Re: [PATCH mm-unstable v18 10/14] mm/khugepaged: introduce collapse_allowable_orders helper function

[Lorenzo Stoakes <ljs@kernel.org>]

On Sun, May 31, 2026 at 10:18:16PM +0200, David Hildenbrand (Arm) wrote:
> On 5/22/26 17:00, Nico Pache wrote:
> > Add collapse_allowable_orders() to generalize THP order eligibility. The
> > function determines which THP orders are permitted based on collapse
> > context (khugepaged vs madv_collapse).
> >
> > This consolidates collapse configuration logic and provides a clean
> > interface for future mTHP collapse support where the orders may be
> > different.
>
> It would have been good to describe here that, for now, it only ever returns
> PMDs, and that it will be extended next.
>
> Logically, this patch belongs to #12, not #11 ... so seeing it before #11 was a bit
>
> ... and there, it is clear that we don't even want to know the orders?
>
> So can we just call this function
>
> "collapse_possible" and make it return a boolean?

Yeah agreed.

But I also don't love the naming, we now have thp_vma_allowable_orders(),
__thp_vma_allowable_orders(), and then collapse_allowable_orders() and we also
have 3 different ways of collapsing, one of which we call MADV_... COLLAPSE,
and the other khugepaged + fault-in too for laughs.

It's like a big circle of confusion.

And of course we call THP collapse 'collapse' in general, so :)

Anyway I'm fine with collapse_possible() so we can move on and then maybe
cleanup later.

Also - if I look at khugepaged.c I see thp_vma_allowable_orders() still used in
hugepage_vma_revalidate().

Wouldn't it be better then to do the abstraction once mTHP order checking is
properly introduced and change this also and have _every_ order check be
consistent in khugepaged.c?

>
> >
> > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  mm/khugepaged.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 4534025bc81d..64ceebc9d8a7 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -552,12 +552,21 @@ void __khugepaged_enter(struct mm_struct *mm)
> >  		wake_up_interruptible(&khugepaged_wait);
> >  }
> >
> > +/* Check what orders are allowed based on the vma and collapse type */

I'd expand this comment to explain that it's explicitly for accounting for
whether mTHP is used, but that also argues for this to be moved to a later
commit as David says.

Otherwise the comment is useless.

> > +static unsigned long collapse_allowable_orders(struct vm_area_struct *vma,
> > +		vm_flags_t vm_flags, enum tva_type tva_flags)
> > +{
> > +	unsigned long orders = BIT(HPAGE_PMD_ORDER);

Could be a const also.

> > +
> > +	return thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
> > +}
> > +
> >  void khugepaged_enter_vma(struct vm_area_struct *vma,
> >  			  vm_flags_t vm_flags)
> >  {
> >  	if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
> >  	    hugepage_pmd_enabled()) {
> > -		if (thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
> > +		if (collapse_allowable_orders(vma, vm_flags, TVA_KHUGEPAGED))

I hate that we separate out the VMA flags like this just for this case, but
that's something for a follow up probably from me as part of a VMA flags
conversion series...

> >  			__khugepaged_enter(vma->vm_mm);
> >  	}
> >  }
> > @@ -2680,7 +2689,7 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
> >  			cc->progress++;
> >  			break;
> >  		}
> > -		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
> > +		if (!collapse_allowable_orders(vma, vma->vm_flags, TVA_KHUGEPAGED)) {
> >  			cc->progress++;
> >  			continue;
> >  		}
> > @@ -2989,7 +2998,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >  	BUG_ON(vma->vm_start > start);
> >  	BUG_ON(vma->vm_end < end);
> >
> > -	if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))
> > +	if (!collapse_allowable_orders(vma, vma->vm_flags, TVA_FORCED_COLLAPSE))
> >  		return -EINVAL;
> >
> >  	cc = kmalloc_obj(*cc);
>
> Having a simple
>
> static bool collapse_possible(...)
> {
> 	return collapse_allowable_orders(...)
> }
>
> Would make the above slightly more readable.

Yup.

>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>
> --
> Cheers,
>
> David

Cheers, Lorenzo