Re: [PATCH v6 1/4] mm: defer THP insertion to khugepaged

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

On Thu, May 15, 2025 at 12:39 PM Nico Pache <npache@redhat.com> wrote:
>
> setting /transparent_hugepages/enabled=always allows applications
> to benefit from THPs without having to madvise. However, the page fault
> handler takes very few considerations to decide weather or not to actually
> use a THP. This can lead to a lot of wasted memory. khugepaged only
> operates on memory that was either allocated with enabled=always or
> MADV_HUGEPAGE.
>
> Introduce the ability to set enabled=defer, which will prevent THPs from
> being allocated by the page fault handler unless madvise is set,
> leaving it up to khugepaged to decide which allocations will collapse to a
> THP. This should allow applications to benefits from THPs, while curbing
> some of the memory waste.
>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Co-developed-by: Rafael Aquini <raquini@redhat.com>
> Signed-off-by: Rafael Aquini <raquini@redhat.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  include/linux/huge_mm.h | 15 +++++++++++++--
>  mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
>  2 files changed, 40 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e3d15c737008..02038e3db829 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -48,6 +48,7 @@ enum transparent_hugepage_flag {
>         TRANSPARENT_HUGEPAGE_UNSUPPORTED,
>         TRANSPARENT_HUGEPAGE_FLAG,
>         TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> +       TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG,
>         TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
>         TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
>         TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG,
> @@ -186,6 +187,7 @@ static inline bool hugepage_global_enabled(void)
>  {
>         return transparent_hugepage_flags &
>                         ((1<<TRANSPARENT_HUGEPAGE_FLAG) |
> +                       (1<<TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG) |
>                         (1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG));
>  }
>
> @@ -195,6 +197,12 @@ static inline bool hugepage_global_always(void)
>                         (1<<TRANSPARENT_HUGEPAGE_FLAG);
>  }
>
> +static inline bool hugepage_global_defer(void)
> +{
> +       return transparent_hugepage_flags &
> +                       (1<<TRANSPARENT_HUGEPAGE_DEFER_PF_FLAG);
> +}
> +
>  static inline int highest_order(unsigned long orders)
>  {
>         return fls_long(orders) - 1;
> @@ -291,13 +299,16 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>                                        unsigned long tva_flags,
>                                        unsigned long orders)
>  {
> +       if ((tva_flags & TVA_IN_PF) && hugepage_global_defer() &&
> +                       !(vm_flags & VM_HUGEPAGE))
> +               return 0;
> +
>         /* Optimization to check if required orders are enabled early. */
>         if ((tva_flags & TVA_ENFORCE_SYSFS) && vma_is_anonymous(vma)) {
>                 unsigned long mask = READ_ONCE(huge_anon_orders_always);
> -
>                 if (vm_flags & VM_HUGEPAGE)
>                         mask |= READ_ONCE(huge_anon_orders_madvise);
> -               if (hugepage_global_always() ||
> +               if (hugepage_global_always() || hugepage_global_defer() ||
>                     ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
>                         mask |= READ_ONCE(huge_anon_orders_inherit);
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 700988a0d5cf..ce0ee74753af 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -297,12 +297,15 @@ static ssize_t enabled_show(struct kobject *kobj,
>         const char *output;
>
>         if (test_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags))
> -               output = "[always] madvise never";
> +               output = "[always] madvise defer never";

a small nit: alphabetical ordering might improve readability here.

-- 
Regards
Yafang