Add opt hpa_hugify_sync to hugify synchronously

Linux 6.1 introduced `MADV_COLLAPSE` flag to perform a best-effort
synchronous collapse of the native pages mapped by the memory range into
transparent huge pages.

Synchronous hugification might be beneficial for at least two reasons:
we are not relying on khugepaged anymore and get an instant feedback if
range wasn't hugified.

If `hpa_hugify_sync` option is on, we'll try to perform synchronously
collapse and if it wasn't successful, we'll fallback to asynchronous
behaviour.

test/unit/hpa.c

@@ -32,6 +32,8 @@ static hpa_shard_opts_t test_hpa_shard_opts_default = {
 	false,
 	/* hugify_delay_ms */
 	10 * 1000,
+	/* hugify_sync */
+	false,
 	/* min_purge_interval_ms */
 	5 * 1000,
 	/* experimental_max_purge_nhp */
@@ -49,6 +51,8 @@ static hpa_shard_opts_t test_hpa_shard_opts_purge = {
 	true,
 	/* hugify_delay_ms */
 	0,
+	/* hugify_sync */
+	false,
 	/* min_purge_interval_ms */
 	5 * 1000,
 	/* experimental_max_purge_nhp */
@@ -371,9 +375,10 @@ defer_test_purge(void *ptr, size_t size) {
 }
 
 static size_t ndefer_hugify_calls = 0;
-static void
-defer_test_hugify(void *ptr, size_t size) {
+static bool
+defer_test_hugify(void *ptr, size_t size, bool sync) {
 	++ndefer_hugify_calls;
+	return false;
 }
 
 static size_t ndefer_dehugify_calls = 0;

test/unit/mallctl.c

@@ -288,6 +288,7 @@ TEST_BEGIN(test_mallctl_opt) {
 	TEST_MALLCTL_OPT(const char *, dss, always);
 	TEST_MALLCTL_OPT(bool, hpa, always);
 	TEST_MALLCTL_OPT(size_t, hpa_slab_max_alloc, always);
+	TEST_MALLCTL_OPT(bool, hpa_hugify_sync, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_nshards, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_max_alloc, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_max_bytes, always);