selftests/mm: save and restore nr_hugepages value

Save and restore nr_hugepages before changing it during the test.  A test
should not change system wide settings.

Link: https://lkml.kernel.org/r/20240125154608.720072-4-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/charge_reserved_hugetlb.sh

@@ -11,6 +11,8 @@ if [[ $(id -u) -ne 0 ]]; then
   exit $ksft_skip
 fi
 
+nr_hugepgs=$(cat /proc/sys/vm/nr_hugepages)
+
 fault_limit_file=limit_in_bytes
 reservation_limit_file=rsvd.limit_in_bytes
 fault_usage_file=usage_in_bytes
@@ -582,3 +584,5 @@ if [[ $do_umount ]]; then
   umount $cgroup_path
   rmdir $cgroup_path
 fi
+
+echo "$nr_hugepgs" > /proc/sys/vm/nr_hugepages

tools/testing/selftests/mm/hugetlb_reparenting_test.sh

@@ -11,6 +11,7 @@ if [[ $(id -u) -ne 0 ]]; then
   exit $ksft_skip
 fi
 
+nr_hugepgs=$(cat /proc/sys/vm/nr_hugepages)
 usage_file=usage_in_bytes
 
 if [[ "$1" == "-cgroup-v2" ]]; then
@@ -252,3 +253,5 @@ if [[ $do_umount ]]; then
   umount $CGROUP_ROOT
   rm -rf $CGROUP_ROOT
 fi
+
+echo "$nr_hugepgs" > /proc/sys/vm/nr_hugepages