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

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Message-ID: <20251002073255.14867-2-lance.yang@linux.dev>
Date: Thu,  2 Oct 2025 15:32:54 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: 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,
	dev.jain@....com,
	baohua@...nel.org,
	ioworker0@...il.com,
	richard.weiyang@...il.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Lance Yang <lance.yang@...ux.dev>
Subject: [PATCH mm-new 1/2] mm/khugepaged: optimize PTE scanning with if-else-if-else-if chain

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))) {
 			++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.
-- 
2.49.0