Linux Weekly News

Using huge pages has been known for years to improve the performance of many workloads. But traditional huge pages, often sized by the CPU at 2MB, can be difficult to allocate and can waste memory due to internal fragmentation. Driven by both the folio transition and hardware improvements, attention to smaller, multi-size transparent huge pages (mTHPs) has been on the rise. In two memory-management-track sessions at the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, developers discussed the kernel's ability to reliably allocate mTHPs and the performance gains that result.

John Garry and Ted Ts'o led a discussion about supporting atomic writes for buffered I/O, without any torn (or partial) writes to the device, at the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit. It is something of a continuation of a discussion at last year's summit. The goal is to help PostgreSQL, which writes its data using 16KB buffered I/O; it currently has to do a lot of extra work to ensure that its data is safe on disk. A promise of non-torn, 16KB buffered writes would allow the database to avoid doing double writes.

The original Linux kernel, posted in 1991, ran on a system with a 4KB page size. Over 30 years later, most of us are still running on systems with 4KB pages, even though the amount of installed memory has grown by a few orders of magnitude. It is generally accepted that using large page sizes results in better performance for most applications, but allocating larger pages is often difficult. During a memory-management session at the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, Yu Zhao presented his ideas on improving the allocation of huge pages in the kernel.

Kui-Feng Lee spoke early in the BPF track at the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit about some of the recent improvements to BPF. These changes were largely driven by the sched_ext work that David Vernet had covered in the previous talk. Lee focused on changes relevant to struct_ops programs, but several of those changes apply to all BPF programs.

With the release of Fedora 40 it's time to start looking ahead to what Fedora 41 has in store. One of the largest changes planned for the next release is a switch to DNF5, a C++ rewrite of the DNF package manager. A previous attempt to make the switch, during the Fedora 39 cycle, was called off, and deferred to Fedora 41. The developers have had nearly a year to address compatibility problems and bring DNF5 to a state suitable to replace DNF4. Signs point to a successful switch in the upcoming release, though there may be a few surprises lurking for Fedora users.

The kernel contains a pair of related filesystems that, among other things, can be used for shared-memory applications; shmem is an internal mechanism used within the kernel, while the tmpfs filesystem is mounted and accessible from user space. As is the case elsewhere in the kernel, these subsystems would benefit from the addition of large-folio support. During a joint storage, filesystem, and memory-management session at the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, Daniel Gomez talked about the work he is doing to add that support.

This Graphite blog post retells the history of the BitKeeper fiasco and the dawn of the Git era.

When we think of history, we often romanticize it as being born of a sudden stroke of inspiration. But the creation of git shows the far harsher reality of invention: a slowly escalating disagreement over a license; the need for a scrappy backup solution to unblock work; and then continued polishing and iteration through years and years, led not by the inventor, but rather a community.

For those who weren't around in those days, a perusal of the LWN coverage from the time might be of interest too, including:

• Our first mention of BitKeeper in October 1998
• Not quite open source, 1999
• Linus tries out BitKeeper, 2002
• The free software community and proprietary packages, 2002
• The kernel and BitKeeper part ways, 2005
• How Tridge reverse engineered BitKeeper, 2005
• The guts of Git, 2005

...and a lot more for those who care to search for it.

Security updates have been issued by Fedora (chromium, libreoffice, and thunderbird), Red Hat (.NET 7.0, .NET 8.0, gdk-pixbuf2, git-lfs, glibc, python3, and xorg-x11-server-Xwayland), SUSE (firefox, opensc, and ucode-intel), and Ubuntu (cjson and gnome-remote-desktop).

Swapping may be a memory-management technique at its core, but its implementation also involves the kernel's filesystem and storage layers. So it is not surprising that a session on the kernel's swap abstraction layer, led by Chris Li at the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, was held jointly by all three of those tracks. Li has some ambitious ideas for an improved subsystem, but getting to a workable implementation may not be easy.

David Vernet's second talk at the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit was a summary of the state of sched_ext, the extensible BPF scheduler that LWN covered in early May. In short, sched_ext is intended as a platform for rapid experimentation with schedulers, and a tool to let performance-minded administrators customize the scheduler to their workload. The patch set has seen several revisions, becoming more generic and powerful over time. Vernet spoke about what has been done in the past year, and what is still missing before sched_ext can be considered pretty much complete.

