[RFC PATCH 06/11] mm/damon/lru_sort: support young page filters

[SeongJae Park <sj@kernel.org>]

DAMON monitors access pattern in the region level, and hence there could
be some page level mismatches.  A few hot pages could be located in cold
region, and vice versa.  Young page filter can be useful for doing
additional page level access check before applying some DAMOS action.

DAMON_LRU_SORT is not using young page filters, though.  Add a parameter
for using it.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/lru_sort.c | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index 7513ebc6aeef..3ccde23a8566 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -41,6 +41,21 @@ static bool enabled __read_mostly;
 static bool commit_inputs __read_mostly;
 module_param(commit_inputs, bool, 0600);
 
+/*
+ * Filter [none-]young pages accordingly for LRU [de]prioritizations.
+ *
+ * If this is set, check page level access (youngness) once again before each
+ * LRU [de]prioritization operation.  LRU prioritization operation is skipped
+ * if the page has not accessed since the last check (not young).  LRU
+ * deprioritization operation is skipped if the page has accessed since the
+ * last check (young).  The feature is enabled or disabled if this parameter is
+ * set as ``Y`` or ``N``, respectively.
+ *
+ * Disabled by default.
+ */
+static bool filter_young_pages __read_mostly;
+module_param(filter_young_pages, bool, 0600);
+
 /*
  * Access frequency threshold for hot memory regions identification in permil.
  *
@@ -186,6 +201,28 @@ static struct damos *damon_lru_sort_new_cold_scheme(unsigned int cold_thres)
 	return damon_lru_sort_new_scheme(&pattern, DAMOS_LRU_DEPRIO);
 }
 
+static int damon_lru_sort_add_filters(struct damos *hot_scheme,
+		struct damos *cold_scheme)
+{
+	struct damos_filter *filter;
+
+	if (!filter_young_pages)
+		return 0;
+
+	/* disallow prioritizing not-young pages */
+	filter = damos_new_filter(DAMOS_FILTER_TYPE_YOUNG, false, false);
+	if (!filter)
+		return -ENOMEM;
+	damos_add_filter(hot_scheme, filter);
+
+	/* disabllow de-prioritizing young pages */
+	filter = damos_new_filter(DAMOS_FILTER_TYPE_YOUNG, true, false);
+	if (!filter)
+		return -ENOMEM;
+	damos_add_filter(cold_scheme, filter);
+	return 0;
+}
+
 static int damon_lru_sort_apply_parameters(void)
 {
 	struct damon_ctx *param_ctx;
@@ -219,6 +256,10 @@ static int damon_lru_sort_apply_parameters(void)
 	damon_set_schemes(param_ctx, &hot_scheme, 1);
 	damon_add_scheme(param_ctx, cold_scheme);
 
+	err = damon_lru_sort_add_filters(hot_scheme, cold_scheme);
+	if (err)
+		goto out;
+
 	err = damon_set_region_biggest_system_ram_default(param_target,
 					&monitor_region_start,
 					&monitor_region_end);
-- 
2.39.5