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

[Alexei Starovoitov]

On Wed, Feb 4, 2026 at 1:02 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 03-02-26 08:31:19, Alexei Starovoitov wrote:
> > On Tue, Feb 3, 2026 at 5:23 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Mon 02-02-26 16:14:37, Roman Gushchin wrote:
> [...]
> > > > 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;
> > }
>
> OK, so my understanding was that BPF can only use exported
> functionality. If those progs can access whatever they get a pointer for
> and than traverse down the road then this is moot from a large part.

bpf could access all kernel internals from day one 10 years ago.
We made it more ergonomic over the years.

> > 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.
>
> It is obviously not clear to me where that line is for BPF progs. Where
> is this documented?

See Documentation/bpf/kfuncs.rst
Especially "kfunc lifecycle expectations" section.