Post Syndicated from corbet original https://lwn.net/Articles/975145/
Linus has released
6.10-rc1 and closed the merge window for this release. For reasons
that have not been spelled out, the codename for the release has been
changed to “Baby Opossum Posse”.
Post Syndicated from corbet original https://lwn.net/Articles/974826/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974636/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974630/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974914/
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:
…and a lot more for those who care to search for it.
Post Syndicated from corbet original https://lwn.net/Articles/974587/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974575/
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.
Post Syndicated from corbet original https://lwn.net/Articles/973888/
The LWN.net Weekly Edition for May 23, 2024 is available.
Post Syndicated from corbet original https://lwn.net/Articles/974517/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974518/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974515/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974512/
The problem of sharing page tables across processes has been discussed
numerous times over the years, Khaled 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.
Post Syndicated from corbet original https://lwn.net/Articles/974491/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974487/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974392/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974390/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974380/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974367/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974223/
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.
Post Syndicated from corbet original https://lwn.net/Articles/974450/
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).