Re: [Qemu-devel] [PATCH v2 2/7] qcow2: Use unsigned int for both members of Qcow2COWRegion

[Eric Blake]

On 06/08/2017 08:06 AM, Alberto Garcia wrote:
> On Wed 07 Jun 2017 06:02:06 PM CEST, Eric Blake wrote:
> 
>>> - The offset of the COW region from the start of the first cluster
>>>   touched by the I/O request. Since it's always going to be positive
>>>   and the maximum request size is at most INT_MAX, we can use a
>>>   regular unsigned int to store this offset.
>>
>> I don't know if we will ever get to the point that we allow a 64-bit
>> request at the block layer (and then the block layer guarantees it is
>> split down to the driver's limits, which works when a driver is still
>> bound by 32-bit limits).  But we ALSO know that a cluster is at most
>> 2M (in our current implementation of qcow2), so the offset of where
>> the COW region starts in relation to the start of a cluster is < 2M.
> 
> That's correct for the offset of the first region (m->cow_start),
> however m->cow_end is also relative to the offset of the first allocated
> cluster, so it can be > 2M if the requests spans several clusters.
> 
> So I guess the maximum theoretical offset would be something like
> INT_MAX + 2*cluster_size (a bit below that actually).

At which point, your earlier claim that we bound requests at INT_MAX
takes over.  But thanks for correcting me that the end cow region can
indeed be more than a cluster away from the beginning region.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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