linux-kernel - [RFC PATCH 07/12] mm/damon/core: implement max_nr

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Message-ID: <20251123184329.85287-8-sj@kernel.org>
Date: Sun, 23 Nov 2025 10:43:21 -0800
From: SeongJae Park <sj@...nel.org>
To: 
Cc: SeongJae Park <sj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	damon@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [RFC PATCH 07/12] mm/damon/core: implement max_nr_snapshots

There are DAMOS use cases that require user-space centric control of its
activation and deactivation.  Having the control plane on the
user-space, or using DAMOS as a way for monitoring results collection
are such examples.

DAMON parameters online commit, DAMOS quotas and watermarks can be
useful for this purpose.  However, those features work only at the
sub-DAMON-snapshot level.  In some use cases, the DAMON-snapshot level
control is required.  For example, in DAMOS-based monitoring results
collection use case, the user online-installs a DAMOS scheme with
DAMOS_STAT action, wait it be applied to whole regions of a single
DAMON-snapshot, retrieves the stats and tried regions information, and
online-uninstall the scheme.  It is efficient to ensure the lifetime of
the scheme as no more no less one snapshot consumption.

To support such use cases, introduce a new DAMOS core API per-scheme
parameter, namely max_nr_snapshots.  As the name implies, it is the
upper limit of nr_snapshots, which is a DAMOS stat that represents the
number of DAMON-snapshots that the scheme has fully applied.  If the
limit is set with a non-zero value and nr_snapshots reaches or exceeds
the limit, the scheme is deactivated.

Signed-off-by: SeongJae Park <sj@...nel.org>
---
 include/linux/damon.h |  5 +++++
 mm/damon/core.c       | 11 ++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 43dfbfe2292f..a67292a2f09d 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -499,6 +499,7 @@ struct damos_migrate_dests {
  * @ops_filters:	ops layer handling &struct damos_filter objects list.
  * @last_applied:	Last @action applied ops-managing entity.
  * @stat:		Statistics of this scheme.
+ * @max_nr_snapshots:	Upper limit of nr_snapshots stat.
  * @list:		List head for siblings.
  *
  * For each @apply_interval_us, DAMON finds regions which fit in the
@@ -533,6 +534,9 @@ struct damos_migrate_dests {
  * finished.
  *
  * After applying the &action to each region, &stat is updated.
+ *
+ * If &max_nr_snapshots is set as non-zero and &stat.nr_snapshots be same to or
+ * greater than it, the scheme is deactivated.
  */
 struct damos {
 	struct damos_access_pattern pattern;
@@ -567,6 +571,7 @@ struct damos {
 	struct list_head ops_filters;
 	void *last_applied;
 	struct damos_stat stat;
+	unsigned long max_nr_snapshots;
 	struct list_head list;
 };

diff --git a/mm/damon/core.c b/mm/damon/core.c
index af922d5e500c..36313cd1ff1c 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -401,6 +401,7 @@ struct damos *damon_new_scheme(struct damos_access_pattern *pattern,
 	INIT_LIST_HEAD(&scheme->core_filters);
 	INIT_LIST_HEAD(&scheme->ops_filters);
 	scheme->stat = (struct damos_stat){};
+	scheme->max_nr_snapshots = 0;
 	INIT_LIST_HEAD(&scheme->list);

 	scheme->quota = *(damos_quota_init(quota));
@@ -1078,7 +1079,11 @@ static int damos_commit(struct damos *dst, struct damos *src)
 		return err;

 	err = damos_commit_filters(dst, src);
-	return err;
+	if (err)
+		return err;
+
+	dst->max_nr_snapshots = src->max_nr_snapshots;
+	return 0;
 }

 static int damon_commit_schemes(struct damon_ctx *dst, struct damon_ctx *src)
@@ -1955,6 +1960,10 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (damos_skip_charged_region(t, &r, s, c->min_sz_region))
 			continue;

+		if (s->max_nr_snapshots &&
+				s->max_nr_snapshots <= s->stat.nr_snapshots)
+			continue;
+
 		if (damos_valid_target(c, t, r, s))
 			damos_apply_scheme(c, t, r, s);

-- 
2.47.3