[PATCH] selftests: mm: fix and speedup "droppable" test

["David Hildenbrand (Arm)" <david@kernel.org>]

The droppable test currently relies on creating memory pressure in a
child process to trigger dropping the droppable pages.

That not only takes a long time on some machines (allocating and filling
all that memory), on large machines this will not work as we hardcode the
area size to 134217728 bytes.

... further, we rely on timeouts to detect that memory was not dropped,
which is really suboptimal.

Instead, let's just use MADV_PAGEOUT on a 2 MiB region. MADV_PAGEOUT works
with droppable memory even without swap.

There is the low chance of MADV_PAGEOUT failing to drop a page because
of speculative references. We'll wait 1s and retry 10 times to
rule that unlikely case out as best as we can.

On a machine without swap:

	$ ./droppable
	TAP version 13
	1..1
	ok 1 madvise(MADV_PAGEOUT) behavior
	# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

Reported-by: Aishwarya TCV <Aishwarya.TCV@arm.com>
Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 tools/testing/selftests/mm/droppable.c | 46 +++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/mm/droppable.c b/tools/testing/selftests/mm/droppable.c
index 30c8be37fcb9..57e1b6fc5569 100644
--- a/tools/testing/selftests/mm/droppable.c
+++ b/tools/testing/selftests/mm/droppable.c
@@ -17,10 +17,10 @@
 
 int main(int argc, char *argv[])
 {
-	size_t alloc_size = 134217728;
-	size_t page_size = getpagesize();
+	const size_t alloc_size = 2 * 1024 * 1024;
+	int retry_count = 10;
+	bool dropped;
 	void *alloc;
-	pid_t child;
 
 	ksft_print_header();
 	ksft_set_plan(1);
@@ -35,26 +35,32 @@ int main(int argc, char *argv[])
 		exit(KSFT_FAIL);
 	}
 	memset(alloc, 'A', alloc_size);
-	for (size_t i = 0; i < alloc_size; i += page_size)
-		assert(*(uint8_t *)(alloc + i));
-
-	child = fork();
-	assert(child >= 0);
-	if (!child) {
-		for (;;)
-			*(char *)malloc(page_size) = 'B';
-	}
 
-	for (bool done = false; !done;) {
-		for (size_t i = 0; i < alloc_size; i += page_size) {
-			if (!*(uint8_t *)(alloc + i)) {
-				done = true;
-				break;
+	while (retry_count--) {
+		if (madvise(alloc, alloc_size, MADV_PAGEOUT)) {
+			if (errno == EINVAL) {
+				ksft_test_result_skip("madvise(MADV_PAGEOUT) not supported\n");
+				exit(KSFT_SKIP);
 			}
+			ksft_test_result_fail("madvise(MADV_PAGEOUT) error: %s\n", strerror(errno));
+			exit(KSFT_FAIL);
 		}
+
+		dropped = memchr(alloc, 'A', alloc_size) == NULL;
+
+		/*
+		 * Speculative reference can temporarily prevent some
+		 * pages from getting dropped. So sleep and retry.
+		 *
+		 * If a page is not droppable for 10s, something
+		 * is seriously messed up and we want to fail.
+		 */
+		if (dropped)
+			break;
+		sleep(1);
 	}
-	kill(child, SIGTERM);
 
-	ksft_test_result_pass("MAP_DROPPABLE: PASS\n");
-	exit(KSFT_PASS);
+	ksft_test_result(dropped, "madvise(MADV_PAGEOUT) behavior\n");
+
+	ksft_finished();
 }

---

base-commit: d401506a8ee8ac6bc4a7767c17da036e9434a4a3

change-id: 20260611-droppable_test-3737bd791dfb

--

Cheers,

David