DAMON's monitoring targets parameters update function,
damon_commit_targets(), is not providing a way to remove a target in the
middle of existing targets list.  Extend the API by adding a field to
struct damon_target.  If the field of a damon_commit_targets() source
target is set, it indicates the matcing target on the existing targets
list is obsolete.  damon_commit_targets() understands that and remove
those from the list, while respecting the index based matching for other
non-obsolete targets.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h |  6 ++++++
 mm/damon/core.c       | 10 +++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 524dea87cac7..8a7b45b9e40d 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -92,17 +92,23 @@ struct damon_region {
  * @nr_regions:		Number of monitoring target regions of this target.
  * @regions_list:	Head of the monitoring target regions of this target.
  * @list:		List head for siblings.
+ * @obsolete:		Whether the commit destination target is obsolete.
  *
  * Each monitoring context could have multiple targets.  For example, a context
  * for virtual memory address spaces could have multiple target processes.  The
  * @pid should be set for appropriate &struct damon_operations including the
  * virtual address spaces monitoring operations.
+ *
+ * @obsolte is used only for damon_commit_targets() source targets, to specify
+ * the matching destination targets are obsolte.  Read damon_commit_targets()
+ * to see how it is handled.
  */
 struct damon_target {
 	struct pid *pid;
 	unsigned int nr_regions;
 	struct list_head regions_list;
 	struct list_head list;
+	bool obsolete;
 };
 
 /**
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 70e66562a1b3..3242a9573db0 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -492,6 +492,7 @@ struct damon_target *damon_new_target(void)
 	t->nr_regions = 0;
 	INIT_LIST_HEAD(&t->regions_list);
 	INIT_LIST_HEAD(&t->list);
+	t->obsolete = false;
 
 	return t;
 }
@@ -1213,7 +1214,11 @@ static int damon_commit_targets(
 
 	damon_for_each_target_safe(dst_target, next, dst) {
 		src_target = damon_nth_target(i++, src);
-		if (src_target) {
+		/*
+		 * If src target is obsolete, do not commit the parameters to
+		 * the dst target, and further remove the dst target.
+		 */
+		if (src_target && !src_target->obsolete) {
 			err = damon_commit_target(
 					dst_target, damon_target_has_pid(dst),
 					src_target, damon_target_has_pid(src),
@@ -1236,6 +1241,9 @@ static int damon_commit_targets(
 	damon_for_each_target_safe(src_target, next, src) {
 		if (j++ < i)
 			continue;
+		/* target to remove has no matching dst */
+		if (src_target->obsolete)
+			return -EINVAL;
 		new_target = damon_new_target();
 		if (!new_target)
 			return -ENOMEM;
-- 
2.47.3