[RFC PATCH v1.2 03/17] mm/damon/core: always update ->last_nr_accesses for intervals change

[SeongJae Park <sj@kernel.org>]

Each iteration of kdamond_fn() main loop caches and use the next
aggregation time (next_aggregation_sis) because it can be updated in the
middle, inside kdamond_call().  If that happens,
damon_update_monitoring_result() is called for scaling the access
frequency information of each region according to the changed intervals.
The function does not update damon_region->last_nr_accesses when it is
at the end of the aggregation, because it will anyway be reset after the
function is executed, in kdamond_reset_aggregated().

Let's suppose damon_nr_accesses_mvsum() is called with the not yet
updated last_nr_accesses.  It will use the fresh next_aggregation_sis in
the context instead of the cached one, unlike kdamond_fn().  As a
result, use of not updated last_nr_acceses with the updated
next_aggregation_sis result in returning wrong value.

There is no such damon_nr_accesses_nvsum() call at the moment, so this
is no problem.  It is planned to add such calls, though.  Prevent the
issue by updating last_nr_accesses always.  This adds overhead, but
that's fine because the overhead is not big, and it is anyway not a fast
path.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 2cc911fa221aa..191533685cf2f 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -873,6 +873,8 @@ static void damon_update_monitoring_result(struct damon_region *r,
 		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs,
 		bool aggregating)
 {
+	r->last_nr_accesses = damon_nr_accesses_for_new_attrs(
+			r->last_nr_accesses, old_attrs, new_attrs);
 	if (!aggregating) {
 		r->nr_accesses = damon_nr_accesses_for_new_attrs(
 				r->nr_accesses, old_attrs, new_attrs);
@@ -884,8 +886,6 @@ static void damon_update_monitoring_result(struct damon_region *r,
 		 * interval.  In other words, make the status like
 		 * kdamond_reset_aggregated() is called.
 		 */
-		r->last_nr_accesses = damon_nr_accesses_for_new_attrs(
-				r->last_nr_accesses, old_attrs, new_attrs);
 		r->nr_accesses_bp = r->last_nr_accesses * 10000;
 		r->nr_accesses = 0;
 	}
-- 
2.47.3