Re: [PATCH 3/4] cgroup: Use __counted_by for cgroup::ancestors

[Michal Koutný]

On Thu, Dec 18, 2025 at 06:09:42AM -1000, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Thu, Dec 18, 2025 at 03:09:32PM +0800, Chen Ridong wrote:
> > Note that this level may already be used in existing BPF programs (e.g.,
> > tools/testing/selftests/bpf/progs/task_ls_uptr.c). Do we need to consider compatibility here?
> 
> That's a good point.

I wouldn't be concerned about this particular aspect. The commit
e6ac2450d6dee ("bpf: Support bpf program calling kernel function")
excludes ABIs, the example program uses ksyms (not kfuncs), so there
could even apply Documentation/process/stable-api-nonsense.rst.
OTOH, the semantics of level is unchanged for BPF helpers (that are the
official API).


> Is __counted_by instrumentation tied to some compiler flag? If so,
> might as well make it an optional extra field specifically for the
> annotation rather than changing the meaning of an existing field.

Honestly, I can see benefit mainly in the first patch of the series
(posted the rest for discussion).

I'd like to ask Gustavo whether __counted_by here buys us anything or
whether it's more useful in other parts of kernel (e.g. flexible
allocations in networking code with outer sources of data).

Thanks,
Michal

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