linux-kernel - Re: [PATCH v11 00/15] khugepaged: mTHP support

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Message-ID: <CAA1CXcA+pb5KvEnJJqdf1eSjaFiBZ82MRB+KDmyhj3DbiQqOxg@mail.gmail.com>
Date: Fri, 12 Sep 2025 17:31:51 -0600
From: Nico Pache <npache@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Kiryl Shutsemau <kas@...nel.org>, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

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

You can take my collapse_max_ptes_none() function in this series and
rework it for the larger sysctl work you are doing.

Cheers,
-- Nico
>
> In between we will have more flexibility on how to set these values.
>
> Likely 9 will be around 50% to not even motivate the user to set
> something that does not make sense (creep).
>
> Of course, the old parameter will have to stick around in compat mode.
>
> --
> Cheers
>
> David / dhildenb
>