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 7:27 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 8 Oct 2025, at 5:04, Yafang Shao wrote:
>
> > 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.
>
> Another issue is that how and who to deal with hierarchical cgroup, where one
> cgroup is a parent of another. Should bpf program to do that or mm code
> to do that?

The cgroup subsystem handles this propagation automatically. When a
BPF program is attached to a cgroup via cgroup_bpf_attach(), it's
automatically inherited by all descendant cgroups.

Note: struct-ops programs aren't supported by cgroup_bpf_attach(),
requiring us to build new attachment mechanisms for cgroup-based
struct-ops.

> I remember hierarchical cgroup is the main reason THP control
> at cgroup level is rejected. If we do per-cgroup bpf control, wouldn't we
> get the same rejection from cgroup folks?

Right, it was rejected by the cgroup maintainers [0]

[0]. https://lore.kernel.org/linux-mm/20241030150851.GB706616@cmpxchg.org/

-- 
Regards
Yafang