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SubjectRe: [PATCH v5 0/7] hugetlb_cgroup: Add hugetlb_cgroup reservation limits

On Thu, Sep 26, 2019 at 05:55:29PM -0700, Mina Almasry <> wrote:
> My guess is that a new controller needs to support cgroups-v2, which
> is fine. But can a new controller also support v1? Or is there a
> requirement that new controllers support *only* v2? I need whatever
> solution here to work on v1.
Here is my view of important criteria:

1) stability, v1 APIs and semantics should not be changed,
2) futureproofness, v1 uses should be convertible to v2 uses,
3) maintainability, the less (similar) code the better.

And here is my braindump of some approaches:

A) new controller, v2 only
- 1) ok
- 2) may be ok
- 3) separate v1 and v2 implementations
- exclusion must be ensured on hybrid hierarchies

B) new controller, version oblivious (see e.g. pid)
- 1) sort of ok
- 2) partially ok
- 3) two v1 implementations
- exclusion must be ensured even on pure v1 hierarchies

C) extending the existing controller, w/out v2 counterpart
- 1) ok with workarounds (new option switching behavior)
- 2) not ok
- 3) likely OK

D) extending the existing controller, with v2 counterpart
- 1) ok with workarounds (new option switching behavior, see cpuset)
- 2) may be ok
- 3) likely OK

AFAIU, the current patchset is variation of C). Personally, I think
something like D) could work, I'm not convinced about A) and B) based on
the breakdown above. But it may induce some other ideas.

My two cents,
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