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Message-ID: <CALOAHbD_tRSyx1LXKfFrUriH6BcRS6Hw9N1=KddCJpgXH8vZug@mail.gmail.com>
Date: Wed, 8 Oct 2025 17:04:44 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: David Hildenbrand <david@...hat.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 Wed, Oct 8, 2025 at 4:28 PM David Hildenbrand <david@...hat.com> wrote:
>
> 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.
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
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