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Message-ID: <CALOAHbBFNNXHdzp1zNuD530r9ZjpQF__wGWyAdR7oDLvemYSMw@mail.gmail.com>
Date: Wed, 8 Oct 2025 16:18:37 +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 4:08 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/. If
we're aligned on this direction, I'll start working on the patches.

-- 
Regards
Yafang

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