ftruncate-mmap: pages are lost after writing to mmaped file.

From: Ying Han
Date: Wed Mar 18 2009 - 15:44:28 EST


We triggered the failure during some internal experiment with
ftruncate/mmap/write/read sequence. And we found that some pages are
"lost" after writing to the mmaped file. which in the following test
cases (count >= 0).

First we deployed the test cases into group of machines and see about
>20% failure rate on average. Then, I did couple of experiment to try
to reproduce it on a single machine. what i found is that:
1. add a fsync after write the file, i can not reproduce this issue.
2. add memory pressure(mmap/mlock) while run the test in infinite
loop, the failure is reproduced quickly. ( background flushing ? )

The "bad pages" count differs each time from one digit to 4,5 digit
for 128M ftruncated file. and what i also found that the bad page
number are contiguous for each segment which total bad pages container
several segments. ext "1-4, 9-20, 48-50" ( batch flushing ? )

(The failure is reproduced based on 2.6.29-rc8, also happened on
2.6.18 kernel. . Here is the simple test case to reproduce it with
memory pressure. )

#include <sys/mman.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

long kMemSize = 128 << 20;
int kPageSize = 4096;

int main(int argc, char **argv) {
int status;
int count = 0;
int i;
char *fname = "/root/test.mmap";
char *mem;

unlink(fname);
int fd = open(fname, O_CREAT | O_EXCL | O_RDWR, 0600);
status = ftruncate(fd, kMemSize);

mem = mmap(0, kMemSize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
// Fill the memory with 1s.
memset(mem, 1, kMemSize);

for (i = 0; i < kMemSize; i++) {
int byte_good = mem[i] != 0;

if (!byte_good && ((i % kPageSize) == 0)) {
//printf("%d ", i / kPageSize);
count++;
}
}

munmap(mem, kMemSize);
close(fd);
unlink(fname);

if (count > 0) {
printf("Running %d bad page\n", count);
return 1;
}
return 0;
}

--Ying
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