[RFC v4] mm/damon: add synchronous commit for commit_inputs

[Liew Rui Yan <aethernet65535@gmail.com>]

Problem
=======
Writing invalid parameters to sysfs followed by 'commit_inputs=Y' fails
silently (no error returned to shell), because the validation happens
asynchronously in the kdamond.

Solution
========
To fix this, the commit_inputs_store() callback now uses damon_call() to
synchronously commit parameters in the kdamond thread's safe context.
This ensures that validation errors are returned immediately to
userspace, following the pattern used by DAMON_SYSFS.

Changes
=======
1. Added commit_inputs_store() and commit_inputs_fn() to commit
   synchronously.
2. Removed handle_commit_inputs().

This change is motivated from another discussion [1].

[1] https://lore.kernel.org/20260318153731.97470-1-aethernet65535@gmail.com

Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
---
Changes from RFC-v3:
- Added checks for 'ctx' and 'damon_is_running()' to prevent NULL
  pointer dereference during early boot. (Found by Sashiko.dev)
- Removed handle_commit_inputs() and its associated polling logic as
  they have become dead code after moving to the synchronous damon_call()
  approach.
- Ensure the 'commit_inputs' is properly updated.
Link to RFC-v3: https://lore.kernel.org/20260322231522.32700-1-aethernet65535@gmail.com

Changes from RFC-v2:
- Removed damon_validate_attrs(), now using damon_commit_ctx() for
  synchronous validation in the kdamond context.
- Following DAMON_SYSFS pattern for synchronous commit via damon_call().
- Link to RFC-v2: https://lore.kernel.org/20260321140926.22163-1-aethernet65535@gmail.com

Changes from RFC-v1:
- Remove question from commit message area.
- Added synchronous validation for DAMON_RECLAIM.
- Rename damon_valid_attrs() -> damon_validate_attrs().
- Exported a new function damon_validate_attrs() and declared it in
  damon.h.
- Link to RFC-v1: https://lore.kernel.org/20260321002642.22712-1-aethernet65535@gmail.com

 mm/damon/lru_sort.c | 49 +++++++++++++++++++++++++++++++++++++++------
 mm/damon/reclaim.c  | 49 +++++++++++++++++++++++++++++++++++++++------
 2 files changed, 86 insertions(+), 12 deletions(-)

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index 554559d72976..37b3c897e822 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -39,7 +39,6 @@ static bool enabled __read_mostly;
  * the re-reading, DAMON_LRU_SORT will be disabled.
  */
 static bool commit_inputs __read_mostly;
-module_param(commit_inputs, bool, 0600);
 
 /*
  * Desired active to [in]active memory ratio in bp (1/10,000).
@@ -349,18 +348,56 @@ static int damon_lru_sort_apply_parameters(void)
 	return err;
 }
 
-static int damon_lru_sort_handle_commit_inputs(void)
+static int damon_lru_sort_commit_inputs_fn(void *arg)
 {
+	return damon_lru_sort_apply_parameters();
+}
+
+static int damon_lru_sort_commit_inputs_store(const char *val,
+		const struct kernel_param *kp)
+{
+	bool yes;
 	int err;
+	struct damon_call_control control = {
+		.fn = damon_lru_sort_commit_inputs_fn,
+		.data = ctx,
+		.repeat = false,
+	};
 
-	if (!commit_inputs)
+	err = kstrtobool(val, &yes);
+	if (err)
+		return err;
+
+	if (commit_inputs == yes)
 		return 0;
 
-	err = damon_lru_sort_apply_parameters();
+	if (!yes) {
+		commit_inputs = false;
+		return 0;
+	}
+
+	commit_inputs = yes;
+
+	/*
+	 * Skip damon_call() during early boot or when kdamond is idle
+	 * to avoid NULL pointer dereference or unexpected -EINVAL.
+	 */
+	if (!ctx || !damon_is_running(ctx))
+		return 0;
+
+	err = damon_call(ctx, &control);
 	commit_inputs = false;
-	return err;
+
+	return err ? err : control.return_code;
 }
 
+static const struct kernel_param_ops commit_inputs_param_ops = {
+	.set = damon_lru_sort_commit_inputs_store,
+	.get = param_get_bool,
+};
+
+module_param_cb(commit_inputs, &commit_inputs_param_ops, &commit_inputs, 0600);
+
 static int damon_lru_sort_damon_call_fn(void *arg)
 {
 	struct damon_ctx *c = arg;
@@ -374,7 +411,7 @@ static int damon_lru_sort_damon_call_fn(void *arg)
 			damon_lru_sort_cold_stat = s->stat;
 	}
 
-	return damon_lru_sort_handle_commit_inputs();
+	return 0;
 }
 
 static struct damon_call_control call_control = {
diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 86da14778658..441d5d9f9eab 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -39,7 +39,6 @@ static bool enabled __read_mostly;
  * re-reading, DAMON_RECLAIM will be disabled.
  */
 static bool commit_inputs __read_mostly;
-module_param(commit_inputs, bool, 0600);
 
 /*
  * Time threshold for cold memory regions identification in microseconds.
@@ -255,18 +254,56 @@ static int damon_reclaim_apply_parameters(void)
 	return err;
 }
 
-static int damon_reclaim_handle_commit_inputs(void)
+static int damon_reclaim_commit_inputs_fn(void *arg)
 {
+	return damon_reclaim_apply_parameters();
+}
+
+static int damon_reclaim_commit_inputs_store(const char *val,
+		const struct kernel_param *kp)
+{
+	bool yes;
 	int err;
+	struct damon_call_control control = {
+		.fn = damon_reclaim_commit_inputs_fn,
+		.data = ctx,
+		.repeat = false,
+	};
 
-	if (!commit_inputs)
+	err = kstrtobool(val, &yes);
+	if (err)
+		return err;
+
+	if (commit_inputs == yes)
 		return 0;
 
-	err = damon_reclaim_apply_parameters();
+	if (!yes) {
+		commit_inputs = false;
+		return 0;
+	}
+
+	commit_inputs = yes;
+
+	/*
+	 * Skip damon_call() during early boot or when kdamond is idle
+	 * to avoid NULL pointer dereference or unexpected -EINVAL.
+	 */
+	if (!ctx || !damon_is_running(ctx))
+		return 0;
+
+	err = damon_call(ctx, &control);
 	commit_inputs = false;
-	return err;
+
+	return err ? err : control.return_code;
 }
 
+static const struct kernel_param_ops commit_inputs_param_ops = {
+	.set = damon_reclaim_commit_inputs_store,
+	.get = param_get_bool,
+};
+
+module_param_cb(commit_inputs, &commit_inputs_param_ops, &commit_inputs, 0600);
+
 static int damon_reclaim_damon_call_fn(void *arg)
 {
 	struct damon_ctx *c = arg;
@@ -276,7 +313,7 @@ static int damon_reclaim_damon_call_fn(void *arg)
 	damon_for_each_scheme(s, c)
 		damon_reclaim_stat = s->stat;
 
-	return damon_reclaim_handle_commit_inputs();
+	return 0;
 }
 
 static struct damon_call_control call_control = {
-- 
2.53.0