DAMON resets the age of a region if its nr_accesses value has
significantly changed. Specifically, the threshold is calculated as 20%
of largest nr_accesses of the current snapshot.  This means that regions
changing the nr_accesses from zero to small non-zero value or from a
small non-zero value to zero will keep the age.  Since many users treat
zero nr_accesses regions special, this can be confusing.  Kernel code
including DAMOS' regions priority calculation and DAMON_STAT's idle time
calculation also treat zero nr_accesses regions special.  Make it
unconfusing by resetting the age when the nr_accesses changes between
zero and a non-zero value.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index be5942435d78..996647caca02 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2261,6 +2261,9 @@ static void damon_merge_regions_of(struct damon_target *t, unsigned int thres,
 	damon_for_each_region_safe(r, next, t) {
 		if (abs(r->nr_accesses - r->last_nr_accesses) > thres)
 			r->age = 0;
+		else if ((!r->nr_accesses && r->last_nr_accesses) ||
+				(r->nr_accesses && !r->last_nr_accesses))
+			r->age = 0;
 		else
 			r->age++;
 

base-commit: 580b0ae8069448252cfea72b75ba5b0857c128e0
-- 
2.39.5