LWN Coverage of Kernel (Linux)

posted by Roy Schestowitz on Apr 03, 2025

• LWN ☛ Development statistics for 6.14

  By the time that Linus Torvalds released the 6.14 kernel, 11,003 non-merge changesets had been pulled into the mainline, making this one of the smallest releases we have seen in some time. Indeed, one must go back to the 4.0 release, which happened almost exactly ten years ago, to find a release with fewer changesets than 6.14. Even so, "small" is relative, and 6.14 contains a lot of significant changes.

  This kernel release included contributions from 1,897 developers; that, too, is a relatively small number — though rather larger than the 1,457 who participated in the 4.0 release. One only needs to go back to 5.15, released in 2021, to find a kernel release made with the participation of fewer developers. Of the developers contributing to 6.14, 228 were first-time contributors.

• LWN ☛ MM medley: huge page allocation, page promotion, KSM, and BPF

  As the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit (LSFMM+BPF) approaches, the density of memory-management patches on the mailing lists has increased. Included among those are patches aimed at improving the reliability and performance of huge-page allocation, implementing page promotion on tiered-memory systems, adding a different approach to deduplicating memory, and replacing the BPF memory allocator. Read on for an overview of each.

• LWN ☛ Multiple memory classes for address-space isolation

  Brendan Jackman has been working to try to get ahead of the next hardware CPU vulnerability before it gets discovered. In January, he posted the second version of a patch set that introduces address-space isolation (ASI) as a way of preventing future CPU vulnerabilities from leaking important information. The core concept is to ensure that data that is not currently needed is not present in memory, so that speculative execution cannot leak it. The work is nowhere near ready to be incorporated into the mainline kernel — not least of all because it has a large performance impact in its current form — but it is likely to once again be a topic of discussion at the 2025 Linux Filesystem, Memory Management, and BPF Summit.

  Jackman's patch set introduces different classes of memory. The classes are effectively isolated from one another, in order to avoid leaking information between them. The first version only differentiated between memory mappings for KVM virtual machines and everything else, but reviewers wanted a demonstration that his approach could handle more than just two classes. Therefore, the most recent version of the patch set has a separate class for kernel code that handles certain system calls that don't require access to sensitive data as well. Each class of memory has its own independent address space, intended to contain only information relevant to a particular part of the system. When the kernel needs to access user-space memory, or run code for a virtual machine, it needs to switch to using that class (and that address space). Speculative execution cannot leak information that is not mapped into memory, and the system keeps track of switches between classes in order to flush any other microarchitectural state that could be used to leak information.

• LWN ☛ The guaranteed contiguous memory allocator

  As a system runs and its memory becomes fragmented, allocating large, physically contiguous regions of memory becomes increasingly difficult. Much effort over the years has gone into avoiding the need to make such allocations whenever possible, but there are times when they simply cannot be avoided. The kernel's contiguous memory allocator (CMA) subsystem attempts to make such allocations possible, but it has never been a perfect solution. Suren Baghdasaryan is is trying to improve that situation with the guaranteed contiguous memory allocator patch set, which includes work from Minchan Kim as well.

  In the distant past, Dan Magenheimer introduced the concept of transcendent memory — memory that is not directly addressable, but which can be used opportunistically by the kernel for caching or other purposes. Most of the transcendent-memory work has since gone unused and been removed from the kernel, but the idea persists, and this patch series makes use of it to provide guaranteed CMA.

• LWN ☛ The 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit

  The Linux Storage, Filesystem, Memory-Management, and BPF Summit is an annual, invitation-only event where about 140 developers gather to address core-kernel problems. The 2025 event was held March 24 to 26 in Montreal, Canada. As usual, LWN was there to report on the discussions that were held.

• LWN ☛ The state of the page in 2025

  The folio transition is one of the most fundamental kernel changes ever made; it can be thought of as being similar to replacing the foundation of a building while it remains open for business. So it is not surprising that, for some years, the annual Linux Storage, Filesystem, Memory-Management, and BPF Summit has included a session on the state of this transition. The 2025 Summit was no exception, with Matthew Wilcox updating the group on what has been accomplished, what remains to be done, and where some of the significant problems are.

  The initial idea behind folios, Wilcox began, was to manage pages in larger blocks; the experience of the last few years shows that it works. Later on, the goal of shrinking the page structure, which represents a single page in memory was added. Even later came objectives like enabling filesystems with block sizes larger than the page size and improving the debugability and clarity of the memory-management subsystem. There has been the accumulation of a lot of cruft in that subsystem over the years, he said; the folio transition is an opportunity to clean some of it out.