Re: [Qemu-devel] [RFC PATCH v2 03/23] qcow2: Change handle_dependency to byte granularity

[Stefan Hajnoczi]

On Thu, Feb 14, 2013 at 04:45:06PM +0100, Kevin Wolf wrote:
> On Thu, Feb 14, 2013 at 03:48:51PM +0100, Stefan Hajnoczi wrote:
> > On Wed, Feb 13, 2013 at 02:21:53PM +0100, Kevin Wolf wrote:
> > > + *
> > > + *   -EAGAIN if we had to wait for another request, previously gathered
> > > + *           information on cluster allocation may be invalid now. The 
> > > caller
> > > + *           must start over anyway, so consider *cur_bytes undefined.
> > >   */
> > >  static int handle_dependencies(BlockDriverState *bs, uint64_t 
> > > guest_offset,
> > > -    unsigned int *nb_clusters)
> > > +    uint64_t *cur_bytes)
> > >  {
> > >      BDRVQcowState *s = bs->opaque;
> > >      QCowL2Meta *old_alloc;
> > > +    uint64_t bytes = *cur_bytes;
> > >  
> > >      QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
> > >  
> > > -        uint64_t start = guest_offset >> s->cluster_bits;
> > > -        uint64_t end = start + *nb_clusters;
> > > -        uint64_t old_start = old_alloc->offset >> s->cluster_bits;
> > > -        uint64_t old_end = old_start + old_alloc->nb_clusters;
> > > +        uint64_t start = guest_offset;
> > 
> > I'm not sure this is safe.  Previously the function caught write
> > requests which overlap at cluster granularity, now it will allow them?
> 
> At this point in the series, old_start and old_end are still aligned to
> cluster boundaries. So the cases in which an overlap is detected stay exactly
> the same with this patch. This is just a more precise description of what we
> really wanted to compare.

Good point.  In an overlap comparison between A and B only one of them
needs to be cluster-aligned.  I've used this trick myself in the past
but missed it in this code review.

Stefan