[RFC PATCH v1.1 0/2] mm/damon/stat: add kdamond_pid parameter

[RFC PATCH v1.1 0/2] mm/damon/stat: add kdamond_pid parameter

[SeongJae Park]

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.

Changes from RFC
- rfc: https://lore.kernel.org/20260414053742.90296-1-sj@kernel.org
- Fix damon_kdamond_pid() failure handling.

SeongJae Park (2):
  mm/damon/stat: add a parameter for reading kdamond pid
  Docs/admin-guide/mm/damon/stat: document kdamond_pid parameter

 Documentation/admin-guide/mm/damon/stat.rst |  7 +++++++
 mm/damon/stat.c                             | 17 +++++++++++++++++
 2 files changed, 24 insertions(+)


base-commit: 02784c37a710fa3c8c3e7be4f27a5cfa3356dc00
-- 
2.47.3

[RFC PATCH v1.1 1/2] mm/damon/stat: add a parameter for reading kdamond pid

[SeongJae Park]

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.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/stat.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index 99ba346f9e325..510c593a313b8 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -44,6 +44,15 @@ module_param(aggr_interval_us, ulong, 0400);
 MODULE_PARM_DESC(aggr_interval_us,
 		"Current tuned aggregation interval in microseconds");
 
+/*
+ * PID of the DAMON thread
+ *
+ * If DAMON_STAT is enabled, this becomes the PID of the worker thread.
+ * Else, -1.
+ */
+static int kdamond_pid __read_mostly = -1;
+module_param(kdamond_pid, int, 0400);
+
 static struct damon_ctx *damon_stat_context;
 
 static unsigned long damon_stat_last_refresh_jiffies;
@@ -260,6 +269,13 @@ static int damon_stat_start(void)
 		damon_stat_context = NULL;
 		return err;
 	}
+	kdamond_pid = damon_kdamond_pid(damon_stat_context);
+	if (kdamond_pid < 0) {
+		err = kdamond_pid;
+		kdamond_pid = -1;
+		damon_destroy_ctx(damon_stat_context);
+		damon_stat_context = NULL;
+		return err;
 
 	damon_stat_last_refresh_jiffies = jiffies;
 	call_control.data = damon_stat_context;
@@ -269,6 +285,7 @@ static int damon_stat_start(void)
 static void damon_stat_stop(void)
 {
 	damon_stop(&damon_stat_context, 1);
+	kdamond_pid = -1;
 	damon_destroy_ctx(damon_stat_context);
 	damon_stat_context = NULL;
 }
-- 
2.47.3

[RFC PATCH v1.1 2/2] Docs/admin-guide/mm/damon/stat: document kdamond_pid parameter

[SeongJae Park]

Update DAMON_STAT usage document for newly added kdamond_pid parameter.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/stat.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/mm/damon/stat.rst b/Documentation/admin-guide/mm/damon/stat.rst
index c4b14daeb2dd6..46c5dd96aa2ed 100644
--- a/Documentation/admin-guide/mm/damon/stat.rst
+++ b/Documentation/admin-guide/mm/damon/stat.rst
@@ -89,3 +89,10 @@ percentiles of the idle time values via this read-only parameter.  Reading the
 parameter returns 101 idle time values in milliseconds, separated by comma.
 Each value represents 0-th, 1st, 2nd, 3rd, ..., 99th and 100th percentile idle
 times.
+
+kdamond_pid
+-----------
+
+PID of the DAMON thread.
+
+If DAMON_STAT is enabled, this becomes the PID of the worker thread.  Else, -1.
-- 
2.47.3

Re: [RFC PATCH v1.1 1/2] mm/damon/stat: add a parameter for reading kdamond pid

[sashiko-bot]

Sashiko AI review found 1 potential issue(s):
- [Low] Missing closing brace `}` in `damon_stat_start()` error path causes compilation failure.
--

commit 04b7136ae6b7cd907673996f57f0a613de1984b9
Author: SeongJae Park <sj@kernel.org>

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

This commit exposes the pid of the DAMON_STAT kdamond via a new read-only
module parameter named kdamond_pid to help user-space management and
monitoring.

> diff --git a/mm/damon/stat.c b/mm/damon/stat.c
> --- a/mm/damon/stat.c
> +++ b/mm/damon/stat.c

[ ... ]

> @@ -260,6 +269,13 @@ static int damon_stat_start(void)
>  		damon_stat_context = NULL;
>  		return err;
>  	}
> +	kdamond_pid = damon_kdamond_pid(damon_stat_context);
> +	if (kdamond_pid < 0) {
> +		err = kdamond_pid;
> +		kdamond_pid = -1;
> +		damon_destroy_ctx(damon_stat_context);
> +		damon_stat_context = NULL;
> +		return err;

Is there a missing closing brace for the if (kdamond_pid < 0) block here?
It looks like the omission will swallow the rest of the function into the
error path and cause a compilation failure.

>  
>  	damon_stat_last_refresh_jiffies = jiffies;
>  	call_control.data = damon_stat_context;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260414235912.98174-1-sj@kernel.org?part=1

Re: [RFC PATCH v1.1 1/2] mm/damon/stat: add a parameter for reading kdamond pid

[SeongJae Park]

On Wed, 15 Apr 2026 00:16:14 +0000 sashiko-bot@kernel.org wrote:

> Sashiko AI review found 1 potential issue(s):
> - [Low] Missing closing brace `}` in `damon_stat_start()` error path causes compilation failure.
> --
> 
> commit 04b7136ae6b7cd907673996f57f0a613de1984b9
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/stat: add a parameter for reading kdamond pid
> 
> This commit exposes the pid of the DAMON_STAT kdamond via a new read-only
> module parameter named kdamond_pid to help user-space management and
> monitoring.
> 
> > diff --git a/mm/damon/stat.c b/mm/damon/stat.c
> > --- a/mm/damon/stat.c
> > +++ b/mm/damon/stat.c
> 
> [ ... ]
> 
> > @@ -260,6 +269,13 @@ static int damon_stat_start(void)
> >  		damon_stat_context = NULL;
> >  		return err;
> >  	}
> > +	kdamond_pid = damon_kdamond_pid(damon_stat_context);
> > +	if (kdamond_pid < 0) {
> > +		err = kdamond_pid;
> > +		kdamond_pid = -1;
> > +		damon_destroy_ctx(damon_stat_context);
> > +		damon_stat_context = NULL;
> > +		return err;
> 
> Is there a missing closing brace for the if (kdamond_pid < 0) block here?
> It looks like the omission will swallow the rest of the function into the
> error path and cause a compilation failure.

Good eye.  I will fix this in the next revision.


Thanks,
SJ

[...]