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

[Roman Gushchin]

Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:

> On Mon, Oct 6, 2025 at 4:52 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>>
>> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
>>
>> > On Fri, Oct 3, 2025 at 7:01 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>> >>
>> >> Martin KaFai Lau <martin.lau@xxxxxxxxx> 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.
>
> Yes, exactly. "theoretically" part is true right now because of how
> things are wired up internally, but this must be fixable

Ok, one more question: do you think it's better to alter the existing
bpf_struct_ops.reg() callback and add the bpf_attr parameter
or add the new .attach() callback?