[PATCH v2 2/3] mm/huge_memory: don't queue folios that can never be underused

[Joanne Koong <joannelkoong@gmail.com>]

Every anonymous PMD-sized folio is put on the deferred split queue when
it is first mapped, so that the shrinker can find it under memory
pressure and split it if it turns out to be mostly zero-filled.

With the default khugepaged/max_ptes_none this is wasted work. The
default is HPAGE_PMD_NR - 1, which tells thp_underused() that any number
of zero-filled pages is tolerable, which means the shrinker will never
split any of these folios for being underused. They take the list_lru
lock at fault time, inflate the object count the shrinker reports, and
are then walked and dropped when they're first scanned.

Skip the queuing for folios that thp_can_be_underused() says can never
qualify. At the default khugepaged/max_ptes_none that is all of them,
and once the knob is lowered only folios with more pages than it are
queued. This matters for a subsequent patch that adds anonymous mTHP
folios to the deferred split list, as it prevents small mTHP orders from
taking the list_lru lock on every anonymous fault.

Please note that the queue has always been best-effort. A folio that was
queued before khugepaged/max_ptes_none is lowered gets dropped from the
queue by the first scan that finds it's not underused, so changing the
sysctl has never retroactively applied to folios that were already
scanned. Requeueing eligible folios when the sysctl changes will be
addressed in a separate patch.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 mm/huge_memory.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c6ca2a541128..e3349f2314cb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4573,8 +4573,18 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
 	if (folio_order(folio) <= 1)
 		return;
 
-	if (!partially_mapped && !split_underused_thp)
-		return;
+	if (!partially_mapped) {
+		if (!split_underused_thp)
+			return;
+		/*
+		 * Nothing will ever split this folio for being underused, so
+		 * keep it off the queue entirely rather than paying for the
+		 * list_lru lock here and a shrinker scan later.
+		 */
+		if (!thp_can_be_underused(folio_nr_pages(folio),
+					  khugepaged_max_ptes_none))
+			return;
+	}
 
 	/*
 	 * Exclude swapcache: originally to avoid a corrupt deferred split
-- 
2.52.0