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

[Nico Pache <npache@redhat.com>]

On Tue, Feb 3, 2026 at 5:09 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Thu, Jan 22, 2026 at 12:28:32PM -0700, Nico Pache wrote:
> > The current mechanism for determining mTHP collapse scales the
> > khugepaged_max_ptes_none value based on the target order. This
> > introduces an undesirable feedback loop, or "creep", when max_ptes_none
> > is set to a value greater than HPAGE_PMD_NR / 2.
> >
> > With this configuration, a successful collapse to order N will populate
> > enough pages to satisfy the collapse condition on order N+1 on the next
> > scan. This leads to unnecessary work and memory churn.
> >
> > To fix this issue introduce a helper function that will limit mTHP
> > collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> > This effectively supports two modes:
> >
> > - max_ptes_none=0: never introduce new none-pages for mTHP collapse.
> > - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
> >   available mTHP order.
> >
> > This removes the possiblilty of "creep", while not modifying any uAPI
> > expectations. A warning will be emitted if any non-supported
> > max_ptes_none value is configured with mTHP enabled.
> >
> > The limits can be ignored by passing full_scan=true, this is useful for
> > madvise_collapse (which ignores limits), or in the case of
> > collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
> > collapse is available.
>
> Thanks, great commit msg!
>
> >
> > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
>
> This LGTM in terms of logic, some nits below, with those addressed feel
> free to add:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Cheers, Lorenzo
>
> > ---
> >  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?

-- Nico

>
> Then instead doing:
>
>         foo(COLLAPSE_MAX_PTES_LIM);
>
> ?
>
> Seems somewhat silly to pass in a boolean that makes it return a set value in
> cases where you know that should be the case at the outset.
>
> > +
> > +     if (is_pmd_order(order))
> > +             return khugepaged_max_ptes_none;
> > +
> > +     /* Zero/non-present collapse disabled. */
> > +     if (!khugepaged_max_ptes_none)
> > +             return 0;
> > +
> > +     if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
>
> Having a define for HPAGE_PMD_NR - 1 would also be handy here...
>
> > +             return (1 << order) - 1;
> > +
> > +     pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
> > +                   HPAGE_PMD_NR - 1);
>
> ...and here.
>
> Also a MICRO nit here - the function returns unsigned int and thus we
> express PTEs in this unit, so maybe use %u rather than %d?
>
> > +     return -EINVAL;
> > +}
>
> Logic of this function looks correct though!
>
> > +
> >  void khugepaged_enter_vma(struct vm_area_struct *vma,
> >                         vm_flags_t vm_flags)
> >  {
> > @@ -548,7 +586,10 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >       int none_or_zero = 0, shared = 0, referenced = 0;
> >       enum scan_result result = SCAN_FAIL;
> >       const unsigned long nr_pages = 1UL << order;
> > -     int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> > +     int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
>
> Yeah, the !cc->is_khugepaged is a bit gross here, so as per the above, maybe do:
>
>         int max_ptes_none;
>
>         if (cc->is_khugepaged)
>                 max_ptes_none = collapse_max_ptes_none(order);
>         else    /* MADV_COLLAPSE is not limited. */
>                 max_ptes_none = COLLAPSE_MAX_PTES_LIM;
>
> > +
> > +     if (max_ptes_none == -EINVAL)
> > +             return result;
> >
> >       for (_pte = pte; _pte < pte + nr_pages;
> >            _pte++, addr += PAGE_SIZE) {
> > --
> > 2.52.0
> >
>