[Qemu-devel] Re: [PATCH] qcow2/virtio corruption: Don't allocate the same cluster twice

[Avi Kivity]

Kevin Wolf wrote:
> Avi Kivity schrieb:
>   
> > What happens if the second request completes before the first?  Then, 
> > when the first request completes, alloc_cluster_link_l2() will call 
> > copy_clusters() and overwrite the second request.
> >     
>
> Ouch, you're right. I should not only check if the image is consistent,
> but also if the data survives.
>
>   

We really want qemu-io fsx.

> > Also, the second request now depends on the first to update its 
> > metadata.  But if the first request fail, it will not update its 
> > metadata, and the second request will complete without error and also 
> > without updating its metadata.
> >     
>
> Hm, right. Need to think about this...
>   

I suggest retaining the part where you use inflight l2metas to layout 
data contiguously, but change alloc_cluster_link_l2() not to rely on 
n_start and nb_available but instead recompute them on completion. 
m->nb_clusters should never be zeroed for this to work.

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.