DAMON generates monitoring results snapshots for every sampling
interval.  DAMOS applies given schemes on the regions of the snapshots,
for every apply interval of the scheme.

DAMOS stat informs a given scheme has tried to how many memory entities
and applied, in the region and byte level.  In some use cases including
user-space oriented tuning and investigations, it is useful to know that
in the DAMON-snapshot level.  Introduce a new stat, namely nr_snapshots
for DAMON core API callers.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h |  3 +++
 mm/damon/core.c       | 13 ++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 3813373a9200..1d8a1515e75a 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -330,6 +330,8 @@ struct damos_watermarks {
  * @sz_ops_filter_passed:
  *		Total bytes that passed ops layer-handled DAMOS filters.
  * @qt_exceeds: Total number of times the quota of the scheme has exceeded.
+ * @nr_snapshots:
+ *		Total number of DAMON snapshots that the scheme has tried.
  *
  * "Tried an action to a region" in this context means the DAMOS core logic
  * determined the region as eligible to apply the action.  The access pattern
@@ -355,6 +357,7 @@ struct damos_stat {
 	unsigned long sz_applied;
 	unsigned long sz_ops_filter_passed;
 	unsigned long qt_exceeds;
+	unsigned long nr_snapshots;
 };
 
 /**
diff --git a/mm/damon/core.c b/mm/damon/core.c
index f9fc0375890a..af922d5e500c 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -157,6 +157,12 @@ void damon_destroy_region(struct damon_region *r, struct damon_target *t)
 	damon_free_region(r);
 }
 
+static bool damon_is_last_region(struct damon_region *r,
+		struct damon_target *t)
+{
+	return list_is_last(&t->regions_list, &r->list);
+}
+
 /*
  * Check whether a region is intersecting an address range
  *
@@ -1949,10 +1955,11 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (damos_skip_charged_region(t, &r, s, c->min_sz_region))
 			continue;
 
-		if (!damos_valid_target(c, t, r, s))
-			continue;
+		if (damos_valid_target(c, t, r, s))
+			damos_apply_scheme(c, t, r, s);
 
-		damos_apply_scheme(c, t, r, s);
+		if (damon_is_last_region(r, t))
+			s->stat.nr_snapshots++;
 	}
 }
 
-- 
2.47.3