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 Tue, Jan 20, 2026 at 06:52:15AM +0000, Matt Bobrowski wrote:
> On Mon, Jan 19, 2026 at 05:29:52PM -0800, Alexei Starovoitov wrote:
> > On Fri, Jan 16, 2026 at 1:18 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> > >
> > >
> > > E.g. in my bpfoom case:
> > >
> > > SEC("struct_ops.s/handle_out_of_memory")
> > > int BPF_PROG(test_out_of_memory, struct oom_control *oc, struct bpf_struct_ops_link *link)
> > > {
> > >         struct task_struct *task;
> > >         struct mem_cgroup *root_memcg = oc->memcg;
> > 
> > And you'll annotate oom_control->memcg with BTF_TYPE_SAFE_TRUSTED_OR_NULL ?
> > 
> > >         struct mem_cgroup *memcg, *victim = NULL;
> > >         struct cgroup_subsys_state *css_pos, *css;
> > >         unsigned long usage, max_usage = 0;
> > >         unsigned long pagecache = 0;
> > >         int ret = 0;
> > >
> > >         if (root_memcg)
> > >                 root_memcg = bpf_get_mem_cgroup(&root_memcg->css);
> > 
> > similar for mem_cgroup and css types ?
> > or as BTF_TYPE_SAFE_RCU_OR_NULL ?
> > 
> > >         else
> > >                 root_memcg = bpf_get_root_mem_cgroup();
> > >
> > >         if (!root_memcg)
> > >                 return 0;
> > >
> > >         css = &root_memcg->css;
> > >         if (css && css->cgroup == link->cgroup)
> > >                 goto exit;
> > >
> > >         bpf_rcu_read_lock();
> > 
> > then this is a bug ? and rcu_read_lock needs to move up?
> > 
> > >         bpf_for_each(css, css_pos, &root_memcg->css, BPF_CGROUP_ITER_DESCENDANTS_POST) {
> > >                 if (css_pos->cgroup->nr_descendants + css_pos->cgroup->nr_dying_descendants)
> > >                         continue;
> > >
> > >                 memcg = bpf_get_mem_cgroup(css_pos);
> > >                 if (!memcg)
> > >                         continue;
> > >
> > >                 < ... >
> > >
> > >                 bpf_put_mem_cgroup(memcg);
> > >         }
> > >         bpf_rcu_read_unlock();
> > >
> > >         < ... >
> > >
> > >         bpf_put_mem_cgroup(victim);
> > > exit:
> > >         bpf_put_mem_cgroup(root_memcg);
> > 
> > Fair enough.
> > Looks like quite a few pieces are still missing for this to work end-to-end,
> > but, sure, let's revert back to acquire semantics.
> > 
> > Matt,
> > please come with a way to fix a selftest. Introduce test kfunc or something.
> 
> Already on it.

Here is the updated selftest which relies on new test specific BPF
kfuncs defined within bpf_testmod instead:

- https://lore.kernel.org/bpf/20260120091630.3420452-1-mattbobrowski@google.com/T/#u