linux-kernel - Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

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Message-ID: <CAA1CXcDgm4EK5sAs6eFa0RiJHaO=H4vCLMKfjZe_GCXvP+c=xw@mail.gmail.com>
Date: Sun, 8 Jun 2025 12:37:56 -0600
From: Nico Pache <npache@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, ryan.roberts@....com, 
	dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled

On Thu, Jun 5, 2025 at 2:01 AM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
> The MADV_COLLAPSE will ignore the system-wide Anon THP sysfs settings, which
> means that even though we have disabled the Anon THP configuration, MADV_COLLAPSE
> will still attempt to collapse into a Anon THP. This violates the rule we have
> agreed upon: never means never.
>
> Another rule for madvise, referring to David's suggestion: “allowing for collapsing
> in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
>
> To address this issue, should check whether the Anon THP configuration is disabled
> in thp_vma_allowable_orders(), even when the TVA_ENFORCE_SYSFS flag is set.
>
> In summary, the current strategy is:
>
> 1. If always & orders == 0, and madvise & orders == 0, and hugepage_global_enabled() == false
> (global THP settings are not enabled), it means mTHP of that orders are prohibited
> from being used, then madvise_collapse() is forbidden for that orders.
>
> 2. If always & orders == 0, and madvise & orders == 0, and hugepage_global_enabled() == true
> (global THP settings are enabled), and inherit & orders == 0, it means mTHP of that
> orders are still prohibited from being used, thus madvise_collapse() is not allowed
> for that orders.
>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  include/linux/huge_mm.h | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2f190c90192d..199ddc9f04a1 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -287,20 +287,35 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>                                        unsigned long orders)
>  {
>         /* 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 (vma_is_anonymous(vma)) {
> +               unsigned long always = READ_ONCE(huge_anon_orders_always);
> +               unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
> +               unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);
> +               unsigned long mask = always | madvise;
> +
> +               /*
> +                * If the system-wide THP/mTHP sysfs settings are disabled,
> +                * then we should never allow hugepages.
> +                */
> +               if (!(mask & orders) && !(hugepage_global_enabled() && (inherit & orders)))
> +                       return 0;
Hi Baolin,

Thanks for making this change so we enforce the 'never' being never
rule. As others have noted, this statement is hard to read-- is there
any improvements we can make to the readability as Lorenzo suggested?

Either way, I've tested this and the functionality works as intended.

Tested-by: Nico Pache <npache@...hat.com>


-- Nico
> +
> +               if (!(tva_flags & TVA_ENFORCE_SYSFS))
> +                       goto skip;
>
> +               mask = always;
>                 if (vm_flags & VM_HUGEPAGE)
> -                       mask |= READ_ONCE(huge_anon_orders_madvise);
> +                       mask |= madvise;
>                 if (hugepage_global_always() ||
>                     ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
> -                       mask |= READ_ONCE(huge_anon_orders_inherit);
> +                       mask |= inherit;
>
>                 orders &= mask;
>                 if (!orders)
>                         return 0;
>         }
>
> +skip:
>         return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
>  }
>
> --
> 2.43.5
>