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

[Lian Wang]

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.

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@xxxxxxxxx>
Signed-off-by: Lian Wang <lianux.wang@xxxxxxxxxxxxxxxxxx>
---
 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;
 	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;
+	}
 #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;
+	}
 #endif
 	scheme->target_order = target_order;