Knowing how frequently accessed a page of memory is (its "hotness") is a key input to many memory-management heuristics. Jonathan Cameron, in a memory-management track at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, pointed out that the number of sources of that kind of data is growing over time. He wanted to explore the questions of what commonality exists between data from those sources, and whether it makes sense to aggregate them all somehow.

Eduard Zingerman presented a daring proposal that "makes sense if you think about it a bit" at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit. He wants to inline performance-sensitive kernel functions into the BPF programs that call them. His prototype does not yet address all of the design problems inherent in that idea, but it did spark a lengthy discussion about the feasibility of his proposal.

Security updates have been issued by AlmaLinux (delve and golang and go-toolset:rhel8), Debian (webkit2gtk), Fedora (openvpn, thunderbird, uboot-tools, and zabbix), SUSE (expat, fontforge, govulncheck-vulndb, and kernel), and Ubuntu (haproxy and libsoup2.4, libsoup3).

Building on the discussion in the two previous sessions on untorn (or atomic) writes, for buffered I/O and for XFS using direct I/O, Ojaswin Mujoo remotely led a session on support for the feature on ext4. That took place in the combined storage and filesystem track at the 2025 Linux Storage, Filesystem, Memory Management, and BPF Summit. Part of the support for the feature is already in the upstream kernel, with more coming. But there are still some challenges that Mujoo wanted to discuss.

Over on the Red Hat Developer site, David Malcolm has an article about improvements in GCC 15, specifically focusing on the diagnostic information that the compiler emits. This includes ASCII art with a "⚠️" warning emoji to display the execution path when it detects a problem (like an infinite loop in one of his examples), better C++ template errors, machine-readable diagnostics using Static Analysis Results Interchange Format (SARIF), better messages regarding C23 compatibility since that is the default C version for GCC 15, and more. Since the changes are focused on messages, there is the inevitable color-scheme update as well:

GCC will use color when emitting its text messages on stderr at a suitably modern terminal, using a few colors that seem to work well in a number of different terminal themes—but the exact rules for choosing which color to use for each aspect of the output have been rather arbitrary.

For GCC 15, I've gone through C and C++'s errors, looking for places where two different things in the source are being contrasted, such as type mismatches. These diagnostics now use color to visually highlight and distinguish the differences.

Compute Express Link (CXL) memory is not like the ordinary RAM that one might install into a computer; it can come and go at any time and is often not present when the kernel is booting. That complicates the management of this memory. During the memory-management track of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, Gregory Price ran a session on the challenges posed by CXL and how they might be addressed.

Greg Kroah-Hartman has announced the release of eight stable kernels: 6.14.2, 6.13.11, 6.12.23, 6.6.87, 6.1.134, 5.15.180, 5.10.236, and 5.4.292. These all contain a large assortment of important kernel fixes throughout the tree.

The Data Access MONitor (DAMON) subsystem provides access to detailed memory-management statistics, along with a set of tools for implementing policies based on those statistics. An update on DAMON by its primary author, SeongJae Park, has been a fixture of the Linux Storage, Filesystem, Memory-Management, and BPF Summit for some years. The 2025 Summit was no exception; Park led two sessions on recent and future DAMON developments, and how DAMON might evolve to facilitate a more access-aware memory-management subsystem in the future.

Security updates have been issued by AlmaLinux (tomcat and webkit2gtk3), Debian (chromium), Fedora (ghostscript), Mageia (atop, docker-containerd, and xz), Red Hat (go-toolset:rhel8), SUSE (apache2-mod_auth_openidc, apparmor, etcd, expat, firefox, kernel, libmozjs-128-0, and libpoppler-cpp2), and Ubuntu (dino-im, linux, linux-aws, linux-aws-hwe, linux-azure, linux-azure-4.15, linux-gcp, linux-gcp-4.15, linux-hwe, linux-kvm, linux-oracle, linux, linux-aws, linux-kvm, linux-lts-xenial, linux-fips, linux-fips, linux-aws-fips, linux-azure-fips, linux-gcp-fips, opensc, and poppler).

Inside this week's LWN.net Weekly Edition:

• Front: Debian project leader election; 6.15 Merge window; Lots of LSFMM coverage; Joplin.
• Briefs: Firefox hardening; OpenSSH 10.0; Supply chain security; FreeDOS 1.4; OpenSSL 3.5.0; Rust 1.86.0; Quotes; ...
• Announcements: Newsletters, conferences, security updates, patches, and more.

Tom Schuster, Frederik Braun, and Christoph Kerschbaumer have published an article on the Firefox Security team's Attack & Defense blog that explains recent work to harden Firefox's frontend code.

