ksm_start() enables ksmd and then polls full_scans until two more full
scans have completed, so that any merging that can happen has happened.

There is no helper to wait for a number of full scans while ksmd is
already running, so a test that needs one has to copy that loop.

Move the loop into ksm_wait_full_scans() and make ksm_start() call it
after enabling ksmd. The count is now read after ksmd starts rather than
before, which can only make ksm_start() wait longer.

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Tested-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
---
 tools/testing/selftests/mm/vm_util.c | 31 ++++++++++++++++++-------------
 tools/testing/selftests/mm/vm_util.h |  1 +
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index caea045fa7d46..8184691211acc 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -992,35 +992,40 @@ int ksm_use_zero_pages(void)
 	return ret == 1 ? 0 : -EIO;
 }
 
+int ksm_wait_full_scans(long nr)
+{
+	long start_scans, end_scans;
+
+	start_scans = ksm_get_full_scans();
+	if (start_scans < 0)
+		return start_scans;
+	do {
+		end_scans = ksm_get_full_scans();
+		if (end_scans < 0)
+			return end_scans;
+	} while (end_scans < start_scans + nr);
+
+	return 0;
+}
+
 int ksm_start(void)
 {
 	int ksm_fd;
 	ssize_t ret;
-	long start_scans, end_scans;
 
 	ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR);
 	if (ksm_fd < 0)
 		return -errno;
 
-	/* Wait for two full scans such that any possible merging happened. */
-	start_scans = ksm_get_full_scans();
-	if (start_scans < 0) {
-		close(ksm_fd);
-		return start_scans;
-	}
 	ret = write(ksm_fd, "1", 1);
 	close(ksm_fd);
 	if (ret < 0)
 		return -errno;
 	if (ret != 1)
 		return -EIO;
-	do {
-		end_scans = ksm_get_full_scans();
-		if (end_scans < 0)
-			return end_scans;
-	} while (end_scans < start_scans + 2);
 
-	return 0;
+	/* Wait for two full scans such that any possible merging happened. */
+	return ksm_wait_full_scans(2);
 }
 
 int ksm_stop(void)
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
index b5d59729d4327..7e00030077d2f 100644
--- a/tools/testing/selftests/mm/vm_util.h
+++ b/tools/testing/selftests/mm/vm_util.h
@@ -162,6 +162,7 @@ void *sys_mremap(void *old_address, unsigned long old_size,
 long ksm_get_self_zero_pages(void);
 long ksm_get_self_merging_pages(void);
 long ksm_get_full_scans(void);
+int ksm_wait_full_scans(long nr);
 int ksm_use_zero_pages(void);
 int ksm_start(void);
 int ksm_stop(void);

-- 
2.43.0