[Ocfs2-devel] Fwd: [PATCH 0/2] Fix aio completion vs unwritten extents

[tristan]

Sunil Mushran wrote:
> Tristan,
>
> The reflink test is missing aio bits. Please could you add it.
> In short, a pread issued after an aio write, must return that
> data. This should be tested for both direct and buffered ios.
>
> Similarly, fill_holes also needs to be enhanced.

Gotta, verifications for odirect and buffered ios have already been 
included in current tests I guess.

What's more, maybe we can also verify the data from aio_read() after 
pwrite() completed.

Tristan.

>
> Thanks
> Sunil
>
> -------- Original Message --------
> Subject: 	[PATCH 0/2] Fix aio completion vs unwritten extents
> Date: 	Tue, 22 Jun 2010 08:21:44 -0400
> From: 	Christoph Hellwig <hch at infradead.org>
> To: 	linux-fsdevel at vger.kernel.org, xfs at oss.sgi.com, 
> linux-ext4 at vger.kernel.org
>
>
>
> 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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