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

[Gregory Price <gourry@gourry.net>]

On Tue, May 26, 2026 at 01:37:04PM +0200, Yiannis Nikolakopoulos wrote:
> From: Alirad Malek <alirad.malek@zptcorp.com>
> 
> 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 <alirad.malek@zptcorp.com>
> Co-developed-by: Yiannis Nikolakopoulos <yiannis.nikolakop@gmail.com>
> Signed-off-by: Yiannis Nikolakopoulos <yiannis.nikolakop@gmail.com>
> ---
>  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.
> +

Do we actually need this config flag or should we just default to this
(if the arch supports NT stores)?

>  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
>