We have rewritten over 600 JavaScript event handlers to mitigate XSS and other injection attacks in the main Firefox user interface. This mitigation will ship in Firefox 138. However, blocking the execution of scripts in the parent process is not the end - we will expand this technique to other contexts in the near future. There is still more work to do as the UI requires JavaScript APIs with a high level of privileges. However: We still eliminated a whole class of attacks, significantly raising the bar for attackers to exploit Firefox.

In a combined storage and filesystem track session at the 2025 Linux Storage, Filesystem, Memory Management, and BPF Summit, John Garry continued the theme of "untorn" (or atomic) writes that started in the previous session. It was also an update on where things have gone for untorn writes since his session at last year's summit. Beyond that, he looked at some of the plans and challenges for the feature in the future.

Four candidates have stepped up to run in the 2025 Debian Project Leader (DPL) election. Andreas Tille, who is in his first term as DPL, is running again. Sruthi Chandran, Gianfranco Costamagna, and Julian Andres Klode are the other candidates running for a chance to serve a term as DPL. The campaigning phase ended on April 5, and Debian members began voting on April 6. Voting ends on April 19. This year, the campaign period has been lively and sometimes contentious, touching on problems with Debian team delegations and finances.

The 6.15 merge window saw the inclusion of a new type of lock for BPF programs: a resilient queued spinlock that Kumar Kartikeya Dwivedi has been working on for some time. Eventually, he hopes to convert all of the spinlocks currently used in the BPF subsystem to his new lock. He gave a remote presentation about the design of the lock at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF summit.

Tiered-memory systems feature multiple types of memory with varying performance characteristics; on such systems, good performance depends on keeping the most frequently used data in the fastest memory. Identifying that data and placing it properly is a challenge that has kept developers busy for years. Bharata Rao, presenting remotely during a memory-management-track session at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, led a discussion on a potential solution he has recently posted; Raghavendra K T was also named on the session proposal. It seems likely, based on the discussion, that developers working in this area will not run out of problems anytime soon.

The kernel samepage merging (KSM) subsystem works by finding pages in memory with the same contents, then replacing the duplicated copies with a single, shared copy. KSM can improve memory utilization in a system, but has some problems as well. In two memory-management-track sessions at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, Mathieu Desnoyers and Sourav Panda proposed improvements to KSM to make it work better for specific use cases.

OpenSSH 10.0 has been released. Support for the DSA signature algorithm, which was disabled by default beginning in 2015, has been removed. Other notable changes include using the post-quantum algorithm mlkem768x25519-sha256 for key agreement by default, support for systemd-style socket activation in Portable OpenSSH, and moving code for user authentication from the sshd-session binary to the new ssh-auth binary:

Splitting this code into a separate binary ensures that the crucial pre-authentication attack surface has an entirely disjoint address space from the code used for the rest of the connection. It also yields a small runtime memory saving as the authentication code will be unloaded after the authentication phase completes. This change should be largely invisible to users, though some log messages may now come from "sshd-auth" instead of "sshd-session". Downstream distributors of OpenSSH will need to package the sshd-auth binary.

The release notes also warn that "software that naively matches versions using patterns like "OpenSSH_1*"" may be confused by the new version number.

Security updates have been issued by Debian (lemonldap-ng, libbssolv-perl, and phpmyadmin), Fedora (augeas, mariadb10.11, and thunderbird), Oracle (gimp, libxslt, python3.11, python3.12, tomcat, and xorg-x11-server), Red Hat (expat, grafana, opentelemetry-collector, and webkit2gtk3), SUSE (azure-cli-core, doomsday, kernel, and poppler), and Ubuntu (dotnet8, dotnet9, erlang, and poppler).

Version 1.4 of FreeDOS has been released. This is the first stable release since 2022, and includes improvements to the Fdisk hard-disk-management program, and reliability updates for the mTCP set of TCP/IP applications for DOS.

This version was much smoother because Jerome Shidel, our distribution manager, had an idea after FreeDOS 1.3 that we could have a rolling test release that collected all of the changes that people make over time. Previous to this, each new FreeDOS distribution (like 1.0, 1.1, 1.2, and 1.3) required bundling up packages into a "release candidate," and we would go through several iterations of updating the release candidates.

Jerome's method of building the FreeDOS distribution made it easier to automate a test release, which we decided to update every month. As the test releases accumulated enough changes to warrant a release, we could then make the next test release a "release candidate" which would iterate to the next version of the FreeDOS distribution. Since 2022, we've released monthly test releases. Thanks Jerome!

LWN covered FreeDOS last year for its 30th anniversary.

Joplin is an open-source note-taking application designed to handle taking many kinds of notes, whether it is managing code snippets, writing documentation, jotting down lecture notes, or drafting a novel. Joplin has Markdown support, a plugin system for extensibility, and accepts multimedia content, allowing users to attach images, videos, and audio files to their notes. It can provide synchronization of content across devices using end-to-end encryption, or users can opt to stick to local storage only. Joplin even offers a command-line version for terminal-based usage. Joplin 3.2, the most recent feature release, brought long-awaited multi-window support, multi-column layouts, enhanced accessibility, and theme detection.

