selftests/damon: add a test for the pid leak of dbgfs_target_ids_write()

Commit ebb3f994 ("mm/damon/dbgfs: fix 'struct pid' leaks in
'dbgfs_target_ids_write()'") fixes a pid leak bug in DAMON debugfs
interface, namely dbgfs_target_ids_write() function.  Add a selftest for
the issue to prevent the problem from mistakenly recurring.

Link: https://lkml.kernel.org/r/20240207203134.69976-8-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 huge_count_read_write
 debugfs_target_ids_read_before_terminate_race
+debugfs_target_ids_pid_leak

tools/testing/selftests/damon/Makefile

@@ -3,6 +3,7 @@
 
 TEST_GEN_FILES += huge_count_read_write
 TEST_GEN_FILES += debugfs_target_ids_read_before_terminate_race
+TEST_GEN_FILES += debugfs_target_ids_pid_leak
 TEST_GEN_FILES += access_memory
 
 TEST_FILES = _chk_dependency.sh _debugfs_common.sh
@@ -11,6 +12,7 @@ TEST_PROGS += debugfs_empty_targets.sh debugfs_huge_count_read_write.sh
 TEST_PROGS += debugfs_duplicate_context_creation.sh
 TEST_PROGS += debugfs_rm_non_contexts.sh
 TEST_PROGS += debugfs_target_ids_read_before_terminate_race.sh
+TEST_PROGS += debugfs_target_ids_pid_leak.sh
 TEST_PROGS += sysfs.sh sysfs_update_removed_scheme_dir.sh
 TEST_PROGS += sysfs_update_schemes_tried_regions_hang.py
 TEST_PROGS += sysfs_update_schemes_tried_regions_wss_estimation.py

tools/testing/selftests/damon/debugfs_target_ids_pid_leak.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Author: SeongJae Park <sj@kernel.org>
+ */
+
+#define _GNU_SOURCE
+
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/time.h>
+#include <unistd.h>
+
+#define DBGFS_TARGET_IDS "/sys/kernel/debug/damon/target_ids"
+
+static void write_targetid_exit(void)
+{
+	int target_ids_fd = open(DBGFS_TARGET_IDS, O_RDWR);
+	char pid_str[128];
+
+	snprintf(pid_str, sizeof(pid_str), "%d", getpid());
+	write(target_ids_fd, pid_str, sizeof(pid_str));
+	close(target_ids_fd);
+	exit(0);
+}
+
+unsigned long msec_timestamp(void)
+{
+	struct timeval tv;
+
+	gettimeofday(&tv, NULL);
+	return tv.tv_sec * 1000UL + tv.tv_usec / 1000;
+}
+
+int main(int argc, char *argv[])
+{
+	unsigned long start_ms;
+	int time_to_run, nr_forks = 0;
+
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s <msecs to run>\n", argv[0]);
+		exit(1);
+	}
+	time_to_run = atoi(argv[1]);
+
+	start_ms = msec_timestamp();
+	while (true) {
+		int pid = fork();
+
+		if (pid < 0) {
+			fprintf(stderr, "fork() failed\n");
+			exit(1);
+		}
+		if (pid == 0)
+			write_targetid_exit();
+		wait(NULL);
+		nr_forks++;
+
+		if (msec_timestamp() - start_ms > time_to_run)
+			break;
+	}
+	printf("%d\n", nr_forks);
+	return 0;
+}

tools/testing/selftests/damon/debugfs_target_ids_pid_leak.sh

+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+before=$(grep "^pid " /proc/slabinfo | awk '{print $2}')
+
+nr_leaks=$(./debugfs_target_ids_pid_leak 1000)
+expected_after_max=$((before + nr_leaks / 2))
+
+after=$(grep "^pid " /proc/slabinfo | awk '{print $2}')
+
+echo > /sys/kernel/debug/damon/target_ids
+
+echo "tried $nr_leaks pid leak"
+echo "number of active pid slabs: $before -> $after"
+echo "(up to $expected_after_max expected)"
+if [ $after -gt $expected_after_max ]
+then
+	echo "maybe pids are leaking"
+	exit 1
+else
+	exit 0
+fi