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

[Nico Pache <npache@redhat.com>]

On Tue, Oct 28, 2025 at 8:10 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/10/29 02:59, Lorenzo Stoakes wrote:
> > On Tue, Oct 28, 2025 at 07:08:38PM +0100, David Hildenbrand wrote:
> >>
> >>>>> 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.
> >>>>
> >>>> Right, after talking to Johannes it got clearer that what we envisioned with
> >>>
> >>> I'm not sure that you meant to say go ahead with the series as-is with this
> >>> silent capping?
> >>
> >> No, "go ahead" as in "let's find some way forward that works for all and is
> >> not too crazy".
> >
> > Right we clearly needed to discuss that further at the time but that's moot now,
> > we're figuring it out now :)
> >
> >>
> >> [...]
> >>
> >>>> "eagerness" would not be like swappiness, and we will really have to be
> >>>> careful here. I don't know yet when I will have time to look into that.
> >>>
> >>> I guess I missed this part of the converastion, what do you mean?
> >>
> >> Johannes raised issues with that on the list and afterwards we had an
> >> offline discussion about some of the details and why something unpredictable
> >> is not good.
> >
> > Could we get these details on-list so we can discuss them? This doesn't have to
> > be urgent, but I would like to have a say in this or at least be part of the
> > converastion please.
> >
> >>
> >>>
> >>> The whole concept is that we have a paramaeter whose value is _abstracted_ and
> >>> which we control what it means.
> >>>
> >>> I'm not sure exactly why that would now be problematic? The fundamental concept
> >>> seems sound no? Last I remember of the conversation this was the case.
> >>
> >> The basic idea was to do something abstracted as swappiness. Turns out
> >> "swappiness" is really something predictable, not something we can randomly
> >> change how it behaves under the hood.
> >>
> >> So we'd have to find something similar for "eagerness", and that's where it
> >> stops being easy.
> >
> > I think we shouldn't be too stuck on
> >
> >>
> >>>
> >>>>
> >>>> If we want to avoid the implicit capping, I think there are the following
> >>>> possible approaches
> >>>>
> >>>> (1) Tolerate creep for now, maybe warning if the user configures it.
> >>>
> >>> I mean this seems a viable option if there is pressure to land this series
> >>> before we have a viable uAPI for configuring this.
> >>>
> >>> A part of me thinks we shouldn't rush series in for that reason though and
> >>> should require that we have a proper control here.
> >>>
> >>> But I guess this approach is the least-worst as it leaves us with the most
> >>> options moving forwards.
> >>
> >> Yes. There is also the alternative of respecting only 0 / 511 for mTHP
> >> collapse for now as discussed in the other thread.
> >
> > Yes I guess let's carry that on over there.
> >
> > I mean this is why I said it's better to try to keep things in one thread :) but
> > anyway, we've forked and can't be helped now.
> >
> > To be clear that was a criticism of - email development - not you.
> >
> > It's _extremely easy_ to have this happen because one thread naturally leads to
> > a broader discussion of a given topic, whereas another has questions from
> > somebody else about the same topic, to which people reply and then... you have a
> > fork and it can't be helped.
> >
> > I guess I'm saying it'd be good if we could say 'ok let's move this to X'.
> >
> > But that's also broken in its own way, you can't stop people from replying in
> > the other thread still and yeah. It's a limitation of this model :)
> >
> >>
> >>>
> >>>> (2) Avoid creep by counting zero-filled pages towards none_or_zero.
> >>>
> >>> Would this really make all that much difference?
> >>
> >> It solves the creep problem I think, but it's a bit nasty IMHO.
> >
> > Ah because you'd end up wtih a bunch of zeroed pages from the prior mTHP
> > collapses, interesting...
> >
> > Scanning for that does seem a bit nasty though yes...
> >
> >>
> >>>
> >>>> (3) Have separate toggles for each THP size. Doesn't quite solve the
> >>>>       problem, only shifts it.
> >>>
> >>> Yeah I did wonder about this as an alternative solution. But of course it then
> >>> makes it vague what the parent values means in respect of the individual levels,
> >>> unless we have an 'inherit' mode there too (possible).
> >>>
> >>> It's going to be confusing though as max_ptes_none sits at the root khugepaged/
> >>> level and I don't think any other parameter from khugepaged/ is exposed at
> >>> individual page size levels.
> >>>
> >>> And of course doing this means we
> >>>
> >>>>
> >>>> Anything else?
> >>>
> >>> Err... I mean I'm not sure if you missed it but I suggested an approach in the
> >>> sub-thread - exposing mthp_max_ptes_none as a _READ-ONLY_ field at:
> >>>
> >>> /sys/kernel/mm/transparent_hugepage/khugepaged/max_mthp_ptes_none
> >>>
> >>> Then we allow the capping, but simply document that we specify what the capped
> >>> value will be here for mTHP.
> >>
> >> I did not have time to read the details on that so far.
> >
> > OK. It is a bit nasty, yes. The idea is to find something that allows the
> > capping to work.
