Re: [PATCH bpf-next v3 10/17] mm: introduce bpf_task_is_oom_victim() kfunc

[Michal Hocko <mhocko@suse.com>]

On Mon 02-02-26 16:14:37, Roman Gushchin wrote:
> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
> 
> > On Sun, Feb 1, 2026 at 9:39 PM Matt Bobrowski <mattbobrowski@google.com> wrote:
> >>
> >> On Mon, Jan 26, 2026 at 06:44:13PM -0800, Roman Gushchin wrote:
> >> > Export tsk_is_oom_victim() helper as a BPF kfunc.
> >> > It's very useful to avoid redundant oom kills.
> >> >
> >> > Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> >> > Suggested-by: Michal Hocko <mhocko@suse.com>
> >> > ---
> >> >  mm/oom_kill.c | 14 ++++++++++++++
> >> >  1 file changed, 14 insertions(+)
> >> >
> >> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> >> > index 8f63a370b8f5..53f9f9674658 100644
> >> > --- a/mm/oom_kill.c
> >> > +++ b/mm/oom_kill.c
> >> > @@ -1381,10 +1381,24 @@ __bpf_kfunc int bpf_out_of_memory(struct mem_cgroup *memcg__nullable,
> >> >       return ret;
> >> >  }
> >> >
> >> > +/**
> >> > + * bpf_task_is_oom_victim - Check if the task has been marked as an OOM victim
> >> > + * @task: task to check
> >> > + *
> >> > + * Returns true if the task has been previously selected by the OOM killer
> >> > + * to be killed. It's expected that the task will be destroyed soon and some
> >> > + * memory will be freed, so maybe no additional actions required.
> >> > + */
> >> > +__bpf_kfunc bool bpf_task_is_oom_victim(struct task_struct *task)
> >> > +{
> >> > +     return tsk_is_oom_victim(task);
> >> > +}
> >>
> >> Why not just do a direct memory read (i.e., task->signal->oom_mm)
> >> within the BPF program? I'm not quite convinced that a BPF kfunc
> >> wrapper for something like tsk_is_oom_victim() is warranted as you can
> >> literally achieve the same semantics without one.
> >
> > +1
> > there is no need for this kfunc.
> 
> It was explicitly asked by Michal Hocko, who is (co)maintaining the oom
> code. I don't have a strong opinion here. I agree that it can be easily
> open-coded without a kfunc, but at the same time the cost of having an
> extra kfunc is not high and it makes the API more consistent.
> 
> Michal, do you feel strongly about having a dedicated kfunc vs the
> direct memory read?

The reason I wanted this an explicit API is that oom states are quite
internal part of the oom synchronization. And I would really like to
have that completely transparent for oom policies. In other words I do
not want to touch all potential oom policies or break them in the worst
case just because we need to change this. So while a trivial interface
now (and hopefully for a long time) it is really an internal thing.

Do I insist? No, I do not but I would like to hear why this is a bad
idea.

-- 
Michal Hocko
SUSE Labs