Message-ID: <CAA1CXcBDq9PucQdfQRh1iqJLPB6Jn6mNy28v_AuHWb9kz1gpqQ@mail.gmail.com>
Date: Thu, 21 Aug 2025 10:46:18 -0600
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support

On Thu, Aug 21, 2025 at 9:40 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Thu, Aug 21, 2025 at 09:27:19AM -0600, Nico Pache wrote:
> > On Thu, Aug 21, 2025 at 9:25 AM Nico Pache <npache@...hat.com> wrote:
> > >
> > > On Thu, Aug 21, 2025 at 9:20 AM Lorenzo Stoakes
> > > <lorenzo.stoakes@...cle.com> wrote:
> > > >
> > > > On Thu, Aug 21, 2025 at 08:43:18PM +0530, Dev Jain wrote:
> > > > >
> > > > > On 21/08/25 8:31 pm, Lorenzo Stoakes wrote:
> > > > > > OK so I noticed in patch 13/13 (!) where you change the documentation that you
> > > > > > essentially state that the whole method used to determine the ratio of PTEs to
> > > > > > collapse to mTHP is broken:
> > > > > >
> > > > > >     khugepaged uses max_ptes_none scaled to the order of the enabled
> > > > > >     mTHP size to determine collapses. When using mTHPs it's recommended
> > > > > >     to set max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (255
> > > > > >     on 4k page size). This will prevent undesired "creep" behavior that
> > > > > >     leads to continuously collapsing to the largest mTHP size; when we
> > > > > >     collapse, we are bringing in new non-zero pages that will, on a
> > > > > >     subsequent scan, cause the max_ptes_none check of the +1 order to
> > > > > >     always be satisfied. By limiting this to less than half the current
> > > > > >     order, we make sure we don't cause this feedback
> > > > > >     loop. max_ptes_shared and max_ptes_swap have no effect when
> > > > > >     collapsing to a mTHP, and mTHP collapse will fail on shared or
> > > > > >     swapped out pages.
> > > > > >
> > > > > > This seems to me to suggest that using
> > > > > > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none as some means
> > > > > > of establishing a 'ratio' to do this calculation is fundamentally flawed.
> > > > > >
> > > > > > So surely we ought to introduce a new sysfs tunable for this? Perhaps
> > > > > >
> > > > > > /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio
> > > > > >
> > > > > > Or something like this?
> > > > > >
> > > > > > It's already questionable that we are taking a value that is expressed
> > > > > > essentially in terms of PTE entries per PMD and then use it implicitly to
> > > > > > determine the ratio for mTHP, but to then say 'oh but the default value is
> > > > > > known-broken' is just a blocker for the series in my opinion.
> > > > > >
> > > > > > This really has to be done a different way I think.
> > > > > >
> > > > > > Cheers, Lorenzo
> > > > >
> > > > > FWIW this was my version of the documentation patch:
> > > > > https://lore.kernel.org/all/20250211111326.14295-18-dev.jain@arm.com/
> > > > >
> > > > > The discussion about the creep problem started here:
> > > > > https://lore.kernel.org/all/7098654a-776d-413b-8aca-28f811620df7@arm.com/
> > > > >
> > > > > and the discussion continuing here:
> > > > > https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com/
> > > > >
> > > > > ending with a summary I gave here:
> > > > > https://lore.kernel.org/all/8114d47b-b383-4d6e-ab65-a0e88b99c873@arm.com/
> > > > >
> > > > > This should help you with the context.
> > > > >
> > > > >
> > > >
> > > > Thanks and I"ll have a look, but this series is unmergeable with a broken
> > > > default in
> > > > /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio
> > > > sorry.
> > > >
> > > > We need to have a new tunable as far as I can tell. I also find the use of
> > > > this PMD-specific value as an arbitrary way of expressing a ratio pretty
> > > > gross.
> > > The first thing that comes to mind is that we can pin max_ptes_none to
> > > 255 if it exceeds 255. It's worth noting that the issue occurs only
> > > for adjacently enabled mTHP sizes.
>
> No! Presumably the default of 511 (for PMDs with 512 entries) is set for a
> reason, arbitrarily changing this to suit a specific case seems crazy no?
We wouldn't be changing it for PMD collapse, just for the new
behavior. At 511, no mTHP collapses would ever occur anyways, unless
you have 2MB disabled and other mTHP sizes enabled. Technically at 511
only the highest enabled order always gets collapsed.

Ive also argued in the past that 511 is a terrible default for
anything other than thp.enabled=always, but that's a whole other can
of worms we dont need to discuss now.

with this cap of 255, the PMD scan/collapse would work as intended,
then in mTHP collapses we would never introduce this undesired
behavior. We've discussed before that this would be a hard problem to
solve without introducing some expensive way of tracking what has
already been through a collapse, and that doesnt even consider what
happens if things change or are unmapped, and rescanning that section
would be helpful. So having a strictly enforced limit of 255 actually
seems like a good idea to me, as it completely avoids the undesired
behavior and does not require the admins to be aware of such an issue.

Another thought similar to what (IIRC) Dev has mentioned before, if we
have max_ptes_none > 255 then we only consider collapses to the
largest enabled order, that way no creep to the largest enabled order
would occur in the first place, and we would get there straight away.

To me one of these two solutions seem sane in the context of what we
are dealing with.
>
> > >
> > > ie)
> > > if order!=HPAGE_PMD_ORDER && khugepaged_max_ptes_none > 255
> > >       temp_max_ptes_none = 255;
> > Oh and my second point, introducing a new tunable to control mTHP
> > collapse may become exceedingly complex from a tuning and code
> > management standpoint.
>
> Umm right now you hve a ratio expressed in PTES per mTHP * ((PTEs per PMD) /
> PMD) 'except please don't set to the usual default when using mTHP' and it's
> currently default-broken.
>
> I'm really not sure how that is simpler than a seprate tunable that can be
> expressed as a ratio (e.g. percentage) that actually makes some kind of sense?
I agree that the current tunable wasn't designed for this, but we
tried to come up with something that leverages the tunable we have to
avoid new tunables and added complexity.
>
> And we can make anything workable from a code management point of view by
> refactoring/developing appropriately.
What happens if max_ptes_none = 0 and the ratio is 50% - 1 pte
(ideally the max number)? seems like we would be saying we want no new
none pages, but also to allow new none pages. To me that seems equally
broken and more confusing than just taking a scale of the current
number (now with a cap).


-- Nico

>
> And given you're now proposing changing the default for even THP pages with a
> cap or perhaps having mTHP being used silently change the cap - that is clearly
> _far_ worse from a tuning standpoint.
>
> With a new tunable you can just set a sensible default and people don't even
> necessarily have to think about it.
>

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