[PATCH mm-new v3 1/3] mm/khugepaged: optimize PTE scanning with if-else-if-else-if chain

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.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. While at it, the redundant pte_present()
check before is_zero_pfn() is also removed.

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.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Suggested-by: Dev Jain <dev.jain@arm.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 mm/khugepaged.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index bec3e268dc76..e3e27223137a 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;
 		}
@@ -1321,8 +1318,7 @@ 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