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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