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    SubjectRe: [PATCH -v8 4/4] The design document for memory-mapped file times update
    2008/1/23, 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;
    > }
    >

    Miklos, thanks for your program. Its output is given below.

    debian:~/miklos# mount | grep mnt
    tmpfs on /mnt type tmpfs (rw)
    debian:~/miklos# ./miklos /mnt/file
    begin 1201089989 1201089989 1201085868
    write 1201089990 1201089990 1201085868
    mmap 1201089990 1201089990 1201089991
    store b 1201089992 1201089992 1201089991
    msync 1201089992 1201089992 1201089991
    store c 1201089994 1201089994 1201089991
    msync 1201089994 1201089994 1201089991
    store d 1201089996 1201089996 1201089991
    munmap 1201089996 1201089996 1201089991
    close 1201089996 1201089996 1201089991
    sync 1201089996 1201089996 1201089991
    debian:~/miklos# ./miklos /mnt/file -r
    begin 1201090025 1201090025 1201089991
    write 1201090026 1201090026 1201089991
    mmap 1201090026 1201090026 1201090027
    fetch x 1201090026 1201090026 1201090027
    store b 1201090026 1201090026 1201090027
    msync 1201090026 1201090026 1201090027
    fetch y 1201090026 1201090026 1201090027
    store c 1201090032 1201090032 1201090027
    msync 1201090032 1201090032 1201090027
    fetch z 1201090032 1201090032 1201090027
    store d 1201090036 1201090036 1201090027
    munmap 1201090036 1201090036 1201090027
    close 1201090036 1201090036 1201090027
    sync 1201090036 1201090036 1201090027
    debian:~/miklos#

    I think that after the patches applied the msync() system call does
    everything, which it is expected to do. The issue you're now talking
    about is one belonging to do_fsync() and the design of the tmpfs
    driver itself, I believe. Indeed, when the first write in the "-r"
    version of the test did not update the stamps, this is obviously not
    an msync() guilt.

    By the way, I would not like to move the "4/4" patch to the earlier
    place, because then it would describe the functionality, which had not
    been introduced yet.


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