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SubjectRe: [PATCH 2/2] updating ctime and mtime at syncing
From
Date
> > 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.

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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