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

[Nir Soffer <nsoffer@redhat.com>]

On Fri, Aug 23, 2019 at 8:53 PM Max Reitz <mreitz@redhat.com> wrote:

> 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?
>

Sure, it should work.

But I think we can make this simpler, always writing MIN(max_size,
MAX_BLOCKSIZE).

vdsm is enforcing now 4k alignment, and there is no way to create images
with unaligned
size. Maybe qemu should adapt this rule?

Nir