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

[Baolin Wang]

On 2025/6/9 02:37, Nico Pache wrote:
> On Thu, Jun 5, 2025 at 2:01 AM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   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?

Yes, will do in next version.

> Either way, I've tested this and the functionality works as intended.
>
> Tested-by: Nico Pache <npache@xxxxxxxxxx>

Thanks for testing.