/*
 *
 */
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <string.h>

#include <sys/eventfd.h>
#include <sys/mman.h>
#include <sys/types.h>

#define STRING_MAX	(512)

void handle_oom(void)
{
	printf("notification received\n");
}

int wait_oom_notifier(int eventfd_fd, void (*handler)(void))
{
	uint64_t ret;
	int err;

	for (;;) {
		err = read(eventfd_fd, &ret, sizeof(ret));
		if (err != sizeof(ret)) {
			fprintf(stderr, "read()\n");
			return err;
		}
		handler();
	}
}

int register_oom_notifier(const char *memcg)
{
	char path[PATH_MAX];
	char control_string[STRING_MAX];
	int event_control_fd;
	int control_fd;
	int eventfd_fd;
	int err = 0;

	err = snprintf(path, PATH_MAX, "%s/memory.oom_control", memcg);
	if (err < 0) {
		fprintf(stderr, "snprintf()\n");
		goto out;
	}

	control_fd = open(path, O_RDONLY);
	if (control_fd == -1) {
		fprintf(stderr, "open(): %d\n", errno);
		err = errno;
		goto out;
	}

	eventfd_fd = eventfd(0, 0);
	if (eventfd_fd == -1) {
		fprintf(stderr, "eventfd(): %d\n", errno);
		err = errno;
		goto out_close_control;
	}

	err = snprintf(control_string, STRING_MAX, "%d %d", eventfd_fd,
		       control_fd);
	if (err < 0) {
		fprintf(stderr, "snprintf()\n");
		goto out_close_eventfd;
	}

	err = snprintf(path, PATH_MAX, "%s/cgroup.event_control", memcg);
	if (err < 0) {
		fprintf(stderr, "snprintf()\n");
		goto out_close_eventfd;
	}

	event_control_fd = open(path, O_WRONLY);
	if (event_control_fd == 1) {
		fprintf(stderr, "open(): %d\n", errno);
		err = errno;
		goto out_close_eventfd;
	}

	write(event_control_fd, control_string, strlen(control_string));
	close(event_control_fd);

	return eventfd_fd;

out_close_eventfd:
	close(eventfd_fd);
out_close_control:
	close(control_fd);
out:
	return err;
}

int main(int argc, char **argv)
{
	int eventfd_fd;
	int err = 0;

	if (argc != 2) {
		fprintf(stderr, "usage: %s <path>\n", argv[0]);
		return -1;
	}

	err = mlockall(MCL_FUTURE);
	if (err) {
		fprintf(stderr, "%d\n", errno);
		return -1;
	}

	eventfd_fd = register_oom_notifier(argv[1]);
	if (eventfd_fd < 0) {
		fprintf(stderr, "%d\n", err);
		goto out;
	}

	err = wait_oom_notifier(eventfd_fd, handle_oom);
	if (err) {
		fprintf(stderr, "wait_oom_notifier()\n");
		goto out;
	}

out:
	munlockall();
	return err;
}