Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP order selection

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Oct 8, 2025 at 4:28 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 08.10.25 10:18, Yafang Shao wrote:
> > On Wed, Oct 8, 2025 at 4:08 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 03.10.25 04:18, Alexei Starovoitov wrote:
> >>> On Mon, Sep 29, 2025 at 10:59 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >>>>
> >>>> +unsigned long bpf_hook_thp_get_orders(struct vm_area_struct *vma,
> >>>> +                                     enum tva_type type,
> >>>> +                                     unsigned long orders)
> >>>> +{
> >>>> +       thp_order_fn_t *bpf_hook_thp_get_order;
> >>>> +       int bpf_order;
> >>>> +
> >>>> +       /* No BPF program is attached */
> >>>> +       if (!test_bit(TRANSPARENT_HUGEPAGE_BPF_ATTACHED,
> >>>> +                     &transparent_hugepage_flags))
> >>>> +               return orders;
> >>>> +
> >>>> +       rcu_read_lock();
> >>>> +       bpf_hook_thp_get_order = rcu_dereference(bpf_thp.thp_get_order);
> >>>> +       if (WARN_ON_ONCE(!bpf_hook_thp_get_order))
> >>>> +               goto out;
> >>>> +
> >>>> +       bpf_order = bpf_hook_thp_get_order(vma, type, orders);
> >>>> +       orders &= BIT(bpf_order);
> >>>> +
> >>>> +out:
> >>>> +       rcu_read_unlock();
> >>>> +       return orders;
> >>>> +}
> >>>
> >>> I thought I explained it earlier.
> >>> Nack to a single global prog approach.
> >>
> >> I agree. We should have the option to either specify a policy globally,
> >> or more refined for cgroups/processes.
> >>
> >> It's an interesting question if a program would ever want to ship its
> >> own policy: I can see use cases for that.
> >>
> >> So I agree that we should make it more flexible right from the start.
> >
> > To achieve per-process granularity, the struct-ops must be embedded
> > within the mm_struct as follows:
> >
> > +#ifdef CONFIG_BPF_MM
> > +struct bpf_mm_ops {
> > +#ifdef CONFIG_BPF_THP
> > +       struct bpf_thp_ops bpf_thp;
> > +#endif
> > +};
> > +#endif
> > +
> >   /*
> >    * Opaque type representing current mm_struct flag state. Must be accessed via
> >    * mm_flags_xxx() helper functions.
> > @@ -1268,6 +1281,10 @@ struct mm_struct {
> >   #ifdef CONFIG_MM_ID
> >                  mm_id_t mm_id;
> >   #endif /* CONFIG_MM_ID */
> > +
> > +#ifdef CONFIG_BPF_MM
> > +               struct bpf_mm_ops bpf_mm;
> > +#endif
> >          } __randomize_layout;
> >
> > We should be aware that this will involve extensive changes in mm/.
>
> That's what we do on linux-mm :)
>
> It would be great to use Alexei's feedback/experience to come up with
> something that is flexible for various use cases.

I'm still not entirely convinced that allowing individual processes or
cgroups to run independent progs is a valid use case. However, since
we have a consensus that this is the right direction, I will proceed
with this approach.

>
> So I think this is likely the right direction.
>
> It would be great to evaluate which scenarios we could unlock with this
> (global vs. per-process vs. per-cgroup) approach, and how
> extensive/involved the changes will be.

1. Global Approach
   - Pros:
     Simple;
     Can manage different THP policies for different cgroups or processes.
  - Cons:
     Does not allow individual processes to run their own BPF programs.

2. Per-Process Approach
    - Pros:
      Enables each process to run its own BPF program.
    - Cons:
      Introduces significant complexity, as it requires handling the
BPF program's lifecycle (creation, destruction, inheritance) within
every mm_struct.

3. Per-Cgroup Approach
    - Pros:
       Allows individual cgroups to run their own BPF programs.
       Less complex than the per-process model, as it can leverage the
existing cgroup operations structure.
    - Cons:
       Creates a dependency on the cgroup subsystem.
       might not be easy to control at the per-process level.

>
> If we need a slot in the bi-weekly mm alignment session to brainstorm,
> we can ask Dave R. for one in the upcoming weeks.

I will draft an RFC to outline the required changes in both the mm/
and bpf/ subsystems and solicit feedback.

--
Regards
Yafang