Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block

[Max Reitz <mreitz@redhat.com>]

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On 23.08.19 18:48, Nir Soffer wrote:
> On Fri, Aug 23, 2019 at 4:58 PM Max Reitz <mreitz@redhat.com
> <mailto:mreitz@redhat.com>> wrote:

[...]

>     If you have a format layer that truncates the image to a fixed size and
>     does not write anything into the first block itself (say because it uses
>     a footer), then (with O_DIRECT) allocate_first_block() will fail
>     (silently, because while it does return an error value, it is never
>     checked and there is no comment that explains why we don’t check it)
> 
> 
> The motivation is that this is an optimization for the special case of using
> empty image, so it does not worth failing image creation.
> I will add a comment about that.

Thanks!

[...]

> Thanks for the example.
> 
> I will need time to play with blockdev and understand the flows when image
> are created. Do you think is would be useful to fix now only image creation
> via qemu-img, and handle blockdev later?

Well, it isn’t about blockdev, it’s simply about the fact that this
function doesn’t work for O_DIRECT files.  I showed how to reproduce the
issue without blockdev (namely block_resize).  Sure, that is an edge
case, but it is a completely valid case.

Also, it seems to me the fix is rather simple.  Just something like:

static int allocate_first_block(int fd, int64_t max_size)
{
    int write_size = MIN(max_size, MAX_BLOCKSIZE);
    void *buf;
    ssize_t n;

    /* Round down to power of two */
    assert(write_size > 0);
    write_size = 1 << (31 - clz32(write_size));

    buf = qemu_memalign(MAX(getpagesize(), write_size), write_size);
    memset(buf, 0, write_size);

    do {
        n = pwrite(fd, buf, write_size, 0);
    } while (n < 0 && errno == EINTR);

    qemu_vfree(buf);

    return n < 0 ? -errno : 0;
}

Wouldn’t that work?

Max


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