kernel | planet

January 20, 2025

Linux 6.13, I WANT A GUITAR PEDAL

Just as 2025 is starting, we got a new Linux release in mid January, tagged as 6.13. In the spirit of holidays, Linus Torvalds even announced during 6.13-rc6 that he would be building and raffling a guitar pedal for a random kernel developer!

As usual, this release comes with a pack of exciting news done by the kernel community:

  • This release has two important improvements for task scheduling: lazy preemption and proxy execution. The goal with lazy preemption is to find a better balance between throughput and response time. A secondary goal is being able to make it the preferred non-realtime scheduling policy for most cases. Tasks that really need a reschedule in a hurry will use the older TIF_NEED_RESCHED flag. A preliminary work for proxy execution was merged, which will let us avoid priority-inversion scenarios when using real time tasks with deadline scheduling, for use cases such as Android.

  • New important Rust abstractions arrived, such as VFS data structures and interfaces, and also abstractions for misc devices.

  • Lightweight guard pages: guard pages are used to raise a fatal signal when accessed. This feature had the drawback of having a heavy performance impact, but in this new release the flag MADV_GUARD_INSTALL was added for the madvise() syscall, offering a lightweight way to guard pages.

To know more about the community improvements, check out the summary made by Kernel Newbies.

Now let’s highlight the contributions made by Igalians for this release.

Case-insensitive support for tmpfs

Case sensitivity has been a traditional difference between Linux distros and MS Windows, with the most popular filesystems been in opposite sides: while ext4 is case sensitive, NTFS is case insensitive. This difference proved to be challenging when Windows apps, mainly games, started to be a common use case for Linux distros (thanks to Wine!). For instance, games running through Steam’s Proton would expect that the path assets/player.png and assets/PLAYER.PNG would point to be the same file, but this is not the case in ext4. To avoid doing workarounds in userspace, ext4 has support for casefolding since Linux 5.2.

Now, tmpfs joins the group of filesystems with case-insensitive support. This is particularly useful for running games inside containers, like the combination of Wine + Flatpak. In such scenarios, the container shares a subset of the host filesystem with the application, mounting it using tmpfs. To keep the filesystem consistent, with the same expectations of the host filesystem about the mounted one, if the host filesystem is case-insensitive we can do the same thing for the container filesystem too. You can read more about the use case in the patchset cover letter.

While the container frameworks implement proper support for this feature, you can play with it and try it yourself:

$ mount -t tmpfs -o casefold fs_name /mytmpfs
$ cd /mytmpfs # case-sensitive by default, we still need to enable it
$ mkdir a
$ touch a; touch A
$ ls
A  a
$ mkdir B; cd b
cd: The directory 'b' does not exist
$ # now let's create a case-insensitive dir
$ mkdir case_dir
$ chattr +F case_dir
$ cd case_dir
$ touch a; touch A
$ ls
a
$ mkdir B; cd b
$ pwd
$ /home/user/mytmpfs/case_dir/B

V3D Super Pages support

As part of Igalia’s effort for enhancing the graphics stack for Raspberry Pi, the V3D DRM driver now has support for Super Pages, improving performance and making memory usage more efficient for Raspberry Pi 4 and 5. Using Linux 6.13, the driver will enable the MMU to allocate not only the default 4KB pages, but also 64KB “Big Pages” and 1MB “Super Pages”.

To measure the difference that Super Pages made to the performance, a series of benchmarks where used, and the highlights are:

  • +8.36% of FPS boost for Warzone 2100 in RPi4
  • +3.62% of FPS boost for Quake 2 in RPi5
  • 10% time reduction for the Mesa CI job v3dv-rpi5-vk-full:arm64
  • Aether SX2 emulator is more fluid to play

You can read a detailed post about this, with all benchmark results, in Maíra’s blog post, including a super cool PlayStation 2 emulation showcase!

