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Date: Wed, 06 May 2009 20:08:33 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] qcow2/virtio corruption: Don't allocate
 the same cluster twice
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To: Kevin Wolf <kwolf@redhat.com>
Cc: markmc@redhat.com, qemu-devel@nongnu.org

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.