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

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Message-ID: <CAA1CXcA1nqt_f+zSOF66eTNWJACCE84hSQn4uNF8CjcBUZ_1oA@mail.gmail.com>
Date: Wed, 29 Oct 2025 15:14:11 -0600
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, 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, 
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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 12:56 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Wed, Oct 29, 2025 at 10:09:43AM +0800, Baolin Wang wrote:
> > I finally finished reading through the discussions across multiple
> > threads:), and it looks like we've reached a preliminary consensus (make
> > 0/511 work). Great and thanks!
>
> Yes we're getting there :) it's a sincere effort to try to find a way to move
> forwards.
>
> >
> > IIUC, the strategy is, configuring it to 511 means always enabling mTHP
> > collapse, configuring it to 0 means collapsing mTHP only if all PTEs are
> > non-none/zero, and for other values, we issue a warning and prohibit mTHP
> > collapse (avoid Lorenzo's concern about silently changing max_ptes_none).
> > Then the implementation for collapse_max_ptes_none() should be as follows:
> >
> > static int collapse_max_ptes_none(unsigned int order, bool full_scan)
> > {
> >         /* ignore max_ptes_none limits */
> >         if (full_scan)
> >                 return HPAGE_PMD_NR - 1;
> >
> >         if (order == HPAGE_PMD_ORDER)
> >                 return khugepaged_max_ptes_none;
> >
> >         /*
> >          * To prevent creeping towards larger order collapses for mTHP
> > collapse,
> >          * we restrict khugepaged_max_ptes_none to only 511 or 0,
> > simplifying the
> >          * logic. This means:
> >          * max_ptes_none == 511 -> collapse mTHP always
> >          * max_ptes_none == 0 -> collapse mTHP only if we all PTEs are
> > non-none/zero
> >          */
> >         if (!khugepaged_max_ptes_none || khugepaged_max_ptes_none ==
> > HPAGE_PMD_NR - 1)
> >                 return khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER -
> > order);
> >
> >         pr_warn_once("mTHP collapse only supports khugepaged_max_ptes_none
> > configured as 0 or %d\n", HPAGE_PMD_NR - 1);
> >         return -EINVAL;
> > }
> >
> > So what do you think?
>
> Yeah I think something like this.
>
> Though I'd implement it more explicitly like:
>
>         /* Zero/non-present collapse disabled. */
>         if (!khugepaged_max_ptes_none)
>            return 0;
>
>         /* Collapse the maximum number of zero/non-present PTEs. */
>         if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
>                 return (1 << order) - 1;
>
> Then we can do away with this confusing (HPAGE_PMD_ORDER - order) stuff.

This looks cleaner/more explicit given the limits we are enforcing!

I'll go for something like that.

>
> A quick check in google sheets suggests my maths is ok here but do correct me if
> I'm wrong :)

LGTM!

Thanks for all the reviews! I'm glad we were able to find a solution :)

-- Nico

>
> Cheers, Lorenzo
>