New transparent_hugepage_shmem= command-line parameter

Igalia contributed new kernel command-line parameters to improve the configuration of multi-size Transparent Huge Pages (mTHP) for shmem. These parameters, transparent_hugepage_shmem= and thp_shmem=, enable more flexible and fine-grained control over the allocation of huge pages when using shmem.

The transparent_hugepage_shmem= parameter allows users to set a global default huge page allocation policy for the internal shmem mount. This is particularly valuable for DRM GPU drivers. Just as CPU architectures, GPUs can also take advantage of huge pages, but this is possible only if DRM GEM objects are backed by huge pages.

Since GEM uses shmem to allocate anonymous pageable memory, having control over the default huge page allocation policy allows for the exploration of huge pages use on GPUs that rely on GEM objects backed by shmem.

In addition, the thp_shmem= parameter provides fine-grained control over the default huge page allocation policy for specific huge page sizes.

By configuring page sizes and policies of huge-page allocations for the internal shmem mount, these changes complement the V3D Super Pages feature, as we can now tailor the size of the huge pages to the needs of our GPUs.

DRM and AMDGPU improvements

As usual in Linux releases, this one collects a list of improvements made by our team in DRM and AMDGPU driver from the last cycle.

Cosmic (the desktop environment behind Pop! OS) users discovered some bugs in the AMD display driver regarding the handling of overlay planes. These issues were pre-existing and came to light with the introduction of cursor overlay mode. They were causing page faults and divide errors. We debugged the issue together with reporters and proposed a set of solutions that were ultimately accepted by AMD developers in time for this release.

In addition, we worked with AMD developers to migrate the driver-specific handling of EDID data to the DRM common code, using drm_edid opaque objects to avoid handling raw EDID data. The first phase was incorporated and allowed the inclusion of new functionality to get EDID from ACPI. However, some dependencies between the AMD the Linux-dependent and OS-agnostic components were left to be resolved in next iterations. It means that next steps will focus on removing the legacy way of handling this data.

Also in the AMD driver, we fixed one out of bounds memory write, fixed one warning on a boot regression and exposed special GPU memory pools via the fdinfo common DRM framework.

In the DRM scheduler code, we added some missing locking, removed a couple of re-lock cycles for slightly reduced command submission overheads and clarified the internal documentation.

In the common dma-fence code, we fixed one memory leak on the failure path and one significant runtime memory leak caused by incorrect merging of fences. The latter was found by the community and was manifesting itself as a system out of memory condition after a few hours of gameplay.

sched_ext

The sched_ext landed in kernel 6.12 to enable the efficient development of BPF-based custom schedulers. During the 6.13 development cycle, the sched_ext community has made efforts to harden the code to make it more reliable and clean up the BPF APIs and documentation for clarity.

Igalia has contributed to hardening the sched_ext core code. We fixed the incorrect use of the scheduler run queue lock, especially during initializing and finalizing the BPF scheduler. Also, we fixed the missing RCU lock protections when the sched_core selects a proper CPU for a task. Without these fixes, the sched_ext core, in the worst case, could crash or raise a kernel oops message.

Other Contributions & Fixes

syzkaller, a kernel fuzzer, has been an important instrument to find kernel bugs. With the help of KASAN, a memory error detector, and syzbot, numerous such bugs have been reported and fixed.

Igalians have contributed to such fixes around a lot of subsystems (like media, network, etc), helping reduce the number of open bugs.

Check the complete list of Igalia’s contributions for the 6.13 release

Authored (70)

André Almeida

Changwoo Min

Christian Gmeiner

Guilherme G. Piccoli

Maíra Canal

Melissa Wen

Thadeu Lima de Souza Cascardo

Tvrtko Ursulin

Reviewed (41)

André Almeida

Christian Gmeiner

Iago Toral Quiroga

Jose Maria Casanova Crespo

Juan A. Suarez

Maíra Canal

Tvrtko Ursulin

Tested (1)

Christian Gmeiner

Acked (5)

Changwoo Min

Maíra Canal

Maintainer SoB (6)

Maíra Canal

© André Almeida 2022
Licensed as CC BY 4.0

Powered by Hugo & Kiss.