linux-kernel - [PATCH] khugepaged: Optimize __collapse_huge_page_copy_succeeded() for large folios by PTE batching

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Message-Id: <20250618102607.10551-1-dev.jain@arm.com>
Date: Wed, 18 Jun 2025 15:56:07 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org,
	david@...hat.com
Cc: ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com,
	npache@...hat.com,
	ryan.roberts@....com,
	baohua@...nel.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Dev Jain <dev.jain@....com>
Subject: [PATCH] khugepaged: Optimize __collapse_huge_page_copy_succeeded() for large folios by PTE batching

Use PTE batching to optimize __collapse_huge_page_copy_succeeded().

On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
Then, calling ptep_clear() for every pte will cause a TLB flush for every
contpte block. Instead, clear_full_ptes() does a
contpte_try_unfold_partial() which will flush the TLB only for the (if any)
starting and ending contpte block, if they partially overlap with the range
khugepaged is looking at.

For all arches, there should be a benefit due to batching atomic operations
on mapcounts due to folio_remove_rmap_ptes().

No issues were observed with mm-selftests.

Signed-off-by: Dev Jain <dev.jain@....com>
---
 mm/khugepaged.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d45d08b521f6..649ccb2670f8 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -700,12 +700,14 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
 						spinlock_t *ptl,
 						struct list_head *compound_pagelist)
 {
+	unsigned long end = address + HPAGE_PMD_SIZE;
 	struct folio *src, *tmp;
-	pte_t *_pte;
+	pte_t *_pte = pte;
 	pte_t pteval;
+	int nr_ptes;
 
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
-	     _pte++, address += PAGE_SIZE) {
+	do {
+		nr_ptes = 1;
 		pteval = ptep_get(_pte);
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
 			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
@@ -719,23 +721,36 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
 				ksm_might_unmap_zero_page(vma->vm_mm, pteval);
 			}
 		} else {
+			const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+			int max_nr_ptes;
+			bool is_large;
+
 			struct page *src_page = pte_page(pteval);
 
 			src = page_folio(src_page);
-			if (!folio_test_large(src))
+			is_large = folio_test_large(src);
+			if (!is_large)
 				release_pte_folio(src);
+
+			max_nr_ptes = (end - address) >> PAGE_SHIFT;
+			if (is_large && max_nr_ptes != 1)
+				nr_ptes = folio_pte_batch(src, address, _pte,
+							  pteval, max_nr_ptes,
+							  flags, NULL, NULL, NULL);
+
 			/*
 			 * ptl mostly unnecessary, but preempt has to
 			 * be disabled to update the per-cpu stats
 			 * inside folio_remove_rmap_pte().
 			 */
 			spin_lock(ptl);
-			ptep_clear(vma->vm_mm, address, _pte);
-			folio_remove_rmap_pte(src, src_page, vma);
+			clear_full_ptes(vma->vm_mm, address, _pte, nr_ptes, false);
+			folio_remove_rmap_ptes(src, src_page, nr_ptes, vma);
 			spin_unlock(ptl);
-			free_folio_and_swap_cache(src);
+			free_swap_cache(src);
+			folio_put_refs(src, nr_ptes);
 		}
-	}
+	} while (_pte += nr_ptes, address += nr_ptes * PAGE_SIZE, address != end);
 
 	list_for_each_entry_safe(src, tmp, compound_pagelist, lru) {
 		list_del(&src->lru);
-- 
2.30.2