The KDE Project has announced the release of KDE Gear 24.05.0, with new features and updates for the more than 200 applications that are part of the project. In addition to new versions of the Dolphin file manager, Kdenlive video editor, and Elisa music player, this release includes five applications new to KDE Gear: the Audex CD-ripper application, an application Accessibility Inspector, the Francis Pomodoro timer, Kalm to teach breathing techniques, and a Sokoban-like game called Skladnik. See the full changelog for a complete list of changes.

Almost immediately after the merging of control groups, kernel developers set their sights on reimplementing them properly. The second version of the control-group API started trickling into the kernel around the 3.16 release in 2014 and users have long since been encouraged to migrate, but support for (and users of) the initial API remain. At the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, memory-management developers discussed whether (and when) it might be possible to remove the version-1 memory controller. The session was led by Shakeel Butt and (participating remotely) Roman Gushchin.

Security updates have been issued by Debian (chromium), Fedora (chromium, libxml2, pgadmin4, and python-libgravatar), Mageia (ghostscript), Red Hat (389-ds:1.4, ansible-core, bind and dhcp, container-tools:rhel8, edk2, exempi, fence-agents, freeglut, frr, ghostscript, glibc, gmp, go-toolset:rhel8, grafana, grub2, gstreamer1-plugins-bad-free, gstreamer1-plugins-base, gstreamer1-plugins-good, harfbuzz, httpd:2.4, idm:DL1, idm:DL1 and idm:client modules, kernel, kernel-rt, krb5, LibRaw, libreoffice, libsndfile, libssh, libtiff, libX11, libxml2, libXpm, linux-firmware, motif, mutt, openssh, osbuild and osbuild-composer, pam, pcp, pcs, perl-Convert-ASN1, perl-CPAN, perl:5.32, pki-core:10.6 and pki-deps:10.6 modules, pmix, poppler, postgresql-jdbc, python-dns, python-jinja2, python-pillow, python27:2.7, python3.11, python3.11-cryptography, python3.11-urllib3, python39:3.9 and python39-devel:3.9 modules, qt5-qtbase, resource-agents, squashfs-tools, sssd, systemd, tigervnc, tomcat, traceroute, varnish:6, virt:rhel and virt-devel:rhel modules, vorbis-tools, webkit2gtk3, xorg-x11-server, xorg-x11-server-Xwayland, and zziplib), SUSE (chromium, perl, postgresql14, and python-sqlparse), and Ubuntu (klibc, linux-aws-hwe, openssl, and vlc).

The LWN.net Weekly Edition for May 23, 2024 is available.

In a combined storage and filesystem session at the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit, Luis Chamberlain led a discussion on filesystem support for block sizes larger than the usual 4KB page size, which followed up on discussion from last year. While the session was meant to look at the intersection of larger block sizes with atomic block writes that avoid torn (partial) writes (which was also discussed last year), it mostly focused on the filesystem side. Over time, the block sizes offered by storage devices have risen from the original 512 bytes; Chamberlain wanted to discuss filesystem support for block sizes larger than 4KB.

The term "memory model" is used in a couple of ways within the kernel. Perhaps the more obscure meaning is the memory-management subsystem's view of how physical memory is organized on a given system. A proper representation of physical memory will be more efficient in terms of memory and CPU use. Since hardware comes in numerous variations, the kernel supports a number of memory models to match; see this article for details. At the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, Oscar Salvador, presenting remotely, made the case for removing one of those models.

Compute Express Link (CXL) is a data-center-oriented memory solution that, according to some in the industry, will yield large cost savings and performance improvements. Others are more skeptical. At the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, two sessions covered CXL and how it will be supported in future kernels.

For every page of memory in the system, the kernel maintains a set of page flags describing how the page is used and various aspects of its current state. Space for page flags has been in chronic short supply, leading to a desire to eliminate or consolidate them whenever possible. That objective, though, is hampered by the fact that the purpose of many page flags is not well understood. In a memory-management-track session at the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, Matthew Wilcox set out to cooperatively update the page-flag documentation to improve that situation.

The problem of sharing page tables across processes has been discussed numerous times over the years, Khalid Aziz said at the beginning of his 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit session on the topic. He was there to, once again, talk about the proposed mshare() system call (which, in its current form, is no longer actually a system call but the feature still goes by that name) and to see what can be done to finally get it into the mainline.

The kernel's hugetlbfs subsystem was the first mechanism by which the kernel made huge pages available to user space; it was added to the 2.5.46 development kernel in 2002. While hugetlbfs remains useful, it is also viewed as a sort of second memory-management subsystem that would be best unified with the rest of the kernel. At the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, Peter Xu raised the question of what that unification would involve and what the first steps might be.