Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

[Roman Gushchin <roman.gushchin@linux.dev>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Fri, Oct 3, 2025 at 7:01 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>>
>> Martin KaFai Lau <martin.lau@linux.dev> writes:
>>
>> > On 9/2/25 10:31 AM, Roman Gushchin wrote:
>> >> Btw, what's the right way to attach struct ops to a cgroup, if there is
>> >> one? Add a cgroup_id field to the struct and use it in the .reg()
>> >
>> > Adding a cgroup id/fd field to the struct bpf_oom_ops will be hard to
>> > attach the same bpf_oom_ops to multiple cgroups.
>> >
>> >> callback? Or there is something better?
>> >
>> > There is a link_create.target_fd in the "union bpf_attr". The
>> > cgroup_bpf_link_attach() is using it as cgroup fd. May be it can be
>> > used here also. This will limit it to link attach only. Meaning the
>> > SEC(".struct_ops.link") is supported but not the older
>> > SEC(".struct_ops"). I think this should be fine.
>>
>> I thought a bit more about it (sorry for the delay):
>> if we want to be able to attach a single struct ops to multiple cgroups
>> (and potentially other objects, e.g. sockets), we can't really
>> use the existing struct ops's bpf_link.
>>
>> So I guess we need to add a new .attach() function beside .reg()
>> which will take the existing link and struct bpf_attr as arguments and
>> return a new bpf_link. And in libbpf we need a corresponding new
>> bpf_link__attach_cgroup().
>>
>> Does it sound right?
>>
>
> Not really, but I also might be missing some details (I haven't read
> the entire thread).
>
> But conceptually, what you describe is not how things work w.r.t. BPF
> links and attachment.
>
> You don't attach a link to some hook (e.g., cgroup). You attach either
> BPF program or (as in this case) BPF struct_ops map to a hook (i.e.,
> cgroup), and get back the BPF link. That BPF link describes that one
> attachment of prog/struct_ops to that hook. Each attachment gets its
> own BPF link FD.
>
> So, there cannot be bpf_link__attach_cgroup(), but there can be (at
> least conceptually) bpf_map__attach_cgroup(), where map is struct_ops
> map.

I see...
So basically when a struct ops map is created we have a fd and then
we can attach it (theoretically multiple times) using BPF_LINK_CREATE.

>
> Having said that, we do have bpf_map__attach_struct_ops() already
> (it's using BPF_LINK_CREATE command under the hood), and so perhaps
> the right way is to have bpf_map__attach_struct_ops_opts() API, which
> will accept optional extra attachment parameters which will be passed
> into bpf_attr.link_create.struct_ops section of UAPI. That thing can
> have target FD, where FD is cgroup/task/whatever we need to specify
> attachment target. Just like we do that for BPF program's
> BPF_LINK_CREATE, really.

Yes, this sounds good to me!

Thanks you for the clarification.