Re: [PATCH RFC 3/3] mm: use non-temporal stores for demotion

From: Alirad Malek <redacted>

Memory demoted to a lower tier is assumed to be cold and most likely out of
the CPU's last level cache. Additionally, in certain demotion targets (e.g.
CXL devices with compressed memory) the bandwidth can be negatively
impacted by the eviction patterns of the last level cache when standard
memcpy is used. When the feature is enabled, use the
MIGRATE_ASYNC_NON_TEMPORAL_STORES flag in demotions to trigger the folio
copy path using non-temporal stores.

Signed-off-by: Alirad Malek <redacted>
Co-developed-by: Yiannis Nikolakopoulos <redacted>
Signed-off-by: Yiannis Nikolakopoulos <redacted>
---
 mm/Kconfig   | 8 ++++++++
 mm/migrate.c | 9 ++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index ebd8ea353687..4b7a75b57f6e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -645,6 +645,14 @@ config MIGRATION
 	  pages as migration can relocate pages to satisfy a huge page
 	  allocation instead of reclaiming.
 
+config DEMOTION_WITH_NON_TEMPORAL_STORES
+	bool "Use non-temporal stores for demotion"
+	default n
+	depends on MIGRATION
+	help
+	  Enable non-temporal stores when migrating pages due to demotion.
+	  If disabled, demotion uses regular migration copy paths.
+

 config DEVICE_MIGRATION
 	def_bool MIGRATION && ZONE_DEVICE
 

diff --git a/mm/migrate.c b/mm/migrate.c
index ff6cf50e7b0b..368d40dc8772 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -862,7 +862,10 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst,
 	if (folio_ref_count(src) != expected_count)
 		return -EAGAIN;
 
-	rc = folio_mc_copy(dst, src);
+	if (mode == MIGRATE_ASYNC_NON_TEMPORAL_STORES)
+		rc = folio_mc_copy_nt(dst, src);
+	else
+		rc = folio_mc_copy(dst, src);
 	if (unlikely(rc))
 		return rc;
 

@@ -2081,6 +2084,10 @@ int migrate_pages(struct list_head *from, new_folio_t get_new_folio,
 	LIST_HEAD(split_folios);
 	struct migrate_pages_stats stats;
 
+	if (IS_ENABLED(CONFIG_DEMOTION_WITH_NON_TEMPORAL_STORES) &&
+		reason == MR_DEMOTION && mode == MIGRATE_ASYNC)
+		mode = MIGRATE_ASYNC_NON_TEMPORAL_STORES;
+
 	trace_mm_migrate_pages_start(mode, reason);
 
 	memset(&stats, 0, sizeof(stats));

-- 
2.43.0