Comment 0 for bug 1964992

[Impact]
ZFS ignores tunable "zfs_arc_max" due to it being below allmem/32
threshold. This prevents users from properly restraining ARC sizes, and can
cause increased memory contention in some systems.

[Test Plan]
1. Deploy test system with ZFS storage and 32GB RAM
2. Add ARC tunables to /etc/modprobe.d/99-zfs-arc.conf
   # cat /etc/modprobe.d/99-zfs-arc.conf
   options zfs zfs_arc_min=536870912
   options zfs zfs_arc_max=966367641
3. Reboot system
4. Verify ARC sizes through "arc_summary"
   # arc_summary | grep -A3 "ARC size"
   ARC size (current): < 0.1 % 1.3 MiB
           Target size (adaptive): 100.0 % 15.7 GiB
           Min size (hard limit): 3.2 % 512.0 MiB
           Max size (high water): 31:1 15.7 GiB

For a 32GB test system, we should be able to set max ARC sizes below 1GB.

[Fix]
This has been fixed by upstream commit:
 - 36a6e2335c45 "Don't ignore zfs_arc_max below allmem/32"

The commit has been introduced in upstream zfs-2.0.0, so it's needed for Bionic
and Focal. Releases starting with Impish already have this commit by default:

$ git describe --contains 36a6e2335c45
zfs-2.0.0-rc1~332
$ rmadison zfs-linux
 zfs-linux | 0.7.5-1ubuntu15 | bionic | source
 zfs-linux | 0.7.5-1ubuntu16.12 | bionic-updates | source
 zfs-linux | 0.8.3-1ubuntu12 | focal | source
 zfs-linux | 0.8.3-1ubuntu12.9 | focal-security | source
 zfs-linux | 0.8.3-1ubuntu12.13 | focal-updates | source
 zfs-linux | 0.8.3-1ubuntu12.14 | focal-proposed | source
 zfs-linux | 2.0.6-1ubuntu2 | impish | source
 zfs-linux | 2.0.6-1ubuntu2.1 | impish-updates | source
 zfs-linux | 2.1.2-1ubuntu3 | jammy | source

[Regression Potential]
The introduced commit essentially removes the limitation of setting ARC tunables
below allmem/32, and re-arranges the order of how some of the tunables are
parsed. Regressions would possibly show up as other tunables being ignored or
not being set correctly due to parsing errors. We should validate whether other
ARC related tunables are still being set correctly, and whether ZFS is using the
set values for the ARC memory thresholds.