Re: [Qemu-devel] [PATCH v3] qcow2: Rewrite qcow2_alloc_bytes()

[Kevin Wolf <kwolf@redhat.com>]

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Am 06.02.2015 um 16:31 hat Eric Blake geschrieben:
> On 02/06/2015 08:27 AM, Eric Blake wrote:
> 
> >>  
> >> -    /* The cluster refcount was incremented, either by qcow2_alloc_clusters()
> >> -     * or explicitly by qcow2_update_cluster_refcount().  Refcount blocks must
> >> -     * be flushed before the caller's L2 table updates.
> >> -     */
> >> +    assert(offset);
> >> +    ret = update_refcount(bs, offset, size, 1, QCOW2_DISCARD_NEVER);
> > 
> > Case 1: This incremented the new cluster. Good
> > Case 2: This incremented the old cluster. Good
> > Case 3: This incremented the new cluster. Good
> > Case 4: This incremented the old cluster. But the new cluster remains at
> > refcount 0.  BAD.
> 
> Wait. Maybe I'm confused.  You are requesting an update_refcount()
> across size bytes, and given the offset, that means that the code will
> round up to cover BOTH clusters in one call.  Does update_refcount()
> properly increment from  [ 1, 0 ] to [ 2, 1 ] when given a 2-cluster
> size (when offset, size is rounded up to cluster boundaries)?  If so,
> then there is no bug after all.

Yes, this is what it should be doing. Letting one update_refcount() call
(more or less) atomically increment both refcounts was the whole point
of my suggestion anyway because that is what enables the simplification.

Kevin

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