Re: [PATCH -v8 4/4] The design document for memory-mapped file times update
From: Anton Salikhmetov
Date: Wed Jan 23 2008 - 07:25:49 EST
2008/1/23, Miklos Szeredi <miklos@xxxxxxxxxx>:
> > 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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