Message-ID: <CALOAHbAY9sjG-M=nwWRdbp3_m2cx_YJCb7DToaXn-kHNV+A5Zg@mail.gmail.com>
Date: Wed, 8 Oct 2025 21:11:39 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Zi Yan <ziy@...dia.com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	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, 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 8:07 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 08.10.25 13:27, Zi Yan wrote:
> > On 8 Oct 2025, at 5:04, Yafang Shao wrote:
> >
> >> 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.
> >
> > 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?
>
> Valid point.
>
> I do wonder if that problem was already encountered elsewhere with bpf
> and if there is already a solution.

Our standard is to run only one instance of a BPF program type
system-wide to avoid conflicts. For example, we can't have both
systemd and a container runtime running bpf-thp simultaneously.

Perhaps Alexei can enlighten us, though we'd need to read between his
characteristically brief lines. ;-)

>
> Focusing on processes instead of cgroups might be easier initially.


-- 
Regards
Yafang

Powered by blists - more mailing lists