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

[Lorenzo Stoakes (ARM)]

On Thu, Aug 06, 2026 at 02:44:10AM +0100, Matthew Wilcox wrote:
> On Wed, Aug 05, 2026 at 11:41:46AM +0100, Lorenzo Stoakes (ARM) wrote:
> > 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;
> >
> >  		/*
>
> I think 'thp_' in all of this is confusing.  DAX isn't THPs.  PFNMAPs
> aren't THPa.  There's other reasons to implement huge_fault that aren't
> THPs.  Can we rename all of this to drop the 'thp_' string, starting
> with thp_vma_allowable_order()?

Matthew, are you baiting the churnmeister 3000 into a refactor here? :)

I despise this function and agree with you entirely.

So maybe I could add a few other patches first before the change to improve
the horror show that is this function and its auxiliaries also :)

--
Cheers, Lorenzo