The logic that finds the struct pid for a given pid number and assigns
it to a damon_target is duplicated in multiple places.  Including
damon_sysfs_add_target() of mm/damon/sysfs.c and the start functions
of the two sample modules, samples/damon/wsse.c and
samples/damon/prcl.c.  Add a function that does the work, and replace
the duplicated code in the places with calls to the function.

Signed-off-by: Enze Li <lienze@kylinos.cn>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
---
Changes from v1
- v1: https://lore.kernel.org/20260817125319.888994-1-lienze@kylinos.cn
- Collect R-b: from SJ.
- Rebase to latest mm-new.

 include/linux/damon.h |  1 +
 mm/damon/core.c       | 12 ++++++++++++
 mm/damon/sysfs.c      |  6 ++----
 samples/damon/prcl.c  |  5 +----
 samples/damon/wsse.c  |  5 +----
 5 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 0c8b7ddef9abb..5607f98ec6306 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -1055,6 +1055,7 @@ int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src);
 struct damon_target *damon_new_target(void);
 void damon_add_target(struct damon_ctx *ctx, struct damon_target *t);
 bool damon_targets_empty(struct damon_ctx *ctx);
+int damon_set_target_pid(struct damon_target *t, int pid);
 void damon_free_target(struct damon_target *t);
 void damon_destroy_target(struct damon_target *t, struct damon_ctx *ctx);
 unsigned int damon_nr_regions(struct damon_target *t);
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 79515ef03fc2a..a5ea3e61e7f41 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -10,6 +10,7 @@
 #include <linux/kthread.h>
 #include <linux/memcontrol.h>
 #include <linux/mm.h>
+#include <linux/pid.h>
 #include <linux/psi.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
@@ -802,6 +803,17 @@ bool damon_targets_empty(struct damon_ctx *ctx)
 	return list_empty(&ctx->adaptive_targets);
 }
 
+/*
+ * Assign the struct pid of the given pid number to the given target.
+ */
+int damon_set_target_pid(struct damon_target *t, int pid)
+{
+	t->pid = find_get_pid(pid);
+	if (!t->pid)
+		return -EINVAL;
+	return 0;
+}
+
 static void damon_del_target(struct damon_target *t)
 {
 	list_del(&t->list);
diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index f05b256c90ee7..dcb739ce0a729 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -3,7 +3,6 @@
  * DAMON sysfs Interface
  */
 
-#include <linux/pid.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
 
@@ -2036,9 +2035,8 @@ static int damon_sysfs_add_target(struct damon_sysfs_target *sys_target,
 		return -ENOMEM;
 	damon_add_target(ctx, t);
 	if (damon_target_has_pid(ctx)) {
-		t->pid = find_get_pid(sys_target->pid);
-		if (!t->pid)
-			/* caller will destroy targets */
+		/* caller will destroy targets */
+		if (damon_set_target_pid(t, sys_target->pid))
 			return -EINVAL;
 	}
 	t->obsolete = sys_target->obsolete;
diff --git a/samples/damon/prcl.c b/samples/damon/prcl.c
index 842099bd62286..83ddf12811d57 100644
--- a/samples/damon/prcl.c
+++ b/samples/damon/prcl.c
@@ -32,7 +32,6 @@ module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
 MODULE_PARM_DESC(enabled, "Enable or disable DAMON_SAMPLE_PRCL");
 
 static struct damon_ctx *ctx;
-static struct pid *target_pidp;
 
 static int damon_sample_prcl_repeat_call_fn(void *data)
 {
@@ -79,12 +78,10 @@ static int damon_sample_prcl_start(void)
 		return -ENOMEM;
 	}
 	damon_add_target(ctx, target);
-	target_pidp = find_get_pid(target_pid);
-	if (!target_pidp) {
+	if (damon_set_target_pid(target, target_pid)) {
 		damon_destroy_ctx(ctx);
 		return -EINVAL;
 	}
-	target->pid = target_pidp;
 
 	scheme = damon_new_scheme(
 			&(struct damos_access_pattern) {
diff --git a/samples/damon/wsse.c b/samples/damon/wsse.c
index 37fd5da201588..53944aea8428e 100644
--- a/samples/damon/wsse.c
+++ b/samples/damon/wsse.c
@@ -33,7 +33,6 @@ module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
 MODULE_PARM_DESC(enabled, "Enable or disable DAMON_SAMPLE_WSSE");
 
 static struct damon_ctx *ctx;
-static struct pid *target_pidp;
 
 static int damon_sample_wsse_repeat_call_fn(void *data)
 {
@@ -79,12 +78,10 @@ static int damon_sample_wsse_start(void)
 		return -ENOMEM;
 	}
 	damon_add_target(ctx, target);
-	target_pidp = find_get_pid(target_pid);
-	if (!target_pidp) {
+	if (damon_set_target_pid(target, target_pid)) {
 		damon_destroy_ctx(ctx);
 		return -EINVAL;
 	}
-	target->pid = target_pidp;
 
 	err = damon_start(&ctx, 1, true);
 	if (err) {
-- 
2.47.3