Message-ID: <rapl4xr55zv2nq3jh5ulk4wvfyxa2kmbnnb4uram6q43y4cbwn@dv5m52ocyqne>
Date: Fri, 12 Sep 2025 15:35:28 +0100
From: Kiryl Shutsemau <kas@...nel.org>
To: David Hildenbrand <david@...hat.com>
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On Fri, Sep 12, 2025 at 04:28:09PM +0200, David Hildenbrand wrote:
> On 12.09.25 15:47, David Hildenbrand 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.
> > 
> > BTW, I thought further about this and I agree: if we count zero-filled
> > pages towards none_or_zero one we can avoid the "creep" problem.
> > 
> > The scanning-for-zero part is rather nasty, though.
> 
> Aaand, thinking again from the other direction, this would mean that just
> because pages became zero after some time that we would no longer collapse
> because none_or_zero would then be higher. Hm ....
> 
> How I hate all of this so very very much :)

This is not new. Shrinker has the same problem: it cannot distinguish
between hot 4k that happened to be zero from the 4k that is there just
because of we faulted in 2M a time.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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