mm/damon: respect core layer filters' allowance decision on ops layer

Filtering decisions are made in filters evaluation order.  Once a decision
is made by a filter, filters that scheduled to be evaluated after the
decision-made filter should just respect it.  This is the intended and
documented behavior.  Since core layer-handled filters are evaluated
before operations layer-handled filters, decisions made on core layer
should respected by ops layer.

In case of reject filters, the decision is respected, since core
layer-rejected regions are not passed to ops layer.  But in case of allow
filters, ops layer filters don't know if the region has passed to them
because it was allowed by core filters or just because it didn't match to
any core layer.  The current wrong implementation assumes it was due to
not matched by any core filters.  As a reuslt, the decision is not
respected.  Pass the missing information to ops layer using a new filed in
'struct damos', and make the ops layer filters respect it.

Link: https://lkml.kernel.org/r/20250228175336.42781-1-sj@kernel.org
Fixes: 491fee2 ("mm/damon/core: support damos_filter->allow")
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

include/linux/damon.h

@@ -470,6 +470,11 @@ struct damos {
 	unsigned long next_apply_sis;
 	/* informs if ongoing DAMOS walk for this scheme is finished */
 	bool walk_completed;
+	/*
+	 * If the current region in the filtering stage is allowed by core
+	 * layer-handled filters.  If true, operations layer allows it, too.
+	 */
+	bool core_filters_allowed;
 /* public: */
 	struct damos_quota quota;
 	struct damos_watermarks wmarks;

mm/damon/core.c

@@ -1429,9 +1429,13 @@ static bool damos_filter_out(struct damon_ctx *ctx, struct damon_target *t,
 {
 	struct damos_filter *filter;
 
+	s->core_filters_allowed = false;
 	damos_for_each_filter(filter, s) {
-		if (damos_filter_match(ctx, t, r, filter))
+		if (damos_filter_match(ctx, t, r, filter)) {
+			if (filter->allow)
+				s->core_filters_allowed = true;
 			return !filter->allow;
+		}
 	}
 	return false;
 }

mm/damon/paddr.c

@@ -236,6 +236,9 @@ static bool damos_pa_filter_out(struct damos *scheme, struct folio *folio)
 {
 	struct damos_filter *filter;
 
+	if (scheme->core_filters_allowed)
+		return false;
+
 	damos_for_each_filter(filter, scheme) {
 		if (damos_pa_filter_match(filter, folio))
 			return !filter->allow;