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

[Avi Kivity]

Kevin Wolf wrote:
> > 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.
> >     
>
> Is there even a reason why we need to copy the unmodified sectors in
> alloc_cluster_link_l2() and cannot do that in alloc_cluster_offset()
> before we write the new data? Then the callback wouldn't need to mess
> around with figuring out which part must be overwritten and which one
> mustn't.
>   

Then you have the inverse problem, you need to only copy sectors which 
aren't under an inflight write, and if such an inflight write fails, you 
do need to copy them.

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