From: xu xin <xu.xin16@zte.com.cn>

The existing tools/testing/selftests/mm/rmap.c has already one testcase
for ksm_rmap_walk in TEST_F(migrate, ksm), which takes use of migration
of page from one NUMA node to another NUMA node. However, it just lacks
the scenario of mremapped VMAs.

We add the calling of mremap() and then trigger KSM to merge pages before
migrating, which is specifically to test an optimization which is
introduced by this patch ("ksm: Optimize rmap_walk_ksm by passing a
suitable address pgoff").

This test can reproduce the issue that Hugh points out at
https://lore.kernel.org/all/02e1b8df-d568-8cbb-b8f6-46d5476d9d75@google.com/

Signed-off-by: xu xin <xu.xin16@zte.com.cn>
---
 tools/testing/selftests/mm/rmap.c | 86 +++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)

diff --git a/tools/testing/selftests/mm/rmap.c b/tools/testing/selftests/mm/rmap.c
index 53f2058b0ef2..1cdc4beb48c2 100644
--- a/tools/testing/selftests/mm/rmap.c
+++ b/tools/testing/selftests/mm/rmap.c
@@ -430,4 +430,90 @@ TEST_F(migrate, ksm)
 	propagate_children(_metadata, data);
 }

+static void prepare_pages(struct global_data *data, int nr_pages)
+{
+	/* Allocate exactly pages for the test */
+	data->mapsize = nr_pages * getpagesize();
+	data->region = mmap(NULL, data->mapsize, PROT_READ | PROT_WRITE,
+			    MAP_PRIVATE | MAP_ANON, -1, 0);
+	if (data->region == MAP_FAILED)
+		ksft_exit_fail_perror("mmap failed");
+
+	/* Fill all pages with identical content to encourage KSM merging */
+	memset(data->region, 0x77, data->mapsize);
+}
+
+static int mremap_merge_and_migrate(struct global_data *data)
+{
+	int ret;
+	void *old_region;
+	void *new_region;
+	int nr_pages = 32;
+	long merging_pages;
+
+	prepare_pages(data, nr_pages);
+
+	if (ksm_start() < 0)
+		return FAIL_ON_CHECK;
+
+	old_region = data->region;
+	/*
+	 * Mremap the second half region to the first half location (FIXED).
+	 */
+	new_region = mremap(old_region + data->mapsize / 2, data->mapsize / 2,
+			    data->mapsize / 2, MREMAP_MAYMOVE | MREMAP_FIXED,
+			    old_region);
+	if (new_region == MAP_FAILED) {
+		ksft_print_msg("mremap failed: %s\n", strerror(errno));
+		return FAIL_ON_CHECK;
+	}
+	data->region = new_region;
+	data->mapsize /= 2;	/* mapping is now half of original */
+
+	if (ksm_start() < 0)
+		return FAIL_ON_CHECK;
+
+	/* Attempt to migrate the merged KSM page */
+	ret = try_to_move_page(data->region);
+	if (ret != 0) {
+		ksft_print_msg("migration of KSM page after mremap failed\n");
+		return FAIL_ON_CHECK;
+	}
+
+	/* Ensure ksmd scan two turns at least to update ksm counters */
+	if (ksm_start() < 0)
+		return FAIL_ON_CHECK;
+
+	merging_pages = ksm_get_self_merging_pages();
+	printf("merging_pages:%ld\n", merging_pages);
+	if (merging_pages != nr_pages / 2) {
+		ksft_print_msg("Unexpected KSM counters: ksm_merging_pages=%ld,expected=%d\n",
+			       merging_pages, nr_pages / 2);
+		return FAIL_ON_CHECK;
+	}
+
+	return 0;
+}
+
+
+TEST_F(migrate, ksm_and_mremap)
+{
+	struct global_data *data = &self->data;
+	int ret;
+
+	/* Skip if KSM is not available */
+	if (ksm_stop() < 0)
+		SKIP(return, "accessing \"/sys/kernel/mm/ksm/run\" failed");
+	if (ksm_get_full_scans() < 0)
+		SKIP(return, "accessing \"/sys/kernel/mm/ksm/full_scan\" failed");
+
+	ret = prctl(PR_SET_MEMORY_MERGE, 1, 0, 0, 0);
+	if (ret < 0 && errno == EINVAL)
+		SKIP(return, "PR_SET_MEMORY_MERGE not supported");
+	else if (ret)
+		ksft_exit_fail_perror("PR_SET_MEMORY_MERGE=1 failed");
+
+	ASSERT_EQ(mremap_merge_and_migrate(data), 0);
+}
+
 TEST_HARNESS_MAIN
-- 
2.25.1