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Kernel: BPF, Memory Use, and "Episode 29 of the Dirk and Linus show"
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Verifier-state pruning in BPF
The BPF verifier works, on a theoretical level, by considering every possible path that a BPF program could take. As a practical matter, however, it needs to do that in a reasonable amount of time. At the 2025 Linux Plumbers Conference, Mahé Tardy and Paul Chaignon gave a detailed explanation (slides; video) of the main mechanism that it uses to accomplish that: state pruning. They focused on two optimizations that help reduce the number of paths the verifier needs to check, and discussed some of the complications the optimizations introduced to the verifier's code.
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A high-memory elimination timeline for the kernel
Arnd Bergmann began his 2025 Linux Plumbers Conference session on the future of 32-bit support in the Linux kernel by saying that it was to be a followup to his September talk on the same topic. The focus this time, though, was on the kernel's "high memory" abstraction, and when it could be removed. It seems that the kernel community will need to support 32-bit systems for some time yet, even if it might be possible to remove some functionality, including support for large amounts of memory on those systems, more quickly.
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A visualizer for BPF program state
When the verifier rejects a BPF program, it produces a verification log with a mixture of different information: the exact BPF instructions executed on the failing path, calls to any kernel functions or BPF subprograms, line numbers from the debugging information in the program, and information about the contents of different registers and stack slots. This technically contains all of the information needed to understand the failure, but in an ""incomprehensible"" form, Solodrai said. The logs don't include information about the previous states of registers and stack slots, for example, so tracing through a log could involve remembering context from a million instructions ago, which humans cannot do.
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The pane showing the current state of the BPF program also uses colors to indicate which registers and stack slots were read from or written to, and includes visualizations for various different kinds of data, such as scalars and values from BPF maps. The BPF verifier uses a kind of static analysis based on abstract interpretation, so a register could hold a specific value such as "4", but it could also hold "an unknown number that is a multiple of 4 between 12 and 340". The visualizer does its best to show the simplest form of the value in a register.
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Episode 29 of the Dirk and Linus show
Linus Torvalds is famously averse to presenting prepared talks, but the wider community is always interested in what he has to say about the condition of the Linux kernel. So, for some time now, his appearances have been in the form of an informal conversation with Dirk Hohndel. At the 2025 Open Source Summit Japan, the pair followed that tradition for the 29th time. Topics covered include the state of the development process, what Torvalds actually does, and how machine-learning tools might fit into the kernel project.
Hohndel began by noting that Torvalds is now a video star. He was referring to the "Linus x Linus" video that was published at the beginning of December, which is rapidly approaching four-million views. Torvalds said that he enjoys being able to ""do these strange things"" on occasion. He hastened to add that once was enough, though.