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

[David Hildenbrand]

On 12.06.25 10:46, Dev Jain wrote:
> On 12/06/25 1:21 pm, Baolin Wang wrote:
> > On 2025/6/11 20:34, David Hildenbrand wrote:
> > > On 05.06.25 10:00, Baolin Wang 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;
> > >
> > > I'm still trying to digest that. Isn't there a way for us to work
> > > with the orders,
> > > essentially masking off all orders that are forbidden globally.
> > > Similar to below, if !orders, then return 0?
> > > /* Orders disabled directly. */
> > > orders &= ~TODO;
> > > /* Orders disabled by inheriting from the global toggle. */
> > > if (!hugepage_global_enabled())
> > >       orders &= ~READ_ONCE(huge_anon_orders_inherit);
> > >
> > > TODO is probably a -1ULL and then clearing always/madvise/inherit.
> > > Could add a simple helper for that
> > >
> > > huge_anon_orders_never
> >
> > I followed Lorenzo's suggestion to simplify the logic. Does that look
> > more readable?
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 2f190c90192d..3087ac7631e0 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -265,6 +265,43 @@ unsigned long __thp_vma_allowable_orders(struct
> > vm_area_struct *vma,
> >                                           unsigned long tva_flags,
> >                                           unsigned long orders);
> >
> > +/* Strictly mask requested anonymous orders according to sysfs
> > settings. */
> > +static inline unsigned long __thp_mask_anon_orders(unsigned long
> > vm_flags,
> > +                               unsigned long tva_flags, unsigned long
> > orders)
> > +{
> > +       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);
> > +       bool inherit_enabled = hugepage_global_enabled();
> > +       bool has_madvise =  vm_flags & VM_HUGEPAGE;
>
> Let us name this honour_madvise to indicate that this VMA must honour
> the madvise setting.

Not clear, because there is also "VM_NOHUGEPAGE" :/

See my reply where we maybe (if I got it right) only need a single check 
and can just avoid a confusing temporary variable completely.

--
Cheers,

David / dhildenb