Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP order selection
From: David Hildenbrand
Date: Wed Oct 08 2025 - 08:07:29 EST
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@xxxxxxxxxx> wrote:
On 08.10.25 10:18, Yafang Shao wrote:
On Wed, Oct 8, 2025 at 4:08 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 03.10.25 04:18, Alexei Starovoitov wrote:
On Mon, Sep 29, 2025 at 10:59 PM Yafang Shao <laoar.shao@xxxxxxxxx> 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.
Focusing on processes instead of cgroups might be easier initially.
--
Cheers
David / dhildenb