin/mkswap {0} && /usr/sbin/swapon {0}", swapDevice);

373 + ServiceProcessLauncher launcher("/bin/sh", {"/bin/sh", "-c", cmd});

374 + launcher.Launch(*m_virtualMachine);

375 + }

376 + CATCH_LOG()

377 + }

378 +

379 deleteVhdOnFailure.release();

380 }

381

@@ -2418,6 +2437,13 @@ try

2437 m_containerdProcess.reset();

2438 m_virtualMachine.reset();

2439

2440 + // Delete the ephemeral swap VHD now that the VM is gone.

2441 + if (!m_swapVhdPath.empty())

2442 + {

2443 + LOG_IF_WIN32_BOOL_FALSE(DeleteFileW(m_swapVhdPath.c_str()));

2444 + m_swapVhdPath.clear();

2445 + }

2446 +

2447 m_terminated = true;

2448 return S_OK;

2449 }

src/windows/wslcsession/WSLCSession.h

@@ -197,6 +197,7 @@ private:

197 wil::unique_event m_dockerdReadyEvent{wil::EventOptions::ManualReset};

198 std::wstring m_displayName;

199 std::filesystem::path m_storageVhdPath;

200 + std::filesystem::path m_swapVhdPath;

201

202 // N.B. m_lock must be acquired before acquiring m_volumesLock, m_containersLock, or m_networksLock.

203 // These locks protect m_volumes / m_containers without requiring an exclusive m_lock.

test/windows/WSLCTests.cpp

+13

@@ -7417,6 +7417,19 @@ class WSLCTests

7417 VERIFY_ARE_EQUAL(result.Output[1], std::format("{}\n", expectedOrder));

7418 }

7419

7420 + WSLC_TEST_METHOD(SwapConfigured)

7421 + {

7422 + // Swap is configured asynchronously (mkswap + swapon runs fire-and-forget), so retry until it's active.

7423 + wsl::shared::retry::RetryWithTimeout<void>(

7424 + [&]() {

7425 + auto result = ExpectCommandResult(m_defaultSession.get(), {"/usr/sbin/swapon", "--show=NAME,SIZE", "--noheadings"}, 0);

7426 +

7427 + THROW_WIN32_IF(ERROR_RETRY, result.Code != 0 || result.Output.size() < 2 || result.Output[1].find("/dev/") == std::string::npos);

7428 + },

7429 + std::chrono::milliseconds{500},

7430 + std::chrono::seconds{30});

7431 + }

7432 +

7433 WSLC_TEST_METHOD(ContainerAutoRemove)

7434 {

7435 // Test that a container with the Rm flag is automatically deleted on Stop().