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POWER 4KiB page size

Summary

On ppc64le, the kernel is currently compiled for 64 KiB page size.

This change proposes using the more common 4 KiB page size.

Some HPC workloads may be disadvantaged slightly. Workstation users are likely to encounter fewer bugs.

Some things, like the AMD Radeon GPU drivers, firmware or related code, appear to be completely non-functional on the 64k page size. Insufficient upstream developers are testing such issues on this architecture.

Owner

Current status

  • Targeted release: Fedora 35
  • Last updated: 2021-02-22
  • FESCo issue: #2581
  • Tracker bug: <will be assigned by the Wrangler>
  • Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Feedback

Discussed several times on devel, latest here

discussed upstream in the Raptor forum

Benefit to Fedora

Better first impression for users of ppc64le workstations.

Users can focus on reporting ppc64le bugs without being sidetracked by page size bugs.

Scope

  • Other developers: please volunteer by adding your name here
  • Release engineering: #9939 (a check of an impact with Release Engineering is needed)
    • wait for 5.12 kernel, verify that it includes the Btrfs patches for arbitrary 4 KiB / 64 KiB sector size, independent of the page size
    • create a kernel with 4 KiB page size to run on the ppc64le build servers
    • ensure the default kernel RPM in the distribution has 4 KiB page size
    • perform the mass rebuild running on the 4 KiB page size
    • create an installer ISO based on the revised kernel with 4 KiB page size


  • Policies and guidelines: no, as it is an arch-specific issues, most other architectures already have a 4k page size
  • Trademark approval: N/A (not needed for this Change)
  • Alignment with Objectives: none of the current objectives relate to this change

Upgrade/compatibility impact

If the user has already formatted their root filesystem with Btrfs and a 64k sector size, they need to be using a Fedora kernel that supports both 4 KiB and 64 KiB. This is anticipated in a future kernel release, 5.12 and will hopefully be ready for F34 or F35[1].

How To Test

User Experience

New GPUs are more likely to just work on this non-x86 architecture, as long as the latest firmware, mesa, llvm are also used.

Btrfs, the default filesystem, will use the sector size identical to the running kernel's page size. As the 4KiB page size is more common, this will ensure Btrfs filesystems created on ppc64le hosts can be used on x86 and other hosts without hassle.

Dependencies

All RPMs must be rebuilt on a server running the final page size (4KiB)

Contingency Plan

  • Contingency mechanism: Prepare a kernel with the original 64k config, install it on the build server, rebuild all the packages for this architecture
  • Contingency deadline: whenever the last time for a full rebuild or kernel change is possible
  • Blocks release? Yes, full rebuild of all packages must be completed before release
  • Blocks product? product

Documentation

Release Notes