Re: Subject: [PATCH bpf-next 2/3] bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()

[Matt Bobrowski <mattbobrowski@google.com>]

On Thu, Jan 15, 2026 at 08:54:42PM -0800, Roman Gushchin wrote:
> 
> > With the BPF verifier now treating pointers to struct types returned
> > from BPF kfuncs as implicitly trusted by default, there is no need for
> > bpf_get_root_mem_cgroup() to be annotated with the KF_ACQUIRE flag.
> 
> > bpf_get_root_mem_cgroup() does not acquire any references, but rather
> > simply returns a NULL pointer or a pointer to a struct mem_cgroup
> > object that is valid for the entire lifetime of the kernel.
> 
> > This simplifies BPF programs using this kfunc by removing the
> > requirement to pair the call with bpf_put_mem_cgroup().
> 
> It's actually the opposite: having the get semantics (which is also
> suggested by the name) allows to treat the root memory cgroup exactly
> as any other. And it makes the code much simpler, otherwise you
> need to have these ugly checks across the codebase:
> 	if (memcg != root_mem_cgroup)
> 		css_put(&memcg->css);

I mean, you're certainly not forced to do this. But, I do also see
what you mean.

> This is why __all__ memcg && cgroup code follows this principle and the
> hides the special handling of the root memory cgroup within
> css_get()/css_put().
>
> I wasn't cc'ed on this series, otherwise I'd nack this patch.
> If the overhead of an extra kfunc call is a concern here (which I
> doubt), we can introduce a non-acquire bpf_root_mem_cgroup()
> version.
> 
> And I strongly suggest to revert this change.

Apologies, I honestly thought I did CC you on this series. Don't know
what happened with that. Anyway, I'm totally OK with reverting this
patch and keeping bpf_get_root_mem_cgroup() with KF_ACQUIRE
semantics. bpf_get_root_mem_cgroup() was selected as it was the very
first BPF kfunc that came to mind where implicit trusted pointer
semantics should be applied by the BPF verifier.

Notably, the follow up selftest patch [0] will also need to be
reverted if so as it relies on bpf_get_root_mem_cgroup() without
KF_ACQUIRE. We can probably 

[0] https://lore.kernel.org/bpf/20260113083949.2502978-2-mattbobrowski@google.com/T/#mfa14fb83b3350c25f961fd43dc4df9b25d00c5f5