[PATCH RFC v2 14/15] mm/khugepaged: check folio referenced state via LRU refs under MGLRU

[Kairui Song via B4 Relay]

From: Kairui Song <kasong@xxxxxxxxxxx>

For MGLRU, PG_referenced is only the lowest bit of the folio LRU refs
count, so folios with a higher refs count no longer have the bit set.
folio_pte_referenced() testing the raw bit misses hot folios at
refs >= 2, under-counting the referenced folios of a candidate range
and aborting otherwise good collapses with SCAN_LACK_REFERENCED_PAGE.

Test the refs count directly when MGLRU is enabled, mirroring the
smaps conversion.  The classical LRU keeps the plain bit test.

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/khugepaged.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1deb74cf28af..a3ce6735bc32 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -597,7 +597,9 @@ static bool folio_pte_referenced(struct folio *folio,
 		struct vm_area_struct *vma, unsigned long addr, pte_t pteval)
 {
 	/* The folio was referenced previously ... */
-	if (folio_test_young(folio) || folio_test_referenced(folio))
+	if (folio_test_young(folio))
+		return true;
+	if (lru_gen_enabled() ? folio_lru_refs(folio) : folio_test_referenced(folio))
 		return true;
 	/* ... or the PTE mapping was recently used */
 	return pte_young(pteval) || mmu_notifier_test_young(vma->vm_mm, addr);

-- 
2.55.0