The bcachefs filesystem, and the
process for getting it upstream, were the topics
of a session led remotely by
Kent Overstreet, creator of bcachefs, at the
2023 Linux Storage, Filesystem,
Memory-Management and BPF Summit. He has also discussed bcachefs in
previous editions of the summit, first
in 2018 and at last year s event;
in both of those cases, the question of getting bcachefs merged
into the mainline kernel came up, but that merge has not happened yet.
This time
around, though, Overstreet seemed
closer than ever to being ready to actually start that process.
The kernel s "direct map" makes the entirety of a system s physical memory
available in the kernel s virtual address space. Normally, huge pages are used for
this mapping, making it relatively efficient to access. Increasingly,
though, there is a need to carve some pages out of the direct map; this
splits up those huge pages and makes the system as a whole less efficient.
During a memory-management session at the
2022
Linux Storage, Filesystem, Memory-management and BPF Summit (LSFMM),
Mike Rapoport led a session on direct-map fragmentation and how it might be
avoided.
The kernel s direct map makes all of a system s physical memory available
to the kernel within its address space on 64-bit systems, at least. This
seemingly simple feature has proved to be hard to maintain, in the face of
the requirements faced by current systems, while keeping good performance.
The latest attempt to address this issue is this patch
set from Mike Rapoport adding more direct-map awareness to the kernel s
page allocator.