Message-ID: <2c5328ee-fb6e-45cf-a151-868cc8e7ff97@lucifer.local>
Date: Mon, 15 Sep 2025 11:35:53 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Kiryl Shutsemau <kas@...nel.org>, Nico Pache <npache@...hat.com>,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On Mon, Sep 15, 2025 at 12:22:07PM +0200, David Hildenbrand wrote:
> On 15.09.25 11:22, Kiryl Shutsemau wrote:
> > On Fri, Sep 12, 2025 at 05:31:51PM -0600, Nico Pache wrote:
> > > On Fri, Sep 12, 2025 at 6:25 AM David Hildenbrand <david@...hat.com> wrote:
> > > >
> > > > On 12.09.25 14:19, Kiryl Shutsemau wrote:
> > > > > On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote:
> > > > > > The following series provides khugepaged with the capability to collapse
> > > > > > anonymous memory regions to mTHPs.
> > > > > >
> > > > > > To achieve this we generalize the khugepaged functions to no longer depend
> > > > > > on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
> > > > > > pages that are occupied (!none/zero). After the PMD scan is done, we do
> > > > > > binary recursion on the bitmap to find the optimal mTHP sizes for the PMD
> > > > > > range. The restriction on max_ptes_none is removed during the scan, to make
> > > > > > sure we account for the whole PMD range. When no mTHP size is enabled, the
> > > > > > legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
> > > > > > by the attempted collapse order to determine how full a mTHP must be to be
> > > > > > eligible for the collapse to occur. If a mTHP collapse is attempted, but
> > > > > > contains swapped out, or shared pages, we don't perform the collapse. It is
> > > > > > now also possible to collapse to mTHPs without requiring the PMD THP size
> > > > > > to be enabled.
> > > > > >
> > > > > > When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
> > > > > > 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
> > > > > > mTHP collapses to prevent collapse "creep" behavior. This prevents
> > > > > > constantly promoting mTHPs to the next available size, which would occur
> > > > > > because a collapse introduces more non-zero pages that would satisfy the
> > > > > > promotion condition on subsequent scans.
> > > > >
> > > > > Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
> > > > > all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.
> > > > >
> > > >
> > > > I am all for not adding any more ugliness on top of all the ugliness we
> > > > added in the past.
> > > >
> > > > I will soon propose deprecating that parameter in favor of something
> > > > that makes a bit more sense.
> > > >
> > > > In essence, we'll likely have an "eagerness" parameter that ranges from
> > > > 0 to 10. 10 is essentially "always collapse" and 0 "never collapse if
> > > > not all is populated".
> > > Hi David,
> > >
> > > Do you have any reason for 0-10, I'm guessing these will map to
> > > different max_ptes_none values.
> > > I suggest 0-5, mapping to 0,32,64,128,255,511
> >
> > That's too x86-64 specific.
> >
> > And the whole idea is not to map to directly, but give kernel wiggle
> > room to play.
>
> Initially we will start out simple and map it directly. But yeah, the idea
> is to give us some more room later.

I think it's less 'wiggle room' and more us being able to _abstract_ what this
measurement means while reserving the right to adjust this.

But maybe we are saying the same thing in different ways.

>
> I had something logarithmic in mind which would roughly be (ignoring the the
> weird -1 for simplicity and expressing it as "used" instead of none-or-zero)
>
> 0 -> ~100% used (~0% none)

So equivalent to 511 today?

> 1 -> ~50% used (~50% none)
> 2 -> ~25% used (~75% none)
> 3 -> ~12.5% used (~87.5% none)
> 4 -> ~11.25% used (~88,75% none)
> ...
> 10 -> ~0% used (~100% none)

So equivalent to 0 today?

And with a logarithmic weighting towards values closer to "0% used"?

This seems sensible given the only reports we've had of non-0/511 uses here are
in that range...

But ofc this interpretation should be something we determine + treated as an
implementation detail that we can modify later.

>
> Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy.

And at different mTHP levels too right?

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo

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