Andy Isaacson, le Tue 28 Jun 2005 11:16:20 -0700, a écrit :
> If your interpretation of the problem is correct, then it should be
> trivial to write a test program demonstrating the problem. Did you
> write the simple test program and run it?
I indeed didn't, trusting both the man page, the source code comments,
and my knowledge of zap_page_range().
> MADV_DONTNEED
> Do not expect access in the near future. (For the time
> being, the application is finished with the given range,
> so the kernel can free resources associated with it.)
> Subsequent accesses of pages in this range will succeed,
> but will result either in reloading of the memory contents
> from the underlying mapped file (see mmap) or
> zero-fill-on-demand pages for mappings without an
> underlying file.
>
> You seem to think that "reloading ... from the underlying mapped file"
> means that changes are lost, but that's not implied.
I didn't say anything precise. What mostly feared me was the
"zero-fill-on-demand pages for mappings without an underlying file."
> Below is the test program I used.
It does indeed work, but this is no proof. It your testcase it does
indeed work, since the page still remains in the page cache (it's a
shared mapping of the file). But now try this one. It uses private
mappings, and fails as expected, getting 0 in the ANONYMOUS case, and
the original file value in the file mapping case.
Regards,
Samuel Thibault
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>
typedef unsigned int u32;
void die(char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
vfprintf(stderr, fmt, ap);
va_end(ap);
exit(1);
}
int check_cookie(char *file, u32 cookie)
{
u32 buf;
int fd;
if((fd = open(file, O_RDONLY, 0)) == -1)
die("%s: %s\n", file, strerror(errno));
if(read(fd, &buf, sizeof(buf)) == -1)
die("read: %s\n", strerror(errno));
close(fd);
return buf == cookie;
}
int dotest(char *file, u32 cookie, int do_anonymous, int do_msync, int do_madvise)
{
void *p;
int fd, len = 16 * 1024, prot = PROT_READ|PROT_WRITE;
u32 newcookie;
if (!do_anonymous) {
if((fd = open(file, O_RDWR|O_CREAT, 0666)) == -1)
die("%s: %s\n", file, strerror(errno));
if(ftruncate(fd, len) == -1)
die("ftruncate: %s\n", strerror(errno));
if(write(fd, "", 1) == -1)
die("write: %s\n", strerror(errno));
}
if((p = mmap(0, len, prot, (do_anonymous ? MAP_ANONYMOUS : 0) | MAP_PRIVATE, do_anonymous ? -1 : fd, 0)) == MAP_FAILED)
die("mmap: %s\n", strerror(errno));
*(u32 *)p = cookie;
if(do_msync)
if(msync(p, len, MS_SYNC) == -1)
die("msync: %s\n", strerror(errno));
if(do_madvise)
if(madvise(p, len, MADV_DONTNEED) == -1)
die("madvise: %s\n", strerror(errno));
newcookie = *(u32 *)p;
printf("c = %08x msync: %s madvise: %s %s\n",
cookie, do_msync ? "YES" : " NO", do_madvise ? "YES" : " NO",
newcookie == cookie ? "ok" : "FAILED");
}
int main(int argc, char **argv)
{
if(argc != 2) die("usage: %s file\n", argv[0]);
lrand48();
dotest(argv[1], lrand48(), 0, 0, 0);
dotest(argv[1], lrand48(), 0, 1, 0);
dotest(argv[1], lrand48(), 0, 0, 1);
dotest(argv[1], lrand48(), 0, 1, 1);
dotest(argv[1], lrand48(), 1, 0, 0);
dotest(argv[1], lrand48(), 1, 1, 0);
dotest(argv[1], lrand48(), 1, 0, 1);
dotest(argv[1], lrand48(), 1, 1, 1);
}
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