selftests: mm: protection_keys: save/restore nr_hugepages value from launch script

The save/restore of nr_hugepages was added to the test itself by using the
atexit() functionality.  But it is broken as parent exits after creating
child.  Hence calling the atexit() function early.  That's not it.  The
child exits after creating its child and so on.

The parent cannot wait to get the termination status for its children as
it'll keep on holding the resources until the new pkey allocation fails. 
It is impossible to wait for exits of all the grand and great grand
children.  Hence the restoring of nr_hugepages value from parent is wrong.

Let's save/restore the nr_hugepages settings in the launch script
instead of doing it in the test.

Link: https://lkml.kernel.org/r/20240419115027.3848958-1-usama.anjum@collabora.com
Fixes: c52eb6db ("selftests: mm: restore settings from only parent process")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reported-by: Joey Gouly <joey.gouly@arm.com>
Closes: https://lore.kernel.org/all/20240418125250.GA2941398@e124191.cambridge.arm.com
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/protection_keys.c

@@ -54,7 +54,6 @@ int test_nr;
 u64 shadow_pkey_reg;
 int dprint_in_signal;
 char dprint_in_signal_buffer[DPRINT_IN_SIGNAL_BUF_SIZE];
-char buf[256];
 
 void cat_into_file(char *str, char *file)
 {
@@ -1745,42 +1744,6 @@ void pkey_setup_shadow(void)
 	shadow_pkey_reg = __read_pkey_reg();
 }
 
-pid_t parent_pid;
-
-void restore_settings_atexit(void)
-{
-	if (parent_pid == getpid())
-		cat_into_file(buf, "/proc/sys/vm/nr_hugepages");
-}
-
-void save_settings(void)
-{
-	int fd;
-	int err;
-
-	if (geteuid())
-		return;
-
-	fd = open("/proc/sys/vm/nr_hugepages", O_RDONLY);
-	if (fd < 0) {
-		fprintf(stderr, "error opening\n");
-		perror("error: ");
-		exit(__LINE__);
-	}
-
-	/* -1 to guarantee leaving the trailing \0 */
-	err = read(fd, buf, sizeof(buf)-1);
-	if (err < 0) {
-		fprintf(stderr, "error reading\n");
-		perror("error: ");
-		exit(__LINE__);
-	}
-
-	parent_pid = getpid();
-	atexit(restore_settings_atexit);
-	close(fd);
-}
-
 int main(void)
 {
 	int nr_iterations = 22;
@@ -1788,7 +1751,6 @@ int main(void)
 
 	srand((unsigned int)time(NULL));
 
-	save_settings();
 	setup_handlers();
 
 	printf("has pkeys: %d\n", pkeys_supported);

tools/testing/selftests/mm/run_vmtests.sh

@@ -385,6 +385,7 @@ CATEGORY="ksm_numa" run_test ./ksm_tests -N -m 0
 CATEGORY="ksm" run_test ./ksm_functional_tests
 
 # protection_keys tests
+nr_hugepgs=$(cat /proc/sys/vm/nr_hugepages)
 if [ -x ./protection_keys_32 ]
 then
 	CATEGORY="pkey" run_test ./protection_keys_32
@@ -394,6 +395,7 @@ if [ -x ./protection_keys_64 ]
 then
 	CATEGORY="pkey" run_test ./protection_keys_64
 fi
+echo "$nr_hugepgs" > /proc/sys/vm/nr_hugepages
 
 if [ -x ./soft-dirty ]
 then