linux-kernel - Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes

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Message-ID: <CAA1CXcDR7pe1jKvPOBv-WVYObYtZNNx0w9vESaUsLe+BPR=Dzg@mail.gmail.com>
Date: Wed, 29 Oct 2025 15:23:27 -0600
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-doc@...r.kernel.org, ziy@...dia.com, baolin.wang@...ux.alibaba.com, 
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Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce
 collapse_max_ptes_none helper function

On Wed, Oct 29, 2025 at 12:59 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Tue, Oct 28, 2025 at 08:47:12PM -0600, Nico Pache wrote:
> > On Tue, Oct 28, 2025 at 1:00 PM Lorenzo Stoakes
> > > Right, well I agree if we can make this 0/511 thing work, let's do that.
> >
> > Ok, great, some consensus! I will go ahead with that solution.
>
> :) awesome.
>
> >
> > Just to make sure we are all on the same page,
>
> I am still stabilising my understanding of the creep issue, see the thread
> where David kindly + patiently goes in detail, I think I am at a
> (pre-examining algorithm itself) broad understanding of this.

I added some details of the creep issue in my other replies, hopefully
that also helps!

>
> >
> > the max_ptes_none value will be treated as 0 for anything other than
> > PMD collapse, or in the case of 511. Or will the max_ptes_none only
> > work for mTHP collapse when it is 0.
>
> 511 implies always collapse zero/none, 0 implies never, as I understand it.

0 implies only collapse if a given mTHP size is fully occupied by
present PTES. Since we start at PMD and work our way down we will
always end up with a PMD range of fully occupied mTHPs, potentially of
all different sizes.

>
> >
> > static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> > {
> > unsigned int max_ptes_none;
> >
> > /* ignore max_ptes_none limits */
> > if (full_scan)
> > return HPAGE_PMD_NR - 1;
> >
> > if (order == HPAGE_PMD_ORDER)
> > return khugepaged_max_ptes_none;
> >
> > if (khugepaged_max_ptes_none != HPAGE_PMD_NR - 1)
> > return 0;
> >
> > return max_ptes_none >> (HPAGE_PMD_ORDER - order);
> > }
> >
> > Here's the implementation for the first approach, looks like Baolin
> > was able to catch up and beat me to the other solution while I was
> > mulling over the thread lol
>
> Broadly looks similar to Baolin's, I made some suggestions over there
> though!

Thanks! They are both based on my current collapse_max_ptes_none! Just
a slight difference in behavior surrounding the two suggested
solutions by David.

I will still have to implement the logic for not attempting mTHP
collapses if it is any intermediate value (i.e. the function returns
-EINVAL).

-- Nico

>
> >
> > Cheers,
> > -- Nico
>
> Thanks, Lorenzo
>