Message-ID: <CAA1CXcCugu0C3s2V1GcZZC=WksiNy8vkomHCcdvtGKfhxhoyfw@mail.gmail.com>
Date: Fri, 12 Sep 2025 17:26:03 -0600
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH v11 06/15] khugepaged: introduce collapse_max_ptes_none
 helper function

On Fri, Sep 12, 2025 at 7:36 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Thu, Sep 11, 2025 at 09:28:01PM -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).
>
> I would say very clearly that this is only in the mTHP case.

ack, I stole most of the verbiage here from other notes I've
previously written, but it can be improved.

>
>
> >
> > Signed-off-by: Nico Pache <npache@...hat.com>
>
> Hmm I thought we were going to wait for David to investigate different
> approaches to this?
>
> This is another issue with quickly going to another iteration. Though I do think
> David explicitly said he'd come back with a solution?

Sorry I thought that was being done in lockstep. The last version was
about a month ago and I had a lot of changes queued up. Now that we
have collapse_max_pte_none() David has a much easier entry point to
work off :)

I think he will still need this groundwork for the solution he is
working on with "eagerness". if 10 -> 511, and 9 ->255, ..., 0 -> 0.
It will still have to do the scaling. Although I believe 0-10 should
be more like 0-5 mapping to 0,32,64,128,255,511

>
> So I'm not sure why we're seeing this solution here? Unless I'm missing
> something?
>
> > ---
> >  mm/khugepaged.c | 22 +++++++++++++++++++++-
> >  1 file changed, 21 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b0ae0b63fc9b..4587f2def5c1 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -468,6 +468,26 @@ void __khugepaged_enter(struct mm_struct *mm)
> >               wake_up_interruptible(&khugepaged_wait);
> >  }
> >
> > +/* Returns the scaled max_ptes_none for a given order.
>
> We don't start comments at the /*, please use a normal comment format like:
ack
>
> /*
>  * xxxx
>  */
>
> > + * Caps the value to HPAGE_PMD_NR/2 - 1 in the case of mTHP collapse to prevent
>
> This is super unclear.
>
> It start with 'caps the xxx' which seems like you're talking generally.
>
> You should say very clearly 'For PMD allocations we apply the
> khugepaged_max_ptes_none parameter as normal. For mTHP ... [details about mTHP].
ack I will clean this up.
>
> > + * a feedback loop. If max_ptes_none is greater than HPAGE_PMD_NR/2, the value
> > + * would lead to collapses that introduces 2x more pages than the original
> > + * number of pages. On subsequent scans, the max_ptes_none check would be
> > + * satisfied and the collapses would continue until the largest order is reached
> > + */
>
> This is a super vauge explanation. Please describe the issue with creep more
> clearly.
ok I will try to come up with something clearer.
>
> Also aren't we saying that 511 or 0 are the sensible choices? But now somehow
> that's not the case?
Oh I stated I wanted to propose this, and although there was some
pushback I still thought it deserved another attempt. This still
allows for some configurability, and with David's eagerness toggle
this still seems to fit nicely.
>
> You're also not giving a kdoc info on what this returns.
Ok I'll add a kdoc here, why this function in particular, I'm trying
to understand why we dont add kdocs on other functions?
>
> > +static int collapse_max_ptes_none(unsigned int order)
>
> It's a problem that existed already, but khugepaged_max_ptes_none is an unsigned
> int and this returns int.
>
> Maybe we should fix this while we're at it...
ack
>
> > +{
> > +     int max_ptes_none;
> > +
> > +     if (order != HPAGE_PMD_ORDER &&
> > +         khugepaged_max_ptes_none >= HPAGE_PMD_NR/2)
> > +             max_ptes_none = HPAGE_PMD_NR/2 - 1;
> > +     else
> > +             max_ptes_none = khugepaged_max_ptes_none;
> > +     return max_ptes_none >> (HPAGE_PMD_ORDER - order);
> > +
> > +}
> > +
>
> I really don't like this formulation, you're making it unnecessarily unclear and
> now, for the super common case of PMD size, you have to figure out 'oh it's this
> second branch and we're subtracting HPAGE_PMD_ORDER from HPAGE_PMD_ORDER so just
> return khugepaged_max_ptes_none'. When we could... just return it no?
>
> So something like:
>
> #define MAX_PTES_NONE_MTHP_CAP (HPAGE_PMD_NR / 2 - 1)
>
> static unsigned int collapse_max_ptes_none(unsigned int order)
> {
>         unsigned int max_ptes_none_pmd;
>
>         /* PMD-sized THPs behave precisely the same as before. */
>         if (order == HPAGE_PMD_ORDER)
>                 return khugepaged_max_ptes_none;
>
>         /*
>         * Bizarrely, this is expressed in terms of PTEs were this PMD-sized.
>         * For the reasons stated above, we cap this value in the case of mTHP.
>         */
>         max_ptes_none_pmd = MIN(MAX_PTES_NONE_MTHP_CAP,
>                 khugepaged_max_ptes_none);
>
>         /* Apply PMD -> mTHP scaling. */
>         return max_ptes_none >> (HPAGE_PMD_ORDER - order);
> }
yeah that's much cleaner thanks!
>
> >  void khugepaged_enter_vma(struct vm_area_struct *vma,
> >                         vm_flags_t vm_flags)
> >  {
> > @@ -554,7 +574,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >       struct folio *folio = NULL;
> >       pte_t *_pte;
> >       int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> > -     int scaled_max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> > +     int scaled_max_ptes_none = collapse_max_ptes_none(order);
> >       const unsigned long nr_pages = 1UL << order;
> >
> >       for (_pte = pte; _pte < pte + nr_pages;
> > --
> > 2.51.0
> >
>
> Thanks, Lorenzo
>

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