Re: [PATCH -v8 4/4] The design document for memory-mapped file times update

[Miklos Szeredi <miklos@szeredi.hu>]

> Ah, OK, this is becuase mmap doesn't actually set up the page tables
> by default.   Try adding MAP_POPULATE to the flags.

Here's an updated version of the program, with an added a '-r' option,
that performs a read access on the mapping before doing the write
(basically equivalent to MAP_POPULATE, but portable).

Please try this on a tmpfs file.

Thanks,
Miklos

---
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/wait.h>

static const char *filename;
static int msync_flag = MS_ASYNC;
static int msync_fork = 0;
static int msync_read = 0;

static void print_times(const char *msg)
{
	struct stat stbuf;
	stat(filename, &stbuf);
	printf("%s\t%li\t%li\t%li\n", msg, stbuf.st_ctime, stbuf.st_mtime,
	       stbuf.st_atime);
}

static void do_msync(void *addr, int len)
{
	int res;
	if (!msync_fork) {
		res = msync(addr, len, msync_flag);
		if (res == -1) {
			perror("msync");
			exit(1);
		}
	} else {
		int pid = fork();
		if (pid == -1) {
			perror("fork");
			exit(1);
		}
		if (!pid) {
			int fd = open(filename, O_RDWR);
			if (fd == -1) {
				perror("open");
				exit(1);
			}
			addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
			if (addr == MAP_FAILED) {
				perror("mmap");
				exit(1);
			}
			res = msync(addr, len, msync_flag);
			if (res == -1) {
				perror("msync");
				exit(1);
			}
			exit(0);
		}
		wait(NULL);
	}
}

static void usage(const char *progname)
{
	fprintf(stderr, "usage: %s filename [-sfr]\n", progname);
	fprintf(stderr, " -s: use MS_SYNC instead of MS_ASYNC\n");
	fprintf(stderr, " -f: fork and perform msync in a different process\n");
	fprintf(stderr, " -r: do a read access before each write access\n");
	exit(1);
}

int main(int argc, char *argv[])
{
	int res;
	char *addr;
	char tmp[32];
	int fd;

	if (argc < 2)
		usage(argv[0]);

	filename = argv[1];
	if (argc > 2) {
		char *s;
		if (argc > 3)
			usage(argv[0]);
		s = argv[2];
		if (s[0] != '-' || !s[1])
			usage(argv[0]);
		for (s++; *s; s++) {
			switch (*s) {
			case 's':
				msync_flag = MS_SYNC;
				break;
			case 'f':
				msync_fork = 1;
				break;
			case 'r':
				msync_read = 1;
				break;
			default:
				usage(argv[0]);
			}
		}
	}

	fd = open(filename, O_RDWR | O_TRUNC | O_CREAT, 0666);
	if (fd == -1) {
		perror(filename);
		return 1;
	}
	print_times("begin");
	sleep(1);
	write(fd, "wxyz\n", 4);
	print_times("write");
	sleep(1);
	addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (addr == MAP_FAILED) {
		perror("mmap");
		return 1;
	}
	print_times("mmap");
	sleep(1);

	if (msync_read) {
		sprintf(tmp, "fetch %c", addr[1]);
		print_times(tmp);
		sleep(1);
	}
	addr[1] = 'b';
	print_times("store b");
	sleep(1);
	do_msync(addr, 4);
	print_times("msync");
	sleep(1);

	if (msync_read) {
		sprintf(tmp, "fetch %c", addr[2]);
		print_times(tmp);
		sleep(1);
	}
	addr[2] = 'c';
	print_times("store c");
	sleep(1);
	do_msync(addr, 4);
	print_times("msync");
	sleep(1);

	if (msync_read) {
		sprintf(tmp, "fetch %c", addr[3]);
		print_times(tmp);
		sleep(1);
	}
	addr[3] = 'd';
	print_times("store d");
	sleep(1);
	res = munmap(addr, 4);
	if (res == -1) {
		perror("munmap");
		return 1;
	}
	print_times("munmap");
	sleep(1);

	res = close(fd);
	if (res == -1) {
		perror("close");
		return 1;
	}
	print_times("close");
	sleep(1);
	sync();
	print_times("sync");

	return 0;
}

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