linux-kernel - Re: [PATCH mm-new 2/2] mm/khugepaged: merge PTE scanning logic into a new helper

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Message-ID: <9DF437D8-E98B-4061-A8C7-521D89070C97@nvidia.com>
Date: Fri, 03 Oct 2025 13:11:23 -0400
From: Zi Yan <ziy@...dia.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org,
 ioworker0@...il.com, richard.weiyang@...il.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH mm-new 2/2] mm/khugepaged: merge PTE scanning logic into a
 new helper

On 2 Oct 2025, at 3:32, Lance Yang wrote:

> From: Lance Yang <lance.yang@...ux.dev>
>
> As David suggested, the PTE scanning logic in hpage_collapse_scan_pmd()
> and __collapse_huge_page_isolate() was almost duplicated.
>
> This patch cleans things up by moving all the common PTE checking logic
> into a new shared helper, thp_collapse_check_pte().
>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>  mm/khugepaged.c | 167 ++++++++++++++++++++++++++++++------------------
>  1 file changed, 104 insertions(+), 63 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 808523f92c7b..2a897cfb1d03 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -61,6 +61,12 @@ enum scan_result {
>  	SCAN_PAGE_FILLED,
>  };
>
> +enum pte_check_result {
> +	PTE_CHECK_SUCCEED,
> +	PTE_CHECK_CONTINUE,
> +	PTE_CHECK_FAIL,
> +};
> +
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/huge_memory.h>
>
> @@ -533,6 +539,87 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>  	}
>  }
>
> +/*
> + * thp_collapse_check_pte - Check if a PTE is suitable for THP collapse
> + * @pte:           PTE to check
> + * @vma:           VMA the PTE belongs to
> + * @cc:            Collapse control settings
> + * @scan_swap_pte: Allow scanning of swap PTEs if true
> + * @none_or_zero:  Counter for none/zero PTEs (must be non-NULL)
> + * @unmapped:      Counter for swap PTEs (must be non-NULL if scan_swap_pte
> + *                 is true)
> + * @scan_result:   Used to return the failure reason (SCAN_*) on a
> + *                 PTE_CHECK_FAIL return. Must be non-NULL
> + *
> + * Returns:
> + *   PTE_CHECK_SUCCEED  - Valid PTE, proceed with collapse
> + *   PTE_CHECK_CONTINUE - Skip this none/zero PTE but continue scanning
> + *   PTE_CHECK_FAIL     - Abort collapse scan
> + */
> +static inline int thp_collapse_check_pte(pte_t pte, struct vm_area_struct *vma,
> +		struct collapse_control *cc, bool scan_swap_pte,
> +		int *none_or_zero, int *unmapped, int *scan_result)
> +{
> +	VM_BUG_ON(!none_or_zero || !scan_result);
> +	VM_BUG_ON(scan_swap_pte && !unmapped);

Please use VM_WARN_ONCE_ON and just return. Or put all stats in a struct,
pass and check the struct pointer instead.


Best Regards,
Yan, Zi