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

[Alberto Garcia]

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).

>> - The size of the COW region in bytes. This is guaranteed to be >= 0,
>>   so we should use an unsigned type instead.
>
> And likewise, since a COW region is a sub-cluster, and clusters are
> bounded at 2M, we also have a sub-int upper bound on the size of the
> region.

That's correct.

Berto