From: SeongJae Park <sj@kernel.org>
Date: Sat, 2 May 2026 02:05:03 +0000 (-0700)
Subject: mm/damon/stat: add a parameter for reading kdamond pid
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3a0bc9568c354357546557d8b969785bc27fd260;p=thirdparty%2Flinux.git

mm/damon/stat: add a parameter for reading kdamond pid

Patch series "mm/damon/stat: add kdamond_pid parameter".

DAMON_STAT doesn't provide the pid of its kdamond, unlike DAMON_RECLAIM
and DAMON_LRU_SORT.  This makes user-space management of DAMON_STAT
unnecessarily complicated.  Provide the information via a new parameter,
namely kdamond_pid, and document it.


This patch (of 2):

Knowing the pid of the kdamonds can help user-space management including
monitoring of DAMON's system resource consumption.  To make it easier,
DAMON_SYSFS, DAMON_RECLAIM and DAMON_LRU_SORT provide the pid information.
DAMON_STAT is not providing it, though.  Expose the pid of DAMON_STAT
kdamond via a new read-only module parameter, namely kdamond_pid.  This
also makes DAMON modules usage more standardized, because DAMON_RECLAIM
and DAMON_LRU_SORT also provide the information via their read-only
parameters of the same name.

Link: https://lore.kernel.org/20260502020505.80822-1-sj@kernel.org
Link: https://lore.kernel.org/20260502020505.80822-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index f4d3203e9263..0e14f5bb8f75 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -266,6 +266,45 @@ static int damon_stat_enabled_load(char *buffer, const struct kernel_param *kp)
 	return sprintf(buffer, "%c\n", damon_stat_enabled() ? 'Y' : 'N');
 }
 
+static int damon_stat_kdamond_pid_store(
+		const char *val, const struct kernel_param *kp)
+{
+	/*
+	 * kdamond_pid is read-only, but kernel command line could write it.
+	 * Do nothing here.
+	 */
+	return 0;
+}
+
+static int damon_stat_kdamond_pid_load(
+		char *buffer, const struct kernel_param *kp)
+{
+	int pid;
+
+	if (!damon_stat_context) {
+		pid = -1;
+	} else {
+		pid = damon_kdamond_pid(damon_stat_context);
+		if (pid < 1)
+			pid = -1;
+	}
+	return sprintf(buffer, "%d\n", pid);
+}
+
+static const struct kernel_param_ops kdamond_pid_param_ops = {
+	.set = damon_stat_kdamond_pid_store,
+	.get = damon_stat_kdamond_pid_load,
+};
+
+/*
+ * PID of the DAMON thread
+ *
+ * If DAMON_STAT is enabled, this becomes the PID of the worker thread.
+ * Else, -1.
+ */
+module_param_cb(kdamond_pid, &kdamond_pid_param_ops, NULL, 0400);
+MODULE_PARM_DESC(kdamond_pid, "pid of the kdamond");
+
 static int __init damon_stat_init(void)
 {
 	int err = 0;