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

[Sunil Mushran]

On Jun 22, 2010, at 6:50 PM, tristan <tristan.ye at oracle.com> wrote:

> 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.

I meant aio+direct and aio+buffered.


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

Not that interesting. As in, if that fails, then that would mean a  
pread() will also fail. And we already test that. The aio_write case  
is interesting because of the possible race with pread. aio_read after  
a pwrite cannot race each other.

>
> 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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>