Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes_none helper function

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Oct 28, 2025 at 07:36:55AM -0600, Nico Pache wrote:
> On Mon, Oct 27, 2025 at 11:54 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > On Wed, Oct 22, 2025 at 12:37:08PM -0600, 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 caps the max_ptes_none
> > > to HPAGE_PMD_NR / 2 - 1 (255 on 4k page size). The function also scales
> > > the max_ptes_none number by the (PMD_ORDER - target collapse order).
> > >
> > > 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.
> > >
> > > Signed-off-by: Nico Pache <npache@redhat.com>
> > > ---
> > >  mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 34 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 4ccebf5dda97..286c3a7afdee 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -459,6 +459,39 @@ 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, scale down the max_ptes_none proportionally to the folio
> > > + * order, but caps it at HPAGE_PMD_NR/2-1 to prevent a collapse feedback loop.
> > > + *
> > > + * Return: Maximum number of empty PTEs allowed for the collapse operation
> > > + */
> > > +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> > > +{
> > > +     unsigned int max_ptes_none;
> > > +
> > > +     /* ignore max_ptes_none limits */
> > > +     if (full_scan)
> > > +             return HPAGE_PMD_NR - 1;
> > > +
> > > +     if (order == HPAGE_PMD_ORDER)
> > > +             return khugepaged_max_ptes_none;
> > > +
> > > +     max_ptes_none = min(khugepaged_max_ptes_none, HPAGE_PMD_NR/2 - 1);
> >
>
> Hey Lorenzo,
>
> > I mean not to beat a dead horse re: v11 commentary, but I thought we were going
> > to implement David's idea re: the new 'eagerness' tunable, and again we're now just
> > implementing the capping at HPAGE_PMD_NR/2 - 1 thing again?
>
> I spoke to David and he said to continue forward with this series; the
> "eagerness" tunable will take some time, and may require further
> considerations/discussion.

It would be good to communicate this in the patch, I wasn't aware he had said go
ahead with it. Maybe I missed the mail.

Also others might not be aware. When you're explicitly ignoring prior
review from 2 version ago you really do need to spell out why, at least for
civility's sake.

Apologies if there was communication I've forgotten about/missed. But
either way please can we very explicitly communicate these things.

>
> >
> > I'm still really quite uncomfortable with us silently capping this value.
> >
> > If we're putting forward theoretical ideas that are to be later built upon, this
> > series should be an RFC.
> >
> > But if we really intend to silently ignore user input the problem is that then
> > becomes established uAPI.
> >
> > I think it's _sensible_ to avoid this mTHP escalation problem, but the issue is
> > visibility I think.
> >
> > I think people are going to find it odd that you set it to something, but then
> > get something else.
>
> The alternative solution is to not support max_ptes_none for mTHP
> collapse and not allow none/zero pages. This is essentially "capping"
> the value too.

No that alternative equally _silently_ ignores the user-specified tunable,
which is my objection.

The problem you have here is max_ptes_none _defaults_ to a value that
violates the cap for mTHP (511).

So neither solution is workable.

>
> >
> > As an alternative we could have a new sysfs field:
> >
> > /sys/kernel/mm/transparent_hugepage/khugepaged/max_mthp_ptes_none
> >
> > That shows the cap clearly.
> >
> > In fact, it could be read-only... and just expose it to the user. That reduces
> > complexity.
>
> I agree with Baolin here; adding another tunable will only increase
> the complexity for our future goals, and also provides needless
> insight into the internals when they can not be customized.

We already have needless insight into internals with max_pte_none which we can
never, ever remove due to uAPI so that ship has sailed I'm afraid.

I don't personally think adding a read-only view of this data really makes
that much worse.

Also if we do go ahead with eagerness, I expect we are going to want to
have different max_pte_none values for mTHP/non-mTHP.

We _will_ need to convert between max_pte_none and eagerness in some way
(though when eagerness comes along, we can start having 'detent' values,
that is if a use specifies max_ptes_none of 237 we could change it to 128
for instance) and as a result show eagerness _in terms of_ max_pte_none.

Since we _have_ to do this for uAPI reasons, it doesn't seem really that
harmful or adding to complexity to do the equivalent for a _read-only_
field for mTHP.

AFAIC this patch right now is not upstreamable for the simple reason of
violating user expectation (even if that expectation might be silly) and
_silently_ updating max_ptes_none for mTHP.

So this suggestion was designed to try to get us towards something
upstreamable.

So it's not a case of 'sorry I don't like that we can't do it' + we go
ahead with things as they are, it's a case of - we really need to find a
way to do this not-silently or AFAICT, the series is blocked on this until
this is resolved.

Perhaps we should have discussed 'what to do for v12' more on-list and
could have avoided this ahead of time...

Thanks, Lorenzo

>
> Cheers,
> -- Nico
>
> >
> > We can then bring in eagerness later and have the same situation of
> > max_ptes_none being a parameter that exists (plus this additional read-only
> > parameter).
> >
> > > +
> > > +     return max_ptes_none >> (HPAGE_PMD_ORDER - order);
> > > +
> > > +}
> > > +
> > >  void khugepaged_enter_vma(struct vm_area_struct *vma,
> > >                         vm_flags_t vm_flags)
> > >  {
> > > @@ -546,7 +579,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > >       pte_t *_pte;
> > >       int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> > >       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);
> > >
> > >       for (_pte = pte; _pte < pte + nr_pages;
> > >            _pte++, addr += PAGE_SIZE) {
> > > --
> > > 2.51.0
> > >
> >
>