linux-kernel - Re: [PATCH v10 00/13] khugepaged: mTHP support

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Message-ID: <CAA1CXcCqhFoGBvFK-ox2sJw7QHaFt+-Lw09BDYsAGKg4qc8nSw@mail.gmail.com>
Date: Thu, 21 Aug 2025 09:25:42 -0600
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Dev Jain <dev.jain@....com>, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
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Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support

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.

ie)
if order!=HPAGE_PMD_ORDER && khugepaged_max_ptes_none > 255
      temp_max_ptes_none = 255;
>
> Thanks, Lorenzo
>