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.

Link: https://lkml.kernel.org/r/20251216080128.42991-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 15 insertions, 1 deletions

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 9d5be7e9b8e0..344773f53f64 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)
@@ -1984,6 +1989,10 @@ 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);