> >
> >>
> >> It would be one solution forward. I dislike it because I think the whole
> >> capping is an intermediate thing that can be (and likely must be, when
> >> considering mTHP underused shrinking I think) solved in the future
> >> differently. That's why I would prefer adding this only if there is no
> >> other, simpler, way forward.
> >
> > Yes I agree that if we could avoid it it'd be great.
> >
> > Really I proposed this solution on the basis that we were somehow ok with the
> > capping.
> >
> > If we can avoid that'd be ideal as it reduces complexity and 'unexpected'
> > behaviour.
> >
> > We'll clarify on the other thread, but the 511/0 was compelling to me before as
> > a simplification, and if we can have a straightforward model of how mTHP
> > collapse across none/zero page PTEs behaves this is ideal.
> >
> > The only question is w.r.t. warnings etc. but we can handle details there.
> >
> >>
> >>>
> >>> That struck me as the simplest way of getting this series landed without
> >>> necessarily violating any future eagerness which:
> >>>
> >>> a. Must still support khugepaged/max_ptes_none - we aren't getting away from
> >>>      this, it's uAPI.
> >>>
> >>> b. Surely must want to do different things for mTHP in eagerness, so if we're
> >>>      exposing some PTE value in max_ptes_none doing so in
> >>>      khugepaged/mthp_max_ptes_none wouldn't be problematic (note again - it's
> >>>      readonly so unlike max_ptes_none we don't have to worry about the other
> >>>      direction).
> >>>
> >>> HOWEVER, eagerness might want want to change this behaviour per-mTHP size, in
> >>> which case perhaps mthp_max_ptes_none would be problematic in that it is some
> >>> kind of average.
> >>>
> >>> Then again we could always revert to putting this parameter as in (3) in that
> >>> case, ugly but kinda viable.
> >>>
> >>>>
> >>>> IIUC, creep is less of a problem when we have the underused shrinker
> >>>> enabled: whatever we over-allocated can (unless longterm-pinned etc) get
> >>>> reclaimed again.
> >>>>
> >>>> So maybe having underused-shrinker support for mTHP as well would be a
> >>>> solution to tackle (1) later?
> >>>
> >>> How viable is this in the short term?
> >>
> >> I once started looking into it, but it will require quite some work, because
> >> the lists will essentially include each and every (m)THP in the system ...
> >> so i think we will need some redesign.
> >
> > Ack.
> >
> > This aligns with non-0/511 settings being non-functional for mTHP atm anyway.
> >
> >>
> >>>
> >>> Another possible solution:
> >>>
> >>> If mthp_max_ptes_none is not workable, we could have a toggle at, e.g.:
> >>>
> >>> /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_cap_collapse_none
> >>>
> >>> As a simple boolean. If switched on then we document that it caps mTHP as
> >>> per Nico's suggestion.
> >>>
> >>> That way we avoid the 'silent' issue I have with all this and it's an
> >>> explicit setting.
> >>
> >> Right, but it's another toggle I wish we wouldn't need. We could of course
> >> also make it some compile-time option, but not sure if that's really any
> >> better.
> >>
> >> I'd hope we find an easy way forward that doesn't require new toggles, at
> >> least for now ...
> >
> > Right, well I agree if we can make this 0/511 thing work, let's do that.
> >
> > Toggle are just 'least worst' workarounds on assumption of the need for capping.
>
> I finally finished reading through the discussions across multiple
> threads:), and it looks like we've reached a preliminary consensus (make
> 0/511 work). Great and thanks!
>
> IIUC, the strategy is, configuring it to 511 means always enabling mTHP
> collapse, configuring it to 0 means collapsing mTHP only if all PTEs are
> non-none/zero, and for other values, we issue a warning and prohibit
> mTHP collapse (avoid Lorenzo's concern about silently changing
> max_ptes_none). Then the implementation for collapse_max_ptes_none()
> should be as follows:
>
> static int collapse_max_ptes_none(unsigned int order, bool full_scan)
> {
>          /* ignore max_ptes_none limits */
>          if (full_scan)
>                  return HPAGE_PMD_NR - 1;
>
>          if (order == HPAGE_PMD_ORDER)
>                  return khugepaged_max_ptes_none;
>
>          /*
>           * To prevent creeping towards larger order collapses for mTHP
> collapse,
>           * we restrict khugepaged_max_ptes_none to only 511 or 0,
> simplifying the
>           * logic. This means:
>           * max_ptes_none == 511 -> collapse mTHP always
>           * max_ptes_none == 0 -> collapse mTHP only if we all PTEs are
> non-none/zero
>           */
>          if (!khugepaged_max_ptes_none || khugepaged_max_ptes_none ==
> HPAGE_PMD_NR - 1)
>                  return khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER -
> order);
>
>          pr_warn_once("mTHP collapse only supports
> khugepaged_max_ptes_none configured as 0 or %d\n", HPAGE_PMD_NR - 1);
>          return -EINVAL;
> }
>
> So what do you think?

Yes i'm glad we finally came to some consensus, despite it being a
less than ideal solution.

Hopefully the eagerness patchset re-introduces all the lost
functionality in the future.

Cheers
-- Nico

>