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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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.

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.