[RFC PATCH v3 9/9] mm/damon/core: allow both primitives disabled when a perf probe is present

[Ravi Jonnalagadda]

The sample control validation requires exactly one software access check
primitive, page_table or page_fault, to be enabled.  A context carrying a
perf-event probe needs neither: the probe drives the access signal
through its report ring and the drain, so region access rates and ages
update with both primitives off.

That check runs in damon_commit_sample_control(), which sees only the
sample control and cannot tell whether the context has such a probe.
Move it to __damon_commit_ctx(), skip it for a context with event-driven
probes, and run it with the other validation of @src before anything is
committed to @dst, so a rejected configuration cannot leave a live commit
with its PMU events already armed.

Add damon_test_both_primitives_disabled_with_probe(), which commits a
context with both primitives off and an event-driven probe and asserts the
commit succeeds.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@xxxxxxxxx>
---
 mm/damon/core.c              | 11 ++++++++---
 mm/damon/tests/drain-kunit.h | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index c2c74d0daf95..04a1cccbc2f5 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2357,9 +2357,6 @@ static int damon_commit_sample_control(
 		struct damon_sample_control *dst,
 		struct damon_sample_control *src)
 {
-	if (damon_primitives_enabled_invalid(&src->primitives_enabled))
-		return -EINVAL;
-
 	dst->primitives_enabled = src->primitives_enabled;
 	return damon_commit_sample_filters(dst, src);
 }
@@ -2390,6 +2387,14 @@ static int __damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src,
 
 	if (!damon_valid_probe_params(src))
 		return -EINVAL;
+	/*
+	 * A probe-driven context needs no software primitive.  Validate this
+	 * before anything is committed to @dst, so a rejected configuration
+	 * cannot leave a live commit with its PMU events already armed.
+	 */
+	if (!damon_has_event_driven_probes(src) &&
+	    damon_primitives_enabled_invalid(&src->sample_control.primitives_enabled))
+		return -EINVAL;
 
 	err = damon_commit_schemes(dst, src);
 	if (err)
diff --git a/mm/damon/tests/drain-kunit.h b/mm/damon/tests/drain-kunit.h
index 8e5023ebf6df..be44311617fc 100644
--- a/mm/damon/tests/drain-kunit.h
+++ b/mm/damon/tests/drain-kunit.h
@@ -722,6 +722,42 @@ static void damon_test_bsearch_unsorted_regions(struct kunit *test)
 	damon_destroy_ctx(ctx);
 }
 
+/*
+ * Test that damon_primitives_enabled_invalid() accepts the both-disabled
+ * configuration when an event-driven probe is present (this commit's
+ * relaxation).
+ *
+ * Before this commit this configuration was rejected (-EINVAL).  After it,
+ * it is valid: the probe drives access-rate updates directly via the report
+ * ring, so neither software primitive is required.
+ */
+static void damon_test_both_primitives_disabled_with_probe(struct kunit *test)
+{
+	struct damon_ctx *ctx;
+	int ret;
+
+	ctx = damon_new_ctx();
+	if (!ctx)
+		kunit_skip(test, "ctx alloc failed");
+
+	if (damon_test_attach_perf_probe(ctx)) {
+		damon_destroy_ctx(ctx);
+		kunit_skip(test, "perf probe alloc failed");
+	}
+
+	/*
+	 * Explicitly disable both primitives.  With an event-driven probe
+	 * present, damon_commit_ctx() must now succeed outright.
+	 */
+	ctx->sample_control.primitives_enabled.page_table = false;
+	ctx->sample_control.primitives_enabled.page_fault  = false;
+
+	ret = damon_commit_ctx(ctx, ctx);
+	KUNIT_EXPECT_EQ(test, ret, 0);
+
+	damon_destroy_ctx(ctx);
+}
+
 static struct kunit_case damon_drain_test_cases[] = {
 	KUNIT_CASE(damon_test_unified_vaddr_match),
 	KUNIT_CASE(damon_test_unified_vaddr_tgid_mismatch),
@@ -733,6 +769,7 @@ static struct kunit_case damon_drain_test_cases[] = {
 	KUNIT_CASE(damon_test_report_addr_space_keyed),
 	KUNIT_CASE(damon_test_ring_full_counter_increments),
 	KUNIT_CASE(damon_test_bsearch_unsorted_regions),
+	KUNIT_CASE(damon_test_both_primitives_disabled_with_probe),
 	{}
 };
 

-- 
Git-157)