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    SubjectRe: [PATCH 2/2] updating ctime and mtime at syncing
    2008/1/14, Miklos Szeredi <miklos@szeredi.hu>:
    > > > http://bugzilla.kernel.org/show_bug.cgi?id=2645
    > > >
    > > > Changes for updating the ctime and mtime fields for memory-mapped files:
    > > >
    > > > 1) new flag triggering update of the inode data;
    > > > 2) new function to update ctime and mtime for block device files;
    > > > 3) new helper function to update ctime and mtime when needed;
    > > > 4) updating time stamps for mapped files in sys_msync() and do_fsync();
    > > > 5) implementing the feature of auto-updating ctime and mtime.
    > >
    > > How exactly is this done?
    > >
    > > Is this catering for this case:
    > >
    > > 1 page is dirtied through mapping
    > > 2 app calls msync(MS_ASYNC)
    > > 3 page is written again through mapping
    > > 4 app calls msync(MS_ASYNC)
    > > 5 ...
    > > 6 page is written back
    > >
    > > What happens at 4? Do we care about this one at all?
    >
    > Oh, and here's a test program I wrote, that can be used to check this
    > behavior. It has two options:
    >
    > -s use MS_SYNC instead of MS_ASYNC
    > -f fork and do the msync on a different mapping
    >
    > Back then I haven't found a single OS, that fully conformed to all the
    > stupid POSIX rules regarding mmaps and ctime/mtime.

    Thank you very much for sharing your code.

    I'll integrate the MS_ASYNC and fork() test cases into my own unit test.

    >
    > 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 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 [-sf]\n", progname);
    > exit(1);
    > }
    >
    > int main(int argc, char *argv[])
    > {
    > int res;
    > char *addr;
    > int fd;
    >
    > if (argc < 2)
    > usage(argv[0]);
    >
    > filename = argv[1];
    > if (argc > 2) {
    > if (argc > 3)
    > usage(argv[0]);
    > if (strcmp(argv[2], "-s") == 0)
    > msync_flag = MS_SYNC;
    > else if (strcmp(argv[2], "-f") == 0)
    > msync_fork = 1;
    > else if (strcmp(argv[2], "-sf") == 0 || strcmp(argv[2], "-fs") == 0) {
    > msync_flag = MS_SYNC;
    > msync_fork = 1;
    > } else
    > 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, "aaaa\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);
    >
    > addr[1] = 'b';
    > print_times("b");
    > sleep(1);
    > do_msync(addr, 4);
    > print_times("msync b");
    > sleep(1);
    >
    > addr[2] = 'c';
    > print_times("c");
    > sleep(1);
    > do_msync(addr, 4);
    > print_times("msync c");
    > sleep(1);
    >
    > addr[3] = 'd';
    > print_times("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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