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

[Zi Yan <ziy@nvidia.com>]

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? 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?


>
>>
>> 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


--
Best Regards,
Yan, Zi