Re: [PATCH v7 mm-new 02/10] mm: thp: add support for BPF based THP order selection

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Sep 25, 2025 at 6:06 PM Lance Yang <lance.yang@linux.dev> wrote:
>
> On Wed, Sep 10, 2025 at 10:53 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > This patch introduces a new BPF struct_ops called bpf_thp_ops for dynamic
> > THP tuning. It includes a hook bpf_hook_thp_get_order(), allowing BPF
> > programs to influence THP order selection based on factors such as:
> > - Workload identity
> >   For example, workloads running in specific containers or cgroups.
> > - Allocation context
> >   Whether the allocation occurs during a page fault, khugepaged, swap or
> >   other paths.
> > - VMA's memory advice settings
> >   MADV_HUGEPAGE or MADV_NOHUGEPAGE
> > - Memory pressure
> >   PSI system data or associated cgroup PSI metrics
> >
> > The kernel API of this new BPF hook is as follows,
> >
> > /**
> >  * @thp_order_fn_t: Get the suggested THP orders from a BPF program for allocation
> >  * @vma: vm_area_struct associated with the THP allocation
> >  * @vma_type: The VMA type, such as BPF_THP_VM_HUGEPAGE if VM_HUGEPAGE is set
> >  *            BPF_THP_VM_NOHUGEPAGE if VM_NOHUGEPAGE is set, or BPF_THP_VM_NONE if
> >  *            neither is set.
> >  * @tva_type: TVA type for current @vma
> >  * @orders: Bitmask of requested THP orders for this allocation
> >  *          - PMD-mapped allocation if PMD_ORDER is set
> >  *          - mTHP allocation otherwise
> >  *
> >  * Return: The suggested THP order from the BPF program for allocation. It will
> >  *         not exceed the highest requested order in @orders. Return -1 to
> >  *         indicate that the original requested @orders should remain unchanged.
> >  */
> > typedef int thp_order_fn_t(struct vm_area_struct *vma,
> >                            enum bpf_thp_vma_type vma_type,
> >                            enum tva_type tva_type,
> >                            unsigned long orders);
> >
> > Only a single BPF program can be attached at any given time, though it can
> > be dynamically updated to adjust the policy. The implementation supports
> > anonymous THP, shmem THP, and mTHP, with future extensions planned for
> > file-backed THP.
> >
> > This functionality is only active when system-wide THP is configured to
> > madvise or always mode. It remains disabled in never mode. Additionally,
> > if THP is explicitly disabled for a specific task via prctl(), this BPF
> > functionality will also be unavailable for that task.
> >
> > This feature requires CONFIG_BPF_GET_THP_ORDER (marked EXPERIMENTAL) to be
> > enabled. Note that this capability is currently unstable and may undergo
> > significant changes—including potential removal—in future kernel versions.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
>
> I've tested this patch on my machine, and it works as expected. Using BPF
> hooks to control THP is a great step forward!
>
> Tested-by: Lance Yang <lance.yang@linux.dev>

Thanks for your test. I will post a new version ASAP.

>
> This work also inspires some ideas for another useful hook for THP that I
> might propose in the future, once this series is settled and merged ;)

Excited to see it!

-- 
Regards
Yafang