linux-kernel - Re: [PATCH mm-new 1/2] mm/khugepaged: optimize PTE scanning with if-else-if-else-if chain

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Message-ID: <b8fc9ab7-a96f-4763-9432-8aa8c3c2a87d@arm.com>
Date: Fri, 3 Oct 2025 22:03:15 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 david@...hat.com, lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....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 1/2] mm/khugepaged: optimize PTE scanning with
 if-else-if-else-if chain


On 02/10/25 1:02 pm, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> As pointed out by Dev, the PTE checks for disjoint conditions in the
> scanning loops can be optimized. is_swap_pte, (pte_none && is_zero_pfn),
> and pte_uffd_wp are mutually exclusive.
>
> This patch refactors the loops in both __collapse_huge_page_isolate() and
> hpage_collapse_scan_pmd() to use a continuous if-else-if-else-if chain
> instead of separate if blocks.
>
> Also, this is a preparatory step to make it easier to merge the
> almost-duplicated scanning logic in these two functions, as suggested
> by David.
>
> Suggested-by: Dev Jain <dev.jain@....com>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   mm/khugepaged.c | 12 ++++--------
>   1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index f4f57ba69d72..808523f92c7b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -548,8 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>   	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
>   	     _pte++, addr += PAGE_SIZE) {
>   		pte_t pteval = ptep_get(_pte);
> -		if (pte_none(pteval) || (pte_present(pteval) &&
> -				is_zero_pfn(pte_pfn(pteval)))) {
> +		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {

Should have mentioned in the description that pte_present() is not required
here, so removing it.

Reviewed-by: Dev Jain <dev.jain@....com>

>   			++none_or_zero;
>   			if (!userfaultfd_armed(vma) &&
>   			    (!cc->is_khugepaged ||
> @@ -560,12 +559,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>   				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
>   				goto out;
>   			}
> -		}
> -		if (!pte_present(pteval)) {
> +		} else if (!pte_present(pteval)) {
>   			result = SCAN_PTE_NON_PRESENT;
>   			goto out;
> -		}
> -		if (pte_uffd_wp(pteval)) {
> +		} else if (pte_uffd_wp(pteval)) {
>   			result = SCAN_PTE_UFFD_WP;
>   			goto out;
>   		}
> @@ -1316,8 +1313,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>   				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
>   				goto out_unmap;
>   			}
> -		}
> -		if (pte_uffd_wp(pteval)) {
> +		} else if (pte_uffd_wp(pteval)) {
>   			/*
>   			 * Don't collapse the page if any of the small
>   			 * PTEs are armed with uffd write protection.