Fix FEX Build Failure On Non-4K Pagesize Systems

by Alex Johnson 49 views

Ever run into a build failure that just makes you scratch your head? You're trying to compile a package, maybe something cool like fex, and all of a sudden, the build process grinds to a halt with a cryptic error. That's precisely the situation we're diving into today, focusing on a build failure of the fex package when encountered on systems that don't use a 4K pagesize. It’s an interesting snag, especially since fex is an emulator that runs on 4K pagesizes, but the build process itself shouldn't be dictated by it. Let's unpack this, explore why it's happening, and what potential solutions look like.

The Core of the Problem: Page Size Mismatch

The build failure we're discussing centers around fex and its interaction with the system's memory pages. In a nutshell, the fex emulator is designed with the expectation that the underlying operating system uses a 4K pagesize. This is a common configuration, and many systems operate perfectly fine with it. However, when you try to build fex on a system with a different pagesize, like the 16K pagesize encountered in the reported issue on an Asahi Linux host, things go awry. The error message jemalloc: Unsupported system page size is a dead giveaway. This indicates that a component used during the build or testing phase of fex (likely jemalloc, a memory allocator often bundled with software for performance) is hardcoded or configured to expect a 4K pagesize and chokes when it encounters something else. It’s crucial to understand that the build environment shouldn't necessarily mimic the runtime environment for all its constraints. We want to compile fex so it can be used later, potentially in a different environment, not necessarily so that its internal tests pass on the build host itself if those tests have specific hardware or OS assumptions.

This leads to a fundamental question: should the build process of a package be blocked by assumptions about the host system's runtime characteristics, especially when those characteristics are only relevant for the package's execution and not its compilation? In this specific case, the error occurs during the test phase of the build, specifically when FEXCore_Tests_Allocator tries to run. The test is failing because the memory allocator (jemalloc) within fex detects an unsupported pagesize. The user correctly points out that while fex runs on 4K pagesize systems, its compilation should ideally be independent of this requirement, especially for the purpose of creating a distributable package. This is akin to cross-compilation scenarios where you might disable host-specific tests to ensure the compilation succeeds, even if the resulting binary can't be immediately executed on the build machine. The expectation here is that the resulting binary can be transferred to a 4K pagesize system for execution, but the build itself should not fail due to a host system difference. This is a common challenge in package management and build systems like NixOS, where the goal is reproducibility and portability across diverse environments. The NixOS ecosystem aims to abstract away as many host system differences as possible, but sometimes, software dependencies or internal checks can leak these assumptions into the build process, causing unexpected failures like this one. Addressing this requires either modifying the fex build process to be more resilient to different host page sizes during testing or finding a way to configure jemalloc or the build system to ignore or adapt to the host's page size for the purpose of compilation.

Investigating the fex Build Failure

When debugging a build failure like the one encountered with fex on non-4K pagesize systems, the first step is always to scrutinize the logs. In this instance, the log output clearly points to jemalloc as the culprit: <jemalloc>: Unsupported system page size. This message is repeated multiple times, indicating that jemalloc is failing to initialize or operate correctly due to the host system's page size not being the expected 4K. The subsequent terminate called without an active exception and the CMake error about failing to discover tests from the executable (FEXCore_Tests_Allocator) are direct consequences of jemalloc’s failure. The test runner, Catch2, is unable to get information from the FEXCore_Tests_Allocator executable because it crashed during startup due to the jemalloc issue.

Digging deeper, fex is a complex piece of software that aims to emulate x86_64 code on different architectures. To achieve this, it likely relies on various system-level features and libraries, including memory management. jemalloc is a high-performance memory allocator that can be integrated into applications to improve memory allocation speed and reduce fragmentation. It’s plausible that jemalloc itself, or fex’s integration with it, has specific compile-time or run-time checks related to the system's memory page size. These checks might be in place to ensure optimal performance or to avoid subtle bugs that could arise from page size mismatches in certain memory management strategies. However, for the purpose of building the package, especially in a reproducible build environment like NixOS, these checks can become problematic.

