mm/damon/sysfs: test commit input against realistic destination

DAMON sysfs interface tests if given online parameters update request is
valid, by committing those using the DAMON kernel API, to a test-purpose
destination context. The test-purpose destination context is constructed
using damon_new_ctx(), so it has no target, no scheme.

If a source target has the obsolete field set, the test-purpose commit
will fail because damon_commit_targets() fails when there is a source
obsolete target that cannot find its matching destination target. DAMON
sysfs interface is not letting users set the field for now, so there is no
problem. However, the following commit will support that. Also there
could be similar future changes that making commit fails based on current
context structure.

Make the test purpose commit destination context similar to the current
running one, by committing the running one to the test purpose context,
before doing the real test-purpose commit.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: SeongJae Park <[e-mail]>
Reviewed-by: Bijan Tabatabai <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: Lorenzo Stoakes <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/damon/sysfs.c

···
 		struct damon_sysfs_context *sys_ctx);
 
 /*
+ * Return a new damon_ctx for testing new parameters to commit.
+ */
+static struct damon_ctx *damon_sysfs_new_test_ctx(
+		struct damon_ctx *running_ctx)
+{
+	struct damon_ctx *test_ctx;
+	int err;
+
+	test_ctx = damon_new_ctx();
+	if (!test_ctx)
+		return NULL;
+	err = damon_commit_ctx(test_ctx, running_ctx);
+	if (err) {
+		damon_destroy_ctx(test_ctx);
+		return NULL;
+	}
+	return test_ctx;
+}
+
+/*
  * damon_sysfs_commit_input() - Commit user inputs to a running kdamond.
  * @kdamond:	The kobject wrapper for the associated kdamond.
  *
···
 	param_ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
 	if (IS_ERR(param_ctx))
 		return PTR_ERR(param_ctx);
-	test_ctx = damon_new_ctx();
+	test_ctx = damon_sysfs_new_test_ctx(kdamond->damon_ctx);
 	if (!test_ctx)
 		return -ENOMEM;
 	err = damon_commit_ctx(test_ctx, param_ctx);