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

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Message-ID: <7723a2c7-3750-44f7-9eb5-4ef64b64fbb8@redhat.com>
Date: Wed, 8 Oct 2025 10:28:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Yafang Shao <laoar.shao@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, ziy@...dia.com,
 baolin.wang@...ux.alibaba.com, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Liam Howlett <Liam.Howlett@...cle.com>, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, Johannes Weiner
 <hannes@...xchg.org>, usamaarif642@...il.com,
 gutierrez.asier@...wei-partners.com, Matthew Wilcox <willy@...radead.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Amery Hung <ameryhung@...il.com>,
 David Rientjes <rientjes@...gle.com>, Jonathan Corbet <corbet@....net>,
 21cnbao@...il.com, Shakeel Butt <shakeel.butt@...ux.dev>,
 Tejun Heo <tj@...nel.org>, lance.yang@...ux.dev,
 Randy Dunlap <rdunlap@...radead.org>, bpf <bpf@...r.kernel.org>,
 linux-mm <linux-mm@...ck.org>,
 "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP
 order selection

On 08.10.25 10:18, Yafang Shao wrote:
> On Wed, Oct 8, 2025 at 4:08 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 03.10.25 04:18, Alexei Starovoitov wrote:
>>> On Mon, Sep 29, 2025 at 10:59 PM Yafang Shao <laoar.shao@...il.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.

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.

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.

-- 
Cheers

David / dhildenb