Re: [Qemu-devel] [PATCH for-2.7 v2 05/17] raw-posix: Implement .bdrv_lockf

[Fam Zheng <famz@redhat.com>]

On Tue, 04/19 14:07, Richard W.M. Jones wrote:
> On Tue, Apr 19, 2016 at 08:37:14PM +0800, Fam Zheng wrote:
> > On Mon, 04/18 09:04, Richard W.M. Jones wrote:
> > > On Mon, Apr 18, 2016 at 09:10:36AM +0800, Fam Zheng wrote:
> > > > On Sun, 04/17 20:27, Richard W.M. Jones wrote:
> > > > > On Fri, Apr 15, 2016 at 11:27:55AM +0800, Fam Zheng wrote:
> > > > > > virtlockd in libvirt locks the first byte, we lock byte 1 to avoid
> > > > > > the intervene.
> > > > > > +static int raw_lockf(BlockDriverState *bs, BdrvLockfCmd cmd)
> > > > > > +{
> > > > > > +
> > > > > > +    BDRVRawState *s = bs->opaque;
> > > > > > +    int ret;
> > > > > > +    struct flock fl = (struct flock) {
> > > > > > +        .l_whence  = SEEK_SET,
> > > > > > +        /* Locking byte 1 avoids interfereing with virtlockd. */
> > > > > > +        .l_start = 1,
> > > > > > +        .l_len = 1,
> > > > > > +    };
> > > > > > +
> > > > > > +    switch (cmd) {
> > > > > > +    case BDRV_LOCKF_RWLOCK:
> > > > > > +        fl.l_type = F_WRLCK;
> > > > > > +        break;
> > > > > > +    case BDRV_LOCKF_ROLOCK:
> > > > > > +        fl.l_type = F_RDLCK;
> > > > > > +        break;
> > > > > > +    case BDRV_LOCKF_UNLOCK:
> > > > > > +        fl.l_type = F_UNLCK;
> > > > > > +        break;
> > > > > 
> > > > > My understanding is this prevents libguestfs from working on live disk
> > > > > images -- we want to be able to read live disk images (using a
> > > > > writable overlay and the real disk image as a read-only backing file).
> > > > 
> > > > Do you lock the live image or the backing file?
> > > 
> > > At the moment we don't need to do anything, but we do have the problem
> > > that if someone uses libguestfs in write mode on a live image, then
> > > obviously they end up with a corrupted guest.
> > > 
> > > However in this email I'm talking about using libguestfs in
> > > "read-only" mode on a live guest, which is a completely different
> > > thing, and should not be prevented.
> > > 
> > > My assumption [with this patch applied] is the live image (being live)
> > > is locked by some other qemu.
> > > 
> > > Now libguestfs creates a temporary overlay with the live image as
> > > backing, using a command similar to:
> > > 
> > >   qemu-img create -f qcow2 -b live.img tmp-overlay.img
> > > 
> > > and opens 'tmp-overlay.img'.  But since the live image has an
> > > exclusive lock, the open will *fail*, and that is a problem.
> > 
> > I'm not sure that is what we want to support. The image is being read-write
> > open by the VM, any other accessing is a bad idea.
> 
> We've done this successfully for years, for people monitoring their
> VMs using virt-df, pulling out files using guestfish and so on.  We
> allow you to do it while the guest is live and running, with the
> proviso that a consistent view cannot always be guaranteed (although
> it works so reliably that it's never really a problem), or users can
> briefly pause VMs if they need a guaranteed consistent view.
> 
> I'm afraid the onus is on you to explain why this existing practice is
> a bad idea.

It is bad idea because it can produce erroneous data. Perhaps it's not
critical and is rare enough to be practically useful.

As a tradeoff, I guess, we can skip the shared lock in this series. Does that
work for you?

Fam