[Bug 1932329] Re: Support compressed kernel modules in initramfs-tools and kernel

[Dimitri John Ledkov]

moving linux tasks to
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2028568

** No longer affects: linux (Ubuntu Jammy)

** No longer affects: linux (Ubuntu)

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https://bugs.launchpad.net/bugs/1932329

Title:
  Support compressed kernel modules in initramfs-tools and kernel

Status in initramfs-tools package in Ubuntu:
  Fix Released
Status in initramfs-tools source package in Focal:
  New
Status in initramfs-tools source package in Impish:
  Won't Fix
Status in initramfs-tools source package in Jammy:
  Fix Released

Bug description:
  --- initramfs-tools

  [Impact]

   * Initramfs-tools already supports compressed kernel modules.
  However, in focal and impish it does so inefficiently. It is always
  better to have a compressed initramfs of uncommpressed kernel modules,
  than a compressed initrd of compressed kernel modules. Thus when
  kernel module compression is turned on, it is prudent for initramfs-
  tools to pre-uncompress kernel modules when building initramfs.

  [Test Plan]

   * Compress all kernel modules with xz for a current kernel, check
  that all of them have .ko.xz extension and no .ko ones available

   * Rerun depmod

   * Update initramfs with update-initramfs -u

   * lsinitramfs contents and check that all kernel modules are present,
  and are uncompressed (.ko extension)

  [Where problems could occur]

   * This optimization for compressed kernel modules will make initramfs
  build time longer (due to decompression) whilst improving bootspeed
  (overall smaller size of the initrd).

  [Other Info]

   * Original bug report re kernel feature

  --- linux

  Symbol: MODULE_COMPRESS_ZSTD [=n]
  Type  : bool

  = Impacts to measure and observe =

  == Disk space ==

  * Inspect linux-modules-* and linux-modules-extra* deb package
  Installed-Size and Download-Size changes, i.e.

  $ apt show linux-modules-5.8.0-53-generic linux-modules-
  extra-5.8.0-53-generic  | grep -e Package: -e Size:

  Package: linux-modules-5.8.0-53-generic
  Installed-Size: 81.5 MB
  Download-Size: 15.5 MB

  Package: linux-modules-extra-5.8.0-53-generic
  Installed-Size: 215 MB
  Download-Size: 41.5 MB

  In theory, there should not be a significant change in the Download-
  size. It is desired that there is a significant reduction in
  Installed-Size. Modules take up about 300MB and normally one has upto
  three kernel version installed, resulting in about of 1GB of disk
  space that one constantly pays for.

  == Boot Speed ==

  In theory, boot speed may either improve or regress. It depends if
  disk IO is slower than decompression speed, meaning loading compressed
  modules is faster.

  Also one has to observe the changes in the initrd size. zstd(zstd)
  compression may result in slight growth, which shouldn't impact boot
  speed too much.

  = Outcomes =

  If installed size savings can be achieved without regressing bootspeed
  we should enable CONFIG_MODULE_COMPRESS_ZSTD=y by default.

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