Re: [PATCH mm-unstable v14 07/16] khugepaged: introduce collapse_max_ptes_none helper function

[Lorenzo Stoakes]

On Fri, Feb 06, 2026 at 10:44:03AM -0700, Nico Pache wrote:
> On Tue, Feb 3, 2026 at 5:09 AM Lorenzo Stoakes
> > > ---
> > >  mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 42 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 0f68902edd9a..9b7e05827749 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -460,6 +460,44 @@ void __khugepaged_enter(struct mm_struct *mm)
> > >               wake_up_interruptible(&khugepaged_wait);
> > >  }
> > >
> > > +/**
> > > + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
> > > + * @order: The folio order being collapsed to
> > > + * @full_scan: Whether this is a full scan (ignore limits)
> > > + *
> > > + * For madvise-triggered collapses (full_scan=true), all limits are bypassed
> > > + * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
> > > + *
> > > + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured
> > > + * khugepaged_max_ptes_none value.
> > > + *
> > > + * For mTHP collapses, we currently only support khugepaged_max_pte_none values
> > > + * of 0 or (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP
> > > + * collapse will be attempted
> > > + *
> > > + * Return: Maximum number of empty PTEs allowed for the collapse operation
> > > + */
> > > +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> > > +{
> > > +     /* ignore max_ptes_none limits */
> > > +     if (full_scan)
> > > +             return HPAGE_PMD_NR - 1;
> >
> > I wonder if, given we are effectively doing:
> >
> >         const unsigned int nr_pages = collapse_max_ptes_none(order, /*full_scan=*/true);
> >
> >         ...
> >
> >         foo(nr_pages);
> >
> > In places where we ignore limits, whether we would be better off putting
> > HPAGE_PMD_NR - 1 into a define and just using that in these cases, like:
> >
> > #define COLLAPSE_MAX_PTES_LIM (HPAGE_PMD_NR - 1)
>
> Would a shorter name be appropriate? COLLAPSE_MAX_PTES_LIM(IT) is
> quite long. Can we call it MAX_PTES_LIMIT or KHUGE_MAX_PTES_LIM?

Yeah sure re: shorter/better name :) to be fair my suggestion is pretty
terrible, kinda just getting at the notion of there being _some_ define.

But MAX_PTES_LIMIT or KHUGE_MAX_PTES_LIM I think are unclear.

MAX_COLLAPSE_PTES?

Cheers, Lorenzo