The user’s hypothesis that unit tests might need to be skipped on hosts where they cannot be guaranteed to run is a sound one. In NixOS, builds are often performed in sandboxed environments, but these environments still reflect certain aspects of the host system, including the kernel's page size. If the fex build process insists on running its unit tests during the build phase, and these tests fail due to host system characteristics unrelated to the core functionality being compiled, then the entire build will fail. This is a common point of contention in software development: balancing thorough testing with the ability to build software in diverse environments. For a package like fex, which is designed to run on specific hardware configurations, it's understandable that its internal tests might have those same assumptions. However, for the package to be available in a distribution like NixOS, the build should ideally be robust enough to handle variations in the build environment, perhaps by conditionally enabling or disabling tests based on detected host capabilities. The fact that Hydra, NixOS's build farm, cannot reproduce this issue suggests that Hydra's build environments likely use a 4K pagesize, highlighting the specific nature of this problem tied to the user's host system.

Potential Solutions and NixOS Considerations

Resolving the fex build failure on non-4K pagesize systems requires a multi-pronged approach, focusing on how the fex project itself handles different system configurations and how NixOS manages package builds. One primary avenue is to modify the fex build system or its dependencies to be more flexible. This could involve:

  1. Conditional Testing: The most direct solution would be to modify fex’s build scripts (likely within its CMake files or its jemalloc configuration) to detect the host system's page size and conditionally skip the memory allocator tests if the page size is not 4K. This aligns with the user’s suggestion of treating it like a cross-compilation scenario where host-specific tests might be disabled. The tests themselves are valuable for fex's development, but they shouldn't block the compilation of the package for users on different systems.
  2. jemalloc Configuration: Investigate if jemalloc can be configured at build time to operate with different page sizes or to disable the page size check altogether for non-runtime contexts. If fex is building jemalloc from source as part of its build process, there might be specific flags or configuration options that can be passed to jemalloc's build system.
  3. Patching fex Source: If neither of the above is straightforward, a patch could be applied to the fex source code to address the page size check. This would involve identifying the specific code responsible for the check within fex or its jemalloc integration and modifying it to be more permissive during the build.

From a NixOS perspective, there are also strategies to manage such issues:

  1. Nixpkgs Overlays and Patches: Nixpkgs, the repository of Nix packages, allows for overlays and patches. A maintainer or user could submit a patch to nixpkgs that either modifies the fex derivation to apply a fix (as described above) or instructs the build process to skip the problematic tests. This is often the preferred method for ensuring that the fix is maintained within the Nix ecosystem.
  2. Build-time Flags: The Nix expression for fex could be modified to pass specific build-time flags to fex’s build system that might disable the strict page size checks. This requires understanding the configure options available for fex.
  3. Disabling Tests in Nixpkgs: If modifying fex upstream is not feasible or desirable, Nixpkgs can be configured to disable certain tests during the build process. This is a common practice for packages where tests are known to be flaky, environment-dependent, or computationally expensive. The doCheck = false; option in Nix derivations is a way to achieve this, though it sacrifices some level of build-time verification.

The user’s assertion that the build result should not differ based on page size is fundamentally correct for a package intended for distribution. The distinction between building a package and running it is critical. While fex may require a 4K pagesize for optimal or correct execution, its compilation should ideally be decoupled from this requirement. This ensures that users on systems with different kernel configurations can still obtain and use the fex emulator, perhaps by deploying it to a suitable runtime environment. This situation underscores the importance of robust build systems and the need for software to be mindful of its build environment, especially in the context of reproducible and portable package management.

Conclusion: Towards a More Resilient Build

This NixOS build failure with the fex package highlights a common challenge in software development and distribution: ensuring that build processes are resilient to variations in the host system's environment. The issue stems from jemalloc's intolerance for non-4K system pagesizes, which halts the fex build, particularly during its test suite execution. While fex itself may require a 4K pagesize for runtime, its compilation should ideally not be hindered by this. The core principle here is to separate the requirements of the build environment from those of the runtime environment whenever possible.

Potential solutions involve modifying fex to conditionally skip tests on systems with different page sizes, configuring jemalloc for broader compatibility, or patching the source code. Within Nixpkgs, these fixes can be implemented through patches, build-time flags, or by disabling problematic tests altogether. The goal is to make fex buildable on a wider range of systems, enabling its use for more users. This issue also serves as a valuable reminder for developers to consider the diverse environments in which their software might be built and to implement appropriate checks or configurations to accommodate them.

For further insights into build systems and reproducible environments, you might find the following resources helpful: