On Wed, 27 Dec 2006, Linus Torvalds wrote:
>
> I think the test-case could probably be improved by having a munmap() and
> page-cache flush in between the writing and the checking, to see whether
> that shows the corruption easier (and possibly without having to start
> paging in order to throw the pages out, which would simplify testing a
> lot).
I think the page-writeout is implicated, because I do seem to need it, but
the page-cache flush does seem to make corruption _easier_ to see. I now
seem about to trigger it with a 100MB file on a 256MB machine in a minute
or so, with this slight modification.
I still don't see _why_, though. But maybe smarter people than me can see
it..
Linus
---
#include <sys/mman.h>
#include <sys/fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <time.h>
#define TARGETSIZE (100 << 20)
#define CHUNKSIZE (1460)
#define NRCHUNKS (TARGETSIZE / CHUNKSIZE)
#define SIZE (NRCHUNKS * CHUNKSIZE)
static void fillmem(void *start, int nr)
{
memset(start, nr, CHUNKSIZE);
}
static void checkmem(void *start, int nr)
{
unsigned char c = nr, *p = start;
int i;
for (i = 0; i < CHUNKSIZE; i++) {
if (*p++ != c) {
printf("Chunk %d corrupted \n", nr);
return;
}
}
}
static char *remap(int fd, char *mapping)
{
if (mapping) {
munmap(mapping, SIZE);
posix_fadvise(fd, 0, SIZE, POSIX_FADV_DONTNEED);
}
return mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
}
int main(int argc, char **argv)
{
char *mapping;
int fd, i;
static int chunkorder[NRCHUNKS];
/*
* Make some random ordering of writing the chunks to the
* memory map..
*
* Start with fully ordered..
*/
for (i = 0; i < NRCHUNKS; i++)
chunkorder[i] = i;
/* ..and then mix it up randomly */
srandom(time(NULL));
for (i = 0; i < NRCHUNKS; i++) {
int index = (unsigned int) random() % NRCHUNKS;
int nr = chunkorder[index];
chunkorder[index] = chunkorder[i];
chunkorder[i] = nr;
}
fd = open("mapfile", O_RDWR | O_TRUNC | O_CREAT, 0666);
if (fd < 0)
return -1;
if (ftruncate(fd, SIZE) < 0)
return -1;
mapping = remap(fd, NULL);
if (-1 == (int)(long)mapping)
return -1;
for (i = 0; i < NRCHUNKS; i++) {
int chunk = chunkorder[i];
printf("Writing chunk %d/%d (%d%%) \r", i, NRCHUNKS,
100*i/NRCHUNKS);
fillmem(mapping + chunk * CHUNKSIZE, chunk);
}
printf("\n");
/* Unmap, drop, and remap.. */
mapping = remap(fd, mapping);
/* .. and check */
for (i = 0; i < NRCHUNKS; i++) {
int chunk = i;
printf("Checking chunk %d/%d (%d%%) \r", i, NRCHUNKS,
100*i/NRCHUNKS);
checkmem(mapping + chunk * CHUNKSIZE, chunk);
}
printf("\n");
return 0;
}
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