Re: [PATCH 2/2] updating ctime and mtime at syncing

["Anton Salikhmetov" <salikhmetov@gmail.com>]

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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