[PATCH v1 1/2] mm/huge_memory: extend thp_underused() to mTHP folios

[Joanne Koong <joannelkoong@gmail.com>]

thp_underused() decides whether a large folio on the deferred split
list is underused and should be split so that its zero subpages can be
reclaimed. However, the logic in it only accounts for PMD-sized folios.

As preparatory work for splitting underused mTHP folios, make the logic
in thp_underused() compatible with mTHP-sized folios. This uses the
existing khugepaged_max_ptes_none sysctl value and scales it
proportionally to the size of the folio as the threshold for how many
zero-filled pages a folio may contain before it's considered underused.

This introduces no functional changes for PMD-size folios.

Suggested-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 mm/huge_memory.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c0892cc533a9..ea5c350b4818 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4413,24 +4413,28 @@ 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;
+	int nr_pages = folio_nr_pages(folio);
+	unsigned int max_ptes_none;
 	int i;
 
-	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
+	max_ptes_none = khugepaged_max_ptes_none * nr_pages / HPAGE_PMD_NR;
+
+	if (max_ptes_none >= nr_pages - 1)
 		return false;
 
 	if (folio_contain_hwpoisoned_page(folio))
 		return false;
 
-	for (i = 0; i < folio_nr_pages(folio); i++) {
+	for (i = 0; i < nr_pages; 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 >= nr_pages - max_ptes_none)
 				return false;
 		}
 	}
-- 
2.52.0