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 4:08 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/. If
we're aligned on this direction, I'll start working on the patches.

-- 
Regards
Yafang