Some filesystems (XFS and ext4) have support for a concept called
unwritten extents, where we can write data into holes / preallocated
space and only mark them as allocated when the data I/O has finished.
Because the transaction to convert the extent can't be submitted from
I/O completion, which normally happens from IRQ context it needs to
be defered to a workqueue. This is not a problem for buffered I/O
where we keep the data in cache at least until the I/O operation has
finished, but it is an issue for direct I/O. XFS avoids that problem
for synchronous direct I/O by waiting for all unwritten extent conversions
to finish if we did one during direct I/O, but so far has ignored the
problem for asynchronous I/O. Unfortunately the race is very easy
to hit by using QEMU with native AIO support on a sparse image, and
the result is filesystem corruption in the guest.
This contains core direct I/O changes to allow the filesystem to delay
AIO completion, as well as a patch to fix XFS. ext4 also has the same
issue, and from a quick look also doesn't properly complete unwritten
extent conversions for synchronous direct I/O, but I'll leave that
for someone more familar to figure out.
Below is a minimal reproducer for the issue. Given that we're dealing
with a race condition it doesn't always fail, but in 2 core laptop
it triggers 100% reproducibly in 20 runs in a loop.
---
#define _GNU_SOURCE
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <libaio.h>
#define BUF_SIZE 4096
#define IO_PATTERN 0xab
int main(int argc, char *argv[])
{
struct io_context *ctx = NULL;
struct io_event ev;
struct iocb iocb, *iocbs[] = { &iocb };
void *buf;
char cmp_buf[BUF_SIZE];
int fd, err = 0;
fd = open(argv[1], O_DIRECT | O_CREAT | O_TRUNC | O_RDWR, 0600);
if (fd == -1) {
perror("open");
return 1;
}
err = posix_memalign(&buf, BUF_SIZE, BUF_SIZE);
if (err) {
fprintf(stderr, "error %s during %s\n",
strerror(-err),
"posix_memalign");
return 1;
}
memset(buf, IO_PATTERN, BUF_SIZE);
memset(cmp_buf, IO_PATTERN, BUF_SIZE);
/*
* Truncate to some random large file size. Just make sure
* it's not smaller than our I/O size.
*/
if (ftruncate(fd, 1024 * 1024 * 1024) < 0) {
perror("ftruncate");
return 1;
}
/*
* Do a simple 4k write into a hole using aio.
*/
err = io_setup(1, &ctx);
if (err) {
fprintf(stderr, "error %s during %s\n",
strerror(-err),
"io_setup");
return 1;
}
io_prep_pwrite(&iocb, fd, buf, BUF_SIZE, 0);
err = io_submit(ctx, 1, iocbs);
if (err != 1) {
fprintf(stderr, "error %s during %s\n",
strerror(-err),
"io_submit");
return 1;
}
err = io_getevents(ctx, 1, 1, &ev, NULL);
if (err != 1) {
fprintf(stderr, "error %s during %s\n",
strerror(-err),
"io_getevents");
return 1;
}
/*
* And then read it back.
*
* Using pread to keep it simple, but AIO has the same effect.
*/
if (pread(fd, buf, BUF_SIZE, 0) != BUF_SIZE) {
perror("pread");
return 1;
}
/*
* And depending on the machine we'll just get zeroes back quite
* often here. That's because the unwritten extent conversion
* hasn't finished.
*/
if (memcmp(buf, cmp_buf, BUF_SIZE)) {
unsigned long long *ubuf = (unsigned long long *)buf;
int i;
for (i = 0; i < BUF_SIZE / sizeof(unsigned long long); i++)
printf("%d: 0x%llx\n", i, ubuf[i]);
return 1;
}
return 0;
}
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