[PATCH 2/4] selftest/arm64: Treat KSM merge_across_nodes as optional

[Muhammad Usama Anjum <usama.anjum@arm.com>]

The MTE KSM test unconditionally saves, enables and restores the
merge_across_nodes sysfs attribute. The kernel only creates this
attribute when CONFIG_NUMA=y. A non-NUMA kernel consequently prints the
following message three times even though all KSM subtests pass:

  # ERR: missing /sys/kernel/mm/ksm/merge_across_nodes

Check whether the attribute exists and only access it when present. This
preserves the existing setup and restore behavior on NUMA kernels without
requiring NUMA or reducing KSM coverage on single-node systems.

Fixes: f981d8fa2646 ("kselftest/arm64: Verify KSM page merge for MTE pages")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
---
 .../testing/selftests/arm64/mte/check_ksm_options.c  | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/arm64/mte/check_ksm_options.c b/tools/testing/selftests/arm64/mte/check_ksm_options.c
index 0cf5faef17248..b36945fc883d3 100644
--- a/tools/testing/selftests/arm64/mte/check_ksm_options.c
+++ b/tools/testing/selftests/arm64/mte/check_ksm_options.c
@@ -6,6 +6,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -22,6 +23,7 @@
 
 static size_t page_sz;
 static unsigned long ksm_sysfs[5];
+static bool has_merge_across_nodes;
 
 static unsigned long read_sysfs(char *str)
 {
@@ -56,8 +58,11 @@ static void write_sysfs(char *str, unsigned long val)
 
 static void mte_ksm_setup(void)
 {
-	ksm_sysfs[0] = read_sysfs(PATH_KSM "merge_across_nodes");
-	write_sysfs(PATH_KSM "merge_across_nodes", 1);
+	has_merge_across_nodes = !access(PATH_KSM "merge_across_nodes", F_OK);
+	if (has_merge_across_nodes) {
+		ksm_sysfs[0] = read_sysfs(PATH_KSM "merge_across_nodes");
+		write_sysfs(PATH_KSM "merge_across_nodes", 1);
+	}
 	ksm_sysfs[1] = read_sysfs(PATH_KSM "sleep_millisecs");
 	write_sysfs(PATH_KSM "sleep_millisecs", 0);
 	ksm_sysfs[2] = read_sysfs(PATH_KSM "run");
@@ -70,7 +75,8 @@ static void mte_ksm_setup(void)
 
 static void mte_ksm_restore(void)
 {
-	write_sysfs(PATH_KSM "merge_across_nodes", ksm_sysfs[0]);
+	if (has_merge_across_nodes)
+		write_sysfs(PATH_KSM "merge_across_nodes", ksm_sysfs[0]);
 	write_sysfs(PATH_KSM "sleep_millisecs", ksm_sysfs[1]);
 	write_sysfs(PATH_KSM "run", ksm_sysfs[2]);
 	write_sysfs(PATH_KSM "max_page_sharing", ksm_sysfs[3]);
-- 
2.47.3