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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

The page structure is a complicated beast, but some parts of it are more intimidating than others. The mapcount field is one of the scarier parts. It allegedly records the number of references to the page in page tables, but, as David Hildenbrand described during the memory-management track at the 2024 Linux Storage, Filesystem, Memory Management, and BPF Summit, things are more complicated than that. Few people truly understand the semantics of this field, but the situation will hopefully get better over time.

Security updates have been issued by AlmaLinux (firefox, nodejs, and thunderbird), Fedora (uriparser), Oracle (firefox and thunderbird), Slackware (mariadb), SUSE (cairo, gdk-pixbuf, krb5, libosinfo, postgresql14, and python310), and Ubuntu (firefox, linux-aws, linux-aws-5.15, and linux-azure).