linux-kernel - Re: [PATCH bpf-next v3 10/17] mm: introduce bpf_task_is_oom

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Message-ID: <CAADnVQ+uF4cTVYYOYBEDWrkLXJJkru_FaRttFskHRUpamktMJQ@mail.gmail.com>
Date: Tue, 3 Feb 2026 08:31:19 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, Matt Bobrowski <mattbobrowski@...gle.com>, 
	bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, JP Kobryn <inwardvessel@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH bpf-next v3 10/17] mm: introduce bpf_task_is_oom_victim() kfunc

On Tue, Feb 3, 2026 at 5:23 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 02-02-26 16:14:37, Roman Gushchin wrote:
> > Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> >
> > > On Sun, Feb 1, 2026 at 9:39 PM Matt Bobrowski <mattbobrowski@...gle.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@...ux.dev>
> > >> > Suggested-by: Michal Hocko <mhocko@...e.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.

It's a bad idea, since it doesn't address your goal.
bpf prog can access task->signal->oom_mm without kfunc just fine
and it will be doing so because performance matters and
static inline bool foo(task)
{
  return task->signal->oom_mm;
}

will be inlined as 2 loads while kfunc is a function call with 6 registers
being scratched.

If anything changes and, say, oom_mm will get renamed whether
it was kfunc or not doesn't change much. progs will adopt to a new
way easily with CORE. kfuncs can also be renamed/deleted, etc.
You're thinking about kfuncs as a stable api. It's definitely not.
It's not a layer of isolation either. kfuncs are necessary only
for the cases where bpf prog cannot do it on its own.

"internal thing" is also a wrong way of thinking of bpf-oom.
bpf-oom _will_ look into oom, cgroup and kernel internals in general.
All bpf progs do because they have to do that to achieve their goals.
Everything in mm/internal.h have been available to access by bpf progs
for a decade now. Did it cause any issue to mm development? No.
So let's not build some non-existent wall or "internal oom thing".