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

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

Song Liu <song@kernel.org> writes:

> On Mon, Oct 6, 2025 at 5:42 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> [...]
>> >> >
>> >> > 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?
>
> IIUC, bpf_struct_ops_link is just for bpf_struct_ops.reg(). The
> attach() operation can be separate, and it doesn't need to be
> implemented in sys_bpf() syscall. BPF TCP congestion control
> uses setsockopt() to do the attach(). Current sched_ext does
> the attach as part of reg(). Tejun is proposing to use reg() for
> sub scheduler [1]. In my earlier patch set for fanotify-bpf, I
> was planning to use ioctl on the fanotify fd [2]. I think these
> all work for the given use case.
>
> I am not sure what is the best option for cgroup oom killer. There
> are multiple options. Technically, it can even be a sysfs entry.
> We can use it as:
>
> # load and pin oom killers first
> $ cat /sys/fs/cgroup/user.slice/oom.killer
> [oom_a] oom_b oom_c
> $ echo oom_b > /sys/fs/cgroup/user.slice/oom.killer
> $ cat /sys/fs/cgroup/user.slice/oom.killer
> oom_a [oom_b] oom_c

It actually looks nice!
But I expect that most users of bpf_oom won't use it directly,
but through some sort of middleware (e.g. systemd), so Idk if
such a user-oriented interface makes a lot of sense.

> Note that, I am not proposing to use sysfs entries for oom killer.
> I just want to say it is an option.
>
> Given attach() can be implemented in different ways, we probably
> don't need to add it to bpf_struct_ops. But if that turns out to be
> the best option, I would not argue against it. OTOH, I think it is
> better to keep reg() and attach() separate, though sched_ext is
> using reg() for both options.

I'm inclining towards a similar approach, except that I don't want
to embed cgroup_id into the struct_ops, but keep it in the link,
as Martin suggested. But I need to implement it end-to-end before I can
be sure that it's the best option. Working on it...

>
> Does this make sense?

Yes, thank you for the great summary!