Re: [PATCH for-5.1 1/2] block: Require aligned image size to avoid assertion failure

[Nir Soffer]

On Wed, Jul 15, 2020 at 4:42 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 15.07.2020 um 15:22 hat Nir Soffer geschrieben:
> > On Mon, Jul 13, 2020 at 7:56 PM Kevin Wolf <kwolf@redhat.com> wrote:
> > >
> > > Am 13.07.2020 um 18:33 hat Nir Soffer geschrieben:
> > > > On Fri, Jul 10, 2020 at 5:22 PM Kevin Wolf <kwolf@redhat.com> wrote:
> > > > >
> > > > > Unaligned requests will automatically be aligned to 
> > > > > bl.request_alignment
> > > > > and we don't want to extend requests to access space beyond the end of
> > > > > the image, so it's required that the image size is aligned.
> > > > >
> > > > > With write requests, this could cause assertion failures like this if
> > > > > RESIZE permissions weren't requested:
> > > > >
> > > > > qemu-img: block/io.c:1910: bdrv_co_write_req_prepare: Assertion 
> > > > > `end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE' 
> > > > > failed.
> > > > >
> > > > > This was e.g. triggered by qemu-img converting to a target image with 
> > > > > 4k
> > > > > request alignment when the image was only aligned to 512 bytes, but 
> > > > > not
> > > > > to 4k.
> > > >
> > > > Was it on NFS? Shouldn't this be fix by the next patch then?
> > >
> > > Patch 2 makes the problem go away for NFS because NFS doesn't even
> > > require the 4k alignment. But on storage that legitimately needs 4k
> > > alignment (or possibly other filesystems that are misdetected), you
> > > would still hit the same problem.
> >
> > I want to add oVirt point of view on this. We enforce raw image
> > alignment of 4k on file based storage, and 128m on block storage, so
> > our raw images cannot have this issue.
>
> Yes, then you won't hit the problem.
>
> > We have an issue with empty qcow2 images which are unaligned size, but
> > we don't create such images in normal flows.
>
> Can you give a reproducer where qcow2 images would be affected?
> Generally speaking, the qcow2 driver either takes both WRITE and RESIZE
> permissions or neither. So it should just automatically resize the image
> as needed instead of crashing.

I think this is a theoretical issue in other programs trying to access
the unaligned
images using direct I/O.