mm/damon/sysfs: use dynamically allocated repeat mode damon_call_control

DAMON sysfs interface is using a single global repeat mode
damon_call_control variable for refresh_ms handling, for all DAMON
contexts.  As a result, when there are more than one context, the single
global damon_call_control is unexpectedly over-written (corrupted). 
Particularly the ->link field is overwritten by the multiple contexts and
this can cause a user hangup, and/or a kernel crash.  Fix it by using
dynamically allocated damon_call_control object per DAMON context.

Link: https://lkml.kernel.org/r/20250908201513.60802-3-sj@kernel.org
Link: https://lore.kernel.org/20250904011738.930-1-yunjeong.mun@sk.com [1]
Link: https://lore.kernel.org/20250905035411.39501-1-sj@kernel.org [2]
Fixes: d809a7c ("mm/damon/sysfs: implement refresh_ms file internal work")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Yunjeong Mun <yunjeong.mun@sk.com>
Closes: https://lore.kernel.org/20250904011738.930-1-yunjeong.mun@sk.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

mm/damon/sysfs.c

@@ -1534,14 +1534,10 @@ static int damon_sysfs_repeat_call_fn(void *data)
 	return 0;
 }
 
-static struct damon_call_control damon_sysfs_repeat_call_control = {
-	.fn = damon_sysfs_repeat_call_fn,
-	.repeat = true,
-};
-
 static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 {
 	struct damon_ctx *ctx;
+	struct damon_call_control *repeat_call_control;
 	int err;
 
 	if (damon_sysfs_kdamond_running(kdamond))
@@ -1554,18 +1550,29 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 		damon_destroy_ctx(kdamond->damon_ctx);
 	kdamond->damon_ctx = NULL;
 
+	repeat_call_control = kmalloc(sizeof(*repeat_call_control),
+			GFP_KERNEL);
+	if (!repeat_call_control)
+		return -ENOMEM;
+
 	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
-	if (IS_ERR(ctx))
+	if (IS_ERR(ctx)) {
+		kfree(repeat_call_control);
 		return PTR_ERR(ctx);
+	}
 	err = damon_start(&ctx, 1, false);
 	if (err) {
+		kfree(repeat_call_control);
 		damon_destroy_ctx(ctx);
 		return err;
 	}
 	kdamond->damon_ctx = ctx;
 
-	damon_sysfs_repeat_call_control.data = kdamond;
-	damon_call(ctx, &damon_sysfs_repeat_call_control);
+	repeat_call_control->fn = damon_sysfs_repeat_call_fn;
+	repeat_call_control->data = kdamond;
+	repeat_call_control->repeat = true;
+	repeat_call_control->dealloc_on_cancel = true;
+	damon_call(ctx, repeat_call_control);
 	return err;
 }