[ mmap corruptions with leveldb and btrfs compression ]
I ran this a number of times with compression off and wasn't able to
trigger problems. With compress=lzo, I see errors on every run.
Compile: gcc -Wall -o mmap-trunc mmap-trunc.c
Run: ./mmap-trunc file_name
The basic idea is to create a 256MB file in steps. Each step ftruncates
the file larger, and then mmaps a region for writing. It dirties some
unaligned bytes (a little more than 8K), and then munmaps.
Then a verify stage goes back through the file to make sure the data we
wrote is really there. I'm using a simple rotating pattern of chars
that compress very well.
I run it in batches of 100 with some memory pressure on the side:
for x in `seq 1 100` ; do (mmap-trunc f$x &) ; done
#define _FILE_OFFSET_BITS 64
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#define FILE_SIZE ((loff_t)256 * 1024 * 1024)
/* make a painfully unaligned chunk size */
#define CHUNK_SIZE (8192 + 932)
#define mmap_align(x) (((x) + 4095) & ~4095)
char *file_name = NULL;
void mmap_one_chunk(int fd, loff_t *cur_size, unsigned char *file_buf)
{
int ret;
loff_t new_size = *cur_size + CHUNK_SIZE;
loff_t pos = *cur_size;
unsigned long map_size = mmap_align(CHUNK_SIZE) + 4096;
char val = file_buf[0];
char *p;
int extra;
/* step one, truncate out a hole */
ret = ftruncate(fd, new_size);
if (ret) {
perror("truncate");
exit(1);
}
if (val == 0 || val == 'z')
val = 'a';
else
val++;
memset(file_buf, val, CHUNK_SIZE);
extra = pos & 4095;
p = mmap(0, map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd,
pos - extra);
if (p == MAP_FAILED) {
perror("mmap");
exit(1);
}
memcpy(p + extra, file_buf, CHUNK_SIZE);
ret = munmap(p, map_size);
if (ret) {
perror("munmap");
exit(1);
}
*cur_size = new_size;
}
void check_chunks(int fd)
{
char *p;
loff_t checked = 0;
char val = 'a';
int i;
int errors = 0;
int ret;
int extra;
unsigned long map_size = mmap_align(CHUNK_SIZE) + 4096;
fprintf(stderr, "checking chunks\n");
while (checked < FILE_SIZE) {
extra = checked & 4095;
p = mmap(0, map_size, PROT_READ,
MAP_SHARED, fd, checked - extra);
if (p == MAP_FAILED) {
perror("mmap");
exit(1);
}
for (i = 0; i < CHUNK_SIZE; i++) {
if (p[i + extra] != val) {
fprintf(stderr, "%s: bad val %x wanted %x
offset 0x%llx\n",
file_name, p[i + extra], val,
(unsigned long long)checked + i);
errors++;
}
}
if (val == 'z')
val = 'a';
else
val++;
ret = munmap(p, map_size);
if (ret) {
perror("munmap");
exit(1);
}
checked += CHUNK_SIZE;
}
printf("%s found %d errors\n", file_name, errors);
if (errors)
exit(1);
}
int main(int ac, char **av)
{
unsigned char *file_buf;
loff_t pos = 0;
int ret;
int fd;
if (ac < 2) {
fprintf(stderr, "usage: mmap-trunc filename\n");
exit(1);
}
ret = posix_memalign((void **)&file_buf, 4096, CHUNK_SIZE);
if (ret) {
perror("cannot allocate memory\n");
exit(1);
}
file_buf[0] = 0;
file_name = av[1];
fprintf(stderr, "running test on %s\n", file_name);
unlink(file_name);
fd = open(file_name, O_RDWR | O_CREAT, 0600);
if (fd < 0) {
perror("open");
exit(1);
}
fprintf(stderr, "writing chunks\n");
while (pos < FILE_SIZE) {
mmap_one_chunk(fd, &pos, file_buf);
}
check_chunks(fd);
return 0;
}
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