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

[Michal Hocko <mhocko@suse.com>]

On Tue 03-02-26 08:31:19, Alexei Starovoitov wrote:
> On Tue, Feb 3, 2026 at 5:23 AM Michal Hocko <mhocko@suse.com> 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.

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

performance is not really crucial in this context. We are OOM, couple of
loads vs. registers will not make much difference. It is really more
about code writers what they can/should be using. OOM is a piece of
complex code with many loose ends that might not be obvious.

> 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?

-- 
Michal Hocko
SUSE Labs