Re: [Qemu-devel] [PATCH v2 2/3] block/mirror: Fix target backing BDS

[Kevin Wolf <kwolf@redhat.com>]

Am 08.06.2016 um 13:28 hat Paolo Bonzini geschrieben:
> 
> 
> ----- Original Message -----
> > From: "Kevin Wolf" <kwolf@redhat.com>
> > To: "Max Reitz" <mreitz@redhat.com>
> > Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, "Fam Zheng" <famz@redhat.com>, nsoffer@redhat.com,
> > eblake@redhat.com, pbonzini@redhat.com
> > Sent: Wednesday, June 8, 2016 11:32:29 AM
> > Subject: Re: [PATCH v2 2/3] block/mirror: Fix target backing BDS
> > 
> > Am 06.06.2016 um 16:42 hat Max Reitz geschrieben:
> > > Currently, we are trying to move the backing BDS from the source to the
> > > target in bdrv_replace_in_backing_chain() which is called from
> > > mirror_exit(). However, mirror_complete() already tries to open the
> > > target's backing chain with a call to bdrv_open_backing_file().
> > > 
> > > First, we should only set the target's backing BDS once. Second, the
> > > mirroring block job has a better idea of what to set it to than the
> > > generic code in bdrv_replace_in_backing_chain() (in fact, the latter's
> > > conditions on when to move the backing BDS from source to target are not
> > > really correct).
> > > 
> > > Therefore, remove that code from bdrv_replace_in_backing_chain() and
> > > leave it to mirror_complete().
> > > 
> > > However, mirror_complete() in turn pursues a questionable strategy by
> > > employing bdrv_open_backing_file(): On the one hand, because this may
> > > open the wrong backing file with drive-mirror in "existing" mode, or
> > > because it will not override a possibly wrong backing file in the
> > > blockdev-mirror case.
> > 
> > Careful, this "wrong" backing file might actually be intended!
> > 
> > Consider a case where you want to move an image with its whole backing
> > chain to different storage. In that case, you would copy all of the
> > backing files (cp is good enough, they are read-only), create the
> > destination image which already points at the copied backing chain, and
> > then mirror in "existing" mode.
> > 
> > The intention is obviously that after the job completion the new backing
> > chain is used and not the old one.
> 
> Yes, this is the intention and it should not be changed.  In addition
> to what Kevin said, you can use drive-mirror to collapse the image to a
> single file; in this case, QEMU should not be using the backing files of
> the source.
> 
> bdrv_open_backing_file() is used because what we want to do is to
> "undo" the BDRV_O_NO_BACKING flag used by qmp_drive_mirror.
> 
> If the contents change under the guest feet, it's the layers above
> QEMU that have screwed up.

We should probably have test cases for both scenarios. They would make
it obvious that changing this behaviour is not okay. Actually, I'm
surprised that our existing cases don't seem to cover this.

Kevin