[PATCH] mm/thp: support mTHP in thp_underused()

[Ye Liu <ye.liu@linux.dev>]

From: Ye Liu <liuye@kylinos.cn>

When khugepaged_max_ptes_none is configured to a non-default value,
scale it for non-PMD-mappable folios to match the logic in
collapse_max_ptes_none(), so mTHP can be classified as underused
instead of being silently skipped due to unscaled PMD-sized thresholds.

Signed-off-by: Ye Liu <liuye@kylinos.cn>
---
 mm/huge_memory.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 25612af72dca..c642bec967fa 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4489,24 +4489,29 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
 static bool thp_underused(struct folio *folio)
 {
 	int num_zero_pages = 0, num_filled_pages = 0;
+	unsigned int max_ptes_none = khugepaged_max_ptes_none;
+	unsigned int folio_nr = folio_nr_pages(folio);
 	int i;
 
-	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
+	if (!folio_test_pmd_mappable(folio)) {
+		if (max_ptes_none == HPAGE_PMD_NR - 1)
+			max_ptes_none = folio_nr - 1;
+		else if (max_ptes_none)
+			return false;
+	}
+
+	if (max_ptes_none == folio_nr - 1)
 		return false;
 
 	if (folio_contain_hwpoisoned_page(folio))
 		return false;
 
-	for (i = 0; i < folio_nr_pages(folio); i++) {
+	for (i = 0; i < folio_nr; i++) {
 		if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {
-			if (++num_zero_pages > khugepaged_max_ptes_none)
+			if (++num_zero_pages > max_ptes_none)
 				return true;
 		} else {
-			/*
-			 * Another path for early exit once the number
-			 * of non-zero filled pages exceeds threshold.
-			 */
-			if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none)
+			if (++num_filled_pages >= folio_nr - max_ptes_none)
 				return false;
 		}
 	}
-- 
2.43.0