Quite a bit of work has been done in recent years to allow the kernel to make more use of large folios. That progress has not yet reached the handling of text (executable code) areas, though. During the memory-management track of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, Ryan Roberts ran a session on how that situation might be improved. It would be a relatively small and contained operation, but can give a measurable performance improvement.

The kernel makes extensive use of per-CPU data as a way to avoid contention between processors and improve scalability. Using the same technique in user space is harder, though, since there is little control over which CPU a process may be running on at any given time. That hasn't stopped Mathieu Desnoyers from trying, though; in the memory-management track of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, he presented a proposal for how user-space per-CPU memory could work.

Security updates have been issued by AlmaLinux (gimp, libxslt, python3.11, python3.12, and tomcat), Debian (ghostscript and libnet-easytcp-perl), Fedora (openvpn, perl-Data-Entropy, and webkitgtk), Red Hat (python-jinja2), SUSE (giflib, pam, and xen), and Ubuntu (apache2, binutils, expat, fis-gtm, linux-azure, linux-azure-6.8, linux-nvidia-lowlatency, linux-azure, linux-azure-fde, linux-azure-5.15, linux-azure-fde-5.15, linux-azure-fips, linux-gcp-fips, linux-hwe-5.4, linux-nvidia, linux-nvidia-tegra-igx, ruby2.7, ruby3.0, ruby3.2, ruby3.3, and vim).

Pahole (originally "Poke-a-hole") is a Swiss Army knife for exploring and editing debug information. Pahole is also currently involved in the kernel's build process to rearrange the information produced by various compilers into a form useful to the BPF verifier, although there are plans to render it unnecessary. Pahole maintainer Arnaldo Carvalho de Melo shared some status updates about the project at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit. Interested readers can find his slides here.

ACM Queue looks at the security problem in the light of a report on Multics security that was published in 1974.

We are all struggling with a massive shift that has happened in the past 10 or 20 years in the software industry. For decades, software reuse was only a lofty goal. Now it's very real. Modern programming environments such as Go, Node, and Rust have made it trivial to reuse work by others, but our instincts about responsible behaviors have not yet adapted to this new reality.

The fact that the 1974 Multics review anticipated many of the problems we face today is evidence that these problems are fundamental and have no easy answers. We must work to make continuous improvements to open source software supply chain security, making attacks more and more difficult and expensive.

The kernel's swap subsystem is complex and highly optimized — though not always optimized for today's workloads. In three adjacent sessions during the memory-management track of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, Kairui Song, Nhat Pham, and Usama Arif all talked about some of the problems that they are trying to solve in the Linux swap subsystem. In the first two cases, the solutions take the form of an additional layer of indirection in the kernel's swap map; the third, which enables swap-in of large folios, may or may not be worthwhile in the end.

Linus Torvalds released 6.15-rc1 and closed the 6.15 merge window on April 6. By that time, 12,633 non-merge changesets had found their way into his repository; that is substantially more than were merged during the entire 6.14 development cycle. Just under 6,000 of those changesets were merged after the first-half merge-window summary was written.

The 6.14.1, 6.13.10, 6.12.22, 6.6.86, and 6.1.133 stable kernels have all been released. They contain a relatively small collection of important fixes across the kernel tree.

Security updates have been issued by Debian (abseil, atop, jetty9, ruby-saml, tomcat10, trafficserver, xz-utils, and zfs-linux), Fedora (chromium, condor, containernetworking-plugins, cri-tools1.29, crosswords-puzzle-sets-xword-dl, exim, ghostscript, matrix-synapse, upx, varnish, and yarnpkg), Gentoo (XZ Utils), Mageia (augeas, corosync, nss & firefox, and thunderbird), Oracle (container-tools:ol8, firefox, freetype, and kernel), Red Hat (firefox), SUSE (chromium, gn, firefox-esr, go1.23-1.23.8, go1.24, go1.24-1.24.2, google-guest-agent, govulncheck-vulndb, gsl, python311-ecdsa, thunderbird, and webkit2gtk3), and Ubuntu (kamailio, libdbd-mysql-perl, linux-nvidia, linux-nvidia-6.8, and tomcat9).

Linus has released 6.15-rc1 and closed the merge window for this release. "As expected, this was one of the bigger merge windows, almost certainly just because we had some pent-up development due to the previous releases being impacted by the holiday season. That said, while it's bigger than normal, it's not some kind of record-breaking thing.". In the end, 12.633 non-merge changesets were pulled into the mainline during this merge window.

