After various long discussions (https://lists.freedesktop.org/archives/systemd-devel/2022-March/047587.html, https://lwn.net/Articles/889610/), there is no clear answer what the minimum version should be. Bumping the version above 3.15 doesn't allow us to make any significant simplifications (unless we went *much* higher). In particular, even renameat2() is not fully supported with latest kernel versions, e.g. nfs still doesn't have it. And the bpf stuff is optional anyway. So let's just say that 4.15 is what we recommend, because it provides fairly complete cgroups-v2, but without any removals of compat in the code.
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