Re: [PATCH] qemu-img: Allow target be aligned to sector size

["Jose R. Ziviani" <jziviani@suse.de>]

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Hello Hanna,

On Thu, Aug 19, 2021 at 12:12:00PM +0200, Hanna Reitz wrote:
> We cannot write to images opened with O_DIRECT unless we allow them to
> be resized so they are aligned to the sector size: Since 9c60a5d1978,
> bdrv_node_refresh_perm() ensures that for nodes whose length is not
> aligned to the request alignment and where someone has taken a WRITE
> permission, the RESIZE permission is taken, too).
> 
> Let qemu-img convert pass the BDRV_O_RESIZE flag (which causes
> blk_new_open() to take the RESIZE permission) when using cache=none for
> the target, so that when writing to it, it can be aligned to the target
> sector size.
> 
> Without this patch, an error is returned:
> 
> $ qemu-img convert -f raw -O raw -t none foo.img /mnt/tmp/foo.img
> qemu-img: Could not open '/mnt/tmp/foo.img': Cannot get 'write'
> permission without 'resize': Image size is not a multiple of request
> alignment
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1994266
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
> As I have written in the BZ linked above, I am not sure what behavior we
> want.  It can be argued that the current behavior is perfectly OK
> because we want the target to have the same size as the source, so if
> this cannot be done, we should just print the above error (which I think
> explains the problem well enough that users can figure out they need to
> resize the source image).
> 
> OTOH, it is difficult for me to imagine a case where the user would
> prefer the above error to just having qemu-img align the target image's
> length.

This is timely convenient because I'm currently investigating an issue detected
by a libvirt-tck test:

https://gitlab.com/libvirt/libvirt-tck/-/blob/master/scripts/qemu/200-qcow2-single-backing-file.t

It fails with the same message:
qemu-system-x86_64: -device virtio-blk-pci,bus=pci.0,addr=0x3,drive=libvirt-1-format,id=virtio-disk1,write-cache=on: Cannot get 'write' permission without 'resize': Image size is not a multiple of request alignment

Libvirt-tck basically wants to make sure that libvirt won't pass a 'backing'
argument if we force a QCOW2 image to be interpreted as a RAW image. But, the
actual size of a (not preallocated) QCOW2 is usually unaligned so we ended up
failing at that requirement.

I crafted a reproducer (tck-main is a QCOW2 image):
 $ ./qemu-system-x86_64 \
  -machine pc-i440fx-6.0,accel=kvm -m 1024 -display none -no-user-config -nodefaults \
  -kernel vmlinuz -initrd initrd \
  -blockdev driver=file,filename=/var/cache/libvirt-tck/storage-fs/tck/tck-main,node-name=a,cache.direct=on \
  -blockdev node-name=name,driver=raw,file=a \
  -device virtio-blk-pci,drive=name

And if I remove 'cache.direct=on' OR if I remove the device virtio-blk-pci I
don't hit the failure.

In order to fix the libvirt-tck test case, I think that creating a preallocated
QCOW2 image is the best approach, considering their test case goal. But, if you
don't mind, could you explain why cache.direct=on doesn't set resize permission
with virtio-blk-pci?

Thank you!

> ---
>  qemu-img.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index 908fd0cce5..d4b29bf73e 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -2628,6 +2628,14 @@ static int img_convert(int argc, char **argv)
>          goto out;
>      }
>  
> +    if (flags & BDRV_O_NOCACHE) {
> +        /*
> +         * If we open the target with O_DIRECT, it may be necessary to
> +         * extend its size to align to the physical sector size.
> +         */
> +        flags |= BDRV_O_RESIZE;
> +    }
> +
>      if (skip_create) {
>          s.target = img_open(tgt_image_opts, out_filename, out_fmt,
>                              flags, writethrough, s.quiet, false);
> -- 
> 2.31.1
> 
> 

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