A typical cloud-computing host will share some of its memory with each guest that it runs. The host retains its access to that memory, though, meaning that it can readily dig through that memory in search of data that the guest would prefer to keep private. The guest_memfd subsystem removes (most of) the host's access to guest memory, making the guest's data more secure. In the memory-management track of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, David Hildenbrand ran a discussion on the state and future of this feature.

Alistair Popple started his session at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit by proclaiming that ZONE_DEVICE is "the ugly stepchild" of the kernel's memory-management subsystem. Ugly or not, the ability to manage memory that is attached to a peripheral device rather than a CPU is increasingly important on current hardware. Popple hoped to cover some of the challenges with ZONE_DEVICE and find ways to make the stepchild a bit more attractive, if not bring it into the family entirely.

At last year's Linux Storage, Filesystem, Memory-Management, and BPF Summit (LSFMM+BPF), there was a discussion about atomic writes that was accompanied by patches to support the feature in the block layer, and for direct I/O on XFS. That work was merged, but another piece of that discussion concerned adding the feature for buffered I/O, in part because the PostgreSQL database currently has to jump through hoops to ensure that its writes are not "torn" (partially written) when there is an error or crash. Luis Chamberlain led a combined storage and filesystem track at this year's summit to revisit the idea of providing atomic (or untorn) writes for buffered I/O.

Yonghong Song brought a story about tracking down the cause of a strange verifier error message to the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit. He then presented some possible ways to improve Clang's user experience for anyone running into the same class of error in the future. Toward the end of his allotted time, he also discussed the problems with optimizations that change the signature of functions — a problem that José Marchesi had also brought up in the previous session.

Security updates have been issued by AlmaLinux (firefox), Debian (atop and thunderbird), Fedora (webkitgtk), Mageia (microcode), Oracle (expat), SUSE (apparmor, assimp-devel, aws-efs-utils, expat, firefox, ghostscript, go1.23, gotosocial, govulncheck-vulndb, GraphicsMagick, headscale, libmozjs-128-0, libsaml-devel, openvpn, perl-Data-Entropy, and xz), and Ubuntu (gnupg2, kernel, linux-azure-fips, linux-iot, openvpn, ruby-saml, and xz-utils).

Address-space isolation may well be, as Brendan Jackman said at the beginning of his memory-management-track session at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, "some security bullshit". But it also holds the potential to protect the kernel from a wide range of vulnerabilities, both known and unknown, while reducing the impact of existing mitigations. Implementing address-space isolation with reasonable performance, though, is going to require some significant changes. Jackman was there to get feedback from the memory-management community on how those changes should be implemented.

The kernel must often step through the page tables of one or more processes to carry out various operations. This "page-table walking" tends to be performed by ad-hoc (duplicated) code all over the kernel. Oscar Salvador used a memory-management-track session at the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit to talk about strategies to unify the kernel's page-table walking code just a little bit by making hugetlb pages look more like ordinary pages.

Version 1.86.0 of the Rust language has been released. Changes include support for trait upcasting, the ability to index multiple elements of HashMaps and slices mutably, and a number of stabilized APIs.

Security updates have been issued by AlmaLinux (expat), Debian (chromium, commons-vfs, firefox-esr, php-horde-editor, php-horde-imp, and thunderbird), Fedora (corosync, firefox, nextcloud, and suricata), Mageia (curl and upx), Oracle (emacs, fence-agents, freetype, kernel, libreoffice, libxml2, nginx:1.24, podman, python-jinja2, and tigervnc), Red Hat (firefox and python-jinja2), SUSE (assimp, ffmpeg-4, firefox, ghostscript, GraphicsMagick, libxslt, and tomcat), and Ubuntu (linux, linux-aws, linux-aws-5.15, linux-gcp, linux-gke, linux-gkeop, linux-ibm, linux-intel-iotg, linux-kvm, linux-lowlatency, linux-lowlatency-hwe-5.15, linux-meta-raspi, linux-nvidia-tegra, linux-oracle, linux-oracle-5.15, linux-raspi, linux, linux-azure, linux-azure-5.4, linux-bluefield, linux-gcp, linux-ibm, linux-kvm, linux-oracle, linux-oracle-5.4, linux-xilinx-zynqmp, linux-fips, linux-fips, linux-aws-fips, linux-gcp-fips, linux-hwe-5.15, and linux-realtime, linux-intel-iot-realtime).

Inside this week's LWN.net Weekly Edition:

• Front: Calibre 8.0; Fedora reproducibility; OpenWrt One; 6.15 Merge Window; LSFMM+BPF coverage including BPF in GCC, Rust merging process, and more.
• Briefs: Ubuntu namespaces; New FPL; PorteuX 2.0; Firefox 137.0; GCC Rust; Rockbox 4.0; Rust specification; Thundermail; Dave Täht RIP; Quotes; ...
• Announcements: Newsletters, conferences, security updates, patches, and more.