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

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Message-ID: <2ab547d2-584b-48fe-9f43-7c14caa7ab05@lucifer.local>
Date: Wed, 29 Oct 2025 18:55:35 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: David Hildenbrand <david@...hat.com>, Nico Pache <npache@...hat.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
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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 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.

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

Cheers, Lorenzo