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

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Message-ID: <02a72419-bc89-481a-ad2a-a3c91713244d@lucifer.local>
Date: Wed, 29 Oct 2025 18:58:04 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Nico Pache <npache@...hat.com>
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce
 collapse_max_ptes_none helper function

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.

>
> 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.

>
> 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!

>
> Cheers,
> -- Nico

Thanks, Lorenzo