Re: [RESEND RFC PATCH v2 4/5] mm/damon: introduce DAMOS_SPLIT action

[SJ Park <sj@kernel.org>]

On Thu,  2 Jul 2026 17:46:32 +0800 Lian Wang <lianux.mm@gmail.com> wrote:

> Add DAMOS_SPLIT to the damos_action enum for splitting large folios
> into smaller mTHP-order folios.  Add a target_order field to struct
> damos to specify the desired split order.

This commit is not adding but reusing the field.

> 
> Expose the action as "split" through the DAMON sysfs interface with
> target_order validation (must be 2..HPAGE_PMD_ORDER-1).
> 
> Signed-off-by: Lian Wang <lianux.mm@gmail.com>
> Signed-off-by: Lian Wang <lianux.wang@processmission.com>

Let's have only one S-o-b: line if there is no reason to have two.

> ---
>  include/linux/damon.h    |  9 +++++++--
>  mm/damon/sysfs-schemes.c | 16 ++++++++++++++++
>  2 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index 5a0587556573..30cf4afb212c 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -121,6 +121,7 @@ struct damon_target {
>   * @DAMOS_HUGEPAGE:	Call ``madvise()`` for the region with MADV_HUGEPAGE.
>   * @DAMOS_NOHUGEPAGE:	Call ``madvise()`` for the region with MADV_NOHUGEPAGE.
>   * @DAMOS_COLLAPSE:	Call ``madvise()`` for the region with MADV_COLLAPSE.
> + * @DAMOS_SPLIT:	Split large folios to the target mTHP order.
>   * @DAMOS_LRU_PRIO:	Prioritize the region on its LRU lists.
>   * @DAMOS_LRU_DEPRIO:	Deprioritize the region on its LRU lists.
>   * @DAMOS_MIGRATE_HOT:  Migrate the regions prioritizing warmer regions.
> @@ -141,6 +142,7 @@ enum damos_action {
>  	DAMOS_HUGEPAGE,
>  	DAMOS_NOHUGEPAGE,
>  	DAMOS_COLLAPSE,
> +	DAMOS_SPLIT,
>  	DAMOS_LRU_PRIO,
>  	DAMOS_LRU_DEPRIO,
>  	DAMOS_MIGRATE_HOT,
> @@ -573,8 +575,11 @@ struct damos {
>  	struct damos_access_pattern pattern;
>  	enum damos_action action;
>  	/*
> -	 * @target_order: target order for mTHP actions (DAMOS_COLLAPSE).
> -	 * 0 means system default (PMD order).  Valid: 0, 2..HPAGE_PMD_ORDER.
> +	 * @target_order: target mTHP order for DAMOS_COLLAPSE and
> +	 * DAMOS_SPLIT.  For COLLAPSE, 0 means PMD order default,
> +	 * valid values: 0, 2..HPAGE_PMD_ORDER.  For SPLIT,
> +	 * valid values: 2..HPAGE_PMD_ORDER-1; 0 and HPAGE_PMD_ORDER
> +	 * are rejected at scheme creation time (defaulting to 2).
>  	 */
>  	unsigned int target_order;

As I commented to an earlier patch, let's use the union.

>  	unsigned long apply_interval_us;
> diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> index 7dcd582ded86..84a4617ca3d3 100644
> --- a/mm/damon/sysfs-schemes.c
> +++ b/mm/damon/sysfs-schemes.c
> @@ -2293,6 +2293,10 @@ static struct damos_sysfs_action_name damos_sysfs_action_names[] = {
>  		.action = DAMOS_COLLAPSE,
>  		.name = "collapse",
>  	},
> +	{
> +		.action = DAMOS_SPLIT,
> +		.name = "split",
> +	},
>  	{
>  		.action = DAMOS_LRU_PRIO,
>  		.name = "lru_prio",
> @@ -3058,11 +3062,23 @@ static struct damos *damon_sysfs_mk_scheme(
>  			HPAGE_PMD_ORDER, HPAGE_PMD_ORDER);
>  		target_order = 0;
>  	}
> +	if (sysfs_scheme->action == DAMOS_SPLIT &&
> +	    (target_order == 0 ||
> +	     target_order >= HPAGE_PMD_ORDER)) {
> +		pr_warn("DAMON split: target_order %u invalid, need 2..%u. Defaulting to 2.\n",
> +			target_order,
> +			HPAGE_PMD_ORDER - 1);
> +		target_order = 2;
> +	}

As I also commented to an earlier patch, this validation will eventually
dropped.

>  #else
>  	if (sysfs_scheme->action == DAMOS_COLLAPSE && target_order != 0) {
>  		pr_warn("DAMON collapse: target_order not supported without THP. Use 0.\n");
>  		target_order = 0;
>  	}
> +	if (sysfs_scheme->action == DAMOS_SPLIT) {
> +		pr_warn("DAMON split: not supported without THP.\n");
> +		target_order = 2;

Why set target_order here?

> +	}
>  #endif
>  	scheme->target_order = target_order;


Thanks,
SJ