Re: [PATCH] mm/huge_memory: let special huge VMAs bypass the THP policy check

[Lorenzo Stoakes (ARM)]

On Wed, Aug 05, 2026 at 06:21:30PM +0200, Cédric Le Goater wrote:
> Thanks for the quick feedback Lorenzo !
> > So I think it should be something more like the attached.
> >
> > That way all the handling remains the same and the override is applied in
> > the right place plus smaps keeps working.
>
> LGTM, do you want me to respin a v2 with what's below ? or you'd rather
> send it yourself.

Thanks, I think as it's somewhat divergent maybe better if I send it with
Reported-by/Closes if that makes sense to you?

I'd like to get David's input on it first however!

>
> C.
>
>
>
> > ----8<----
> >  From d6537260722c8741586e6295c8eea68d06087efa Mon Sep 17 00:00:00 2001
> > From: "Lorenzo Stoakes (ARM)" <ljs@xxxxxxxxxx>
> > Date: Wed, 5 Aug 2026 11:35:13 +0100
> > Subject: [PATCH] ideas
> >
> > ---
> >   mm/huge_memory.c | 32 +++++++++++++++++++++++++++++---
> >   1 file changed, 29 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index abc65d608c23..5fa01364f089 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -111,6 +111,34 @@ static bool vma_is_special_huge(const struct vm_area_struct *vma)
> >   	return vma_test_any(vma, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT);
> >   }
> >
> > +static bool should_obey_thp_file_tunables(const struct vm_area_struct *vma,
> > +					  bool forced_collapse)
> > +{
> > +	if (forced_collapse)
> > +		return false;
> > +	VM_WARN_ON_ONCE(vma_is_anonymous(vma));
> > +	/* Huge PFN mappings allocate no folios so the policy doesn't apply. */
> > +	if (vma_test(vma, VMA_PFNMAP_BIT) && vma->vm_ops->huge_fault)
> > +		return false;
> > +	return true;
> > +}
> > +
> > +static bool can_thp_collapse_file(const struct vm_area_struct *vma,
> > +		vm_flags_t vm_flags, bool forced_collapse)
> > +{
> > +	/* Override THP tunables? */
> > +	if (!should_obey_thp_file_tunables(vma, forced_collapse))
> > +		return true;
> > +	/* THP=always? */
> > +	if (hugepage_global_always())
> > +		return true;
> > +	/* THP=madvise? */
> > +	if (!hugepage_global_enabled())
> > +		return false;
> > +	/* Has VMA had madvise(..., MADV_HUGEPAGE) applied to it? */
> > +	return vm_flags & VM_HUGEPAGE;
> > +}
> > +
> >   unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> >   					 vm_flags_t vm_flags,
> >   					 enum tva_type type,
> > @@ -188,9 +216,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> >   		 * Enforce THP collapse requirements as necessary. Anonymous vmas
> >   		 * were already handled in thp_vma_allowable_orders().
> >   		 */
> > -		if (!forced_collapse &&
> > -		    (!hugepage_global_enabled() || (!(vm_flags & VM_HUGEPAGE) &&
> > -						    !hugepage_global_always())))
> > +		if (!can_thp_collapse_file(vma, vm_flags, forced_collapse))
> >   			return 0;
> >
> >   		/*
> > --
> > 2.55.0
> >
>

--
Cheers, Lorenzo