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Aujourd’hui — 23 mai 2024LWN

[$] The twilight of the version-1 memory controller

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Par : corbet
23 mai 2024 à 14:03
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 for Thursday

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Par : jake
23 mai 2024 à 13:58
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).
Hier — 22 mai 2024LWN

[$] Supporting larger block sizes in filesystems

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Par : jake
22 mai 2024 à 20:10
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 path to deprecating SPARSEMEM

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Par : corbet
22 mai 2024 à 18:58
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.

[$] Two sessions on CXL memory

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Par : corbet
22 mai 2024 à 18:56
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.

[$] Documenting page flags by committee

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Par : corbet
22 mai 2024 à 17:26
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.

[$] Merging msharefs

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Par : corbet
22 mai 2024 à 17:25
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.

[$] Toward the unification of hugetlbfs

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Par : corbet
22 mai 2024 à 16:53
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.

[$] The KeePassXC kerfuffle

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Par : jzb
22 mai 2024 à 16:30

KeePassXC is an open-source (GPLv3), cross-platform password manager with local-only data storage. The project comes with a number of build options that can be used to toggle optional features, such as browser integration and password database sharing. However, controversy ensued when Debian Developer Julian Klode decided to make use of these compile flags to disable these features to improve security in the keepassxc package uploaded to Debian unstable for the upcoming Debian 13 ("Trixie") release.

[$] The interaction between memory reclaim and RCU

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Par : corbet
22 mai 2024 à 15:40
The 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit was a development conference, where discussion was prioritized and presentations with a lot of slides were discouraged. Paul McKenney seemingly flouted this convention in a joint session of the storage, filesystem, and memory-management tracks where he presented about 50 slides — in five minutes, twice. The subject was the use of the read-copy-update (RCU) mechanism in the memory-reclaim process, and whether changes to RCU would be needed for that purpose.

[$] Faster page faults with RCU-protected VMA walks

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Par : corbet
22 mai 2024 à 13:55
Looking up a virtual memory area (VMA) in a process's address space, for the handling of page faults or any of a number of other tasks, in multi-threaded processes has long been bedeviled by lock contention in the kernel. As a result, developer gatherings have been subjected to many sessions on how to improve the situation. At the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit, developers in the memory-management track met, in a session led by Liam Howlett, to talk about a situation that has improved considerably in recent times, but which still offers opportunities for optimization.

Security updates for Wednesday

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Par : jzb
22 mai 2024 à 13:28
Security updates have been issued by Debian (webkit2gtk), Fedora (kernel), Mageia (chromium-browser-stable, djvulibre, gdk-pixbuf2.0, nss & firefox, postgresql15 & postgresql13, python-pymongo, python-sqlparse, stb, thunderbird, and vim), Red Hat (go-toolset:rhel8, nodejs, and varnish:6), SUSE (gitui, glibc, and kernel), and Ubuntu (libspreadsheet-parseexcel-perl, linux-aws, linux-aws-5.15, linux-gke, linux-gcp, python-idna, and thunderbird).

[$] Virtual machine scheduling with BPF

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Par : daroc
22 mai 2024 à 13:18

Vineeth Pillai gave a remote talk at the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit explaining how BPF could be used to improve the performance of virtual machines (VMs). Pillai has a patch set designed to let guest and host machines share scheduling information in order to eliminate some of the overhead of running in a VM. The assembled developers had several comments on the design, but seemed overall to approve of the prospect.

À partir d’avant-hierLWN

[$] Another try for address-space isolation

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Par : corbet
21 mai 2024 à 22:35
Brendan Jackman started his memory-management-track session at the 2024 Linux Storage, Filesystem, Memory-Management and BPF Summit by saying that, for some years now, the kernel community has been stuck in a reactive posture with regard to hardware vulnerabilities. Each problem shows up with its own scary name, and kernel developers find a way to mitigate it, usually losing performance in the process. Jackman said that it is time to take back the initiative against these vulnerabilities by reconsidering the more general use of address-space isolation.

[$] Memory-allocation profiling for the kernel

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Par : corbet
21 mai 2024 à 20:34
Optimizing the kernel's memory use is made much easier if developers have an accurate idea of how memory is being used, but the kernel's instrumentation is not as good as it could be. When Suren Baghdasaryan and Kent Overstreet presented their memory-allocation profiling work, which is meant to address this shortcoming, at the 2023 Linux Storage, Filesystem, Memory Management, and BPF Summit, their objective was uncontroversial but the proposed solution ran into opposition that played out at length on the mailing lists (example) over the last year. So it may be a bit surprising that, when the two returned to the memory-management track in the 2024 gathering, the controversy was gone and the discussion focused on improving details of the implementation.

AlmaLinux forms engineering steering committee

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Par : jzb
21 mai 2024 à 16:11

The AlmaLinux project has announced the formation of the AlmaLinux Engineering Steering Committee (ALESCo):

[It] is dedicated to guiding the technical direction of the AlmaLinux distribution on a day-to-day basis within the guidelines set forth by the board, ensuring its robustness, reliability, sustainability, and relevance in the open-source ecosystem. ALESCo will work collaboratively with, and oversee relevant technical-focused Special Interest Groups (SIGs) to achieve these goals. It is "air traffic control" for engineering matters.

The initial members of ALESCo appointed by the AlmaLinux OS Foundation board are Andrew Lukoshko, Ben Thomas, Cody Robertson, Elkhan Mammadli, Jonathan Wright, and Neal Gompa. The AlmaLinux Wiki has more information on the committee's activities and how to get involved.

[$] Dynamically sizing the kernel stack

✇LWN
Par : corbet
21 mai 2024 à 15:37
The kernel stack is a scarce and tightly constrained resource; kernel developers often have to go far out of their way to avoid using too much stack space. The size of the stack is also fixed, leading to situations where it is too small for some code paths, while wastefully large for others. At the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit, Pasha Tatashin proposed making the kernel stack size dynamic, making more space available when needed while saving memory overall. This change is not as easy to implement as it might seem, though.
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