[PATCH v2 10/17] khugepaged: Exit early on fully-mapped aligned mTHP

[Dev Jain <dev.jain@arm.com>]

Since mTHP orders under consideration by khugepaged are also candidates for the
fault handler, the case we hit frequently is that khugepaged scans a region for
order-x, whereas an order-x folio was already installed by the fault handler there.
Therefore, exit early; this prevents a timeout in the khugepaged selftest. Earlier
this was not a problem because a PMD-hugepage will get checked by find_pmd_or_thp_or_none(),
and the previous patch does not solve this problem because it will do the entire PTE
scan to exit.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/khugepaged.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0d0d8f415a2e..baa5b44968ac 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -626,6 +626,11 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
 
+		if (_pte == pte && (order != HPAGE_PMD_ORDER) && (folio_order(folio) == order) &&
+		    test_bit(PG_head, &folio->page.flags) && !folio_test_partially_mapped(folio)) {
+			result = SCAN_PTE_MAPPED_THP;
+			goto out;
+		}
 		/* See hpage_collapse_scan_pmd(). */
 		if (folio_likely_mapped_shared(folio)) {
 			++shared;
@@ -1532,6 +1537,16 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 			goto out_unmap;
 		}
 
+		/* Exit early: There is high chance of this due to faulting */
+		if (_pte == pte && (order != HPAGE_PMD_ORDER) && (folio_order(folio) == order) &&
+		    test_bit(PG_head, &folio->page.flags) && !folio_test_partially_mapped(folio)) {
+			pte_unmap_unlock(pte, ptl);
+			_address = address + (PAGE_SIZE << order);
+			_pte = pte + (1UL << order);
+			result = SCAN_PTE_MAPPED_THP;
+			goto decide_order;
+		}
+
 		/*
 		 * We treat a single page as shared if any part of the THP
 		 * is shared. "False negatives" from
-- 
2.30.2