[systemd-devel] Required kernel version for pid1 only?

[Zbigniew Jędrzejewski-Szmek]

On Tue, Jan 09, 2018 at 08:10:55AM +0000, 林自均 wrote:
> Hi folks,
> 
> I saw the following lines in README:
> 
> > REQUIREMENTS:
> >         Linux kernel >= 3.13
> >         Linux kernel >= 4.2 for unified cgroup hierarchy support
> 
> I guess it means that some part of the whole systemd project requires Linux
> kernel 3.13. What if I don't need that part and only pid1 is relevant to
> me? Is such information provided somewhere?
Generally it's pid1 that has the strongest requirements. Sometimes
we need a specific system call, in which case the requirement applies
to library code and all of the project.

Usually you can find _why_ this specific version is required by
looking at the commit that raised the requirement in docs, it should
contain a brief justification. But it's not guaranteed to list all the
reasons, and new uses could have been added in the meantime, and as
long as the latest required kernel version supports them, this
wouldn't be mentioned anywhere. It should be possible to compile and
use systemd with older kernel versions using some workarounds or by
omitting features, but this is not supported and you're basically on
your own.

Zbyszek