Re: [PATCH v2 06/23] mm: introduce BPF struct ops for OOM handling

[Roman Gushchin]

Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:

> On Mon, Oct 27, 2025 at 4:18 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>>
>> +bool bpf_handle_oom(struct oom_control *oc)
>> +{
>> +       struct bpf_oom_ops *bpf_oom_ops = NULL;
>> +       struct mem_cgroup __maybe_unused *memcg;
>> +       int idx, ret = 0;
>> +
>> +       /* All bpf_oom_ops structures are protected using bpf_oom_srcu */
>> +       idx = srcu_read_lock(&bpf_oom_srcu);
>> +
>> +#ifdef CONFIG_MEMCG
>> +       /* Find the nearest bpf_oom_ops traversing the cgroup tree upwards */
>> +       for (memcg = oc->memcg; memcg; memcg = parent_mem_cgroup(memcg)) {
>> +               bpf_oom_ops = READ_ONCE(memcg->bpf_oom);
>> +               if (!bpf_oom_ops)
>> +                       continue;
>> +
>> +               /* Call BPF OOM handler */
>> +               ret = bpf_ops_handle_oom(bpf_oom_ops, memcg, oc);
>> +               if (ret && oc->bpf_memory_freed)
>> +                       goto exit;
>> +       }
>> +#endif /* CONFIG_MEMCG */
>> +
>> +       /*
>> +        * System-wide OOM or per-memcg BPF OOM handler wasn't successful?
>> +        * Try system_bpf_oom.
>> +        */
>> +       bpf_oom_ops = READ_ONCE(system_bpf_oom);
>> +       if (!bpf_oom_ops)
>> +               goto exit;
>> +
>> +       /* Call BPF OOM handler */
>> +       ret = bpf_ops_handle_oom(bpf_oom_ops, NULL, oc);
>> +exit:
>> +       srcu_read_unlock(&bpf_oom_srcu, idx);
>> +       return ret && oc->bpf_memory_freed;
>> +}
>
> ...
>
>> +static int bpf_oom_ops_reg(void *kdata, struct bpf_link *link)
>> +{
>> +       struct bpf_struct_ops_link *ops_link = container_of(link, struct bpf_struct_ops_link, link);
>> +       struct bpf_oom_ops **bpf_oom_ops_ptr = NULL;
>> +       struct bpf_oom_ops *bpf_oom_ops = kdata;
>> +       struct mem_cgroup *memcg = NULL;
>> +       int err = 0;
>> +
>> +       if (IS_ENABLED(CONFIG_MEMCG) && ops_link->cgroup_id) {
>> +               /* Attach to a memory cgroup? */
>> +               memcg = mem_cgroup_get_from_ino(ops_link->cgroup_id);
>> +               if (IS_ERR_OR_NULL(memcg))
>> +                       return PTR_ERR(memcg);
>> +               bpf_oom_ops_ptr = bpf_oom_memcg_ops_ptr(memcg);
>> +       } else {
>> +               /* System-wide OOM handler */
>> +               bpf_oom_ops_ptr = &system_bpf_oom;
>> +       }
>
> I don't like the fallback and special case of cgroup_id == 0.
> imo it would be cleaner to require CONFIG_MEMCG for this feature
> and only allow attach to a cgroup.
> There is always a root cgroup that can be attached to and that
> handler will be acting as "system wide" oom handler.

I thought about it, but then it can't be used on !CONFIG_MEMCG
configurations and also before cgroupfs is mounted, root cgroup
is created etc. This is why system-wide things are often handled in a
special way, e.g. in by PSI (grep system_group_pcpu).

I think supporting !CONFIG_MEMCG configurations might be useful for
some very stripped down VM's, for example.