Re: [Qemu-devel] [PATCH v2] qcow2: Rewrite qcow2_alloc_bytes()

[Eric Blake <eblake@redhat.com>]

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On 02/05/2015 08:58 AM, Max Reitz wrote:
> qcow2_alloc_bytes() is a function with insufficient error handling and
> an unnecessary goto. This patch rewrites it.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
> v2:
> - s/free_cluster_index/free_byte_index/ [Eric]
> - added an assertion at the start of the function that
>   s->free_byte_offset is either 0 or points to the tail of a cluster
>   (but never to the start)
> - use ROUND_UP() instead of start_of_cluster() + cluster_size [Eric]
> - added an assertion that s->free_byte_offset is set before using it
>   [Eric]

Looks nicer.

> +        cluster_end = ROUND_UP(s->free_byte_offset, s->cluster_size);
> +        if (!s->free_byte_offset || cluster_end != new_cluster) {

Here, I can prove that the left side of the || makes no difference to
the outcome (using just the right half is sufficient, since
!s->free_byte_offset implies cluster_end == 0, but new_cluster will be
non-zero because we never allocate the header cluster). I'd be fine if
you wanted to respin that and add the comment for the micro-optimized
shorter code; but for maintenance purposes, it's easier to rely on the
explicit condition (even if redundant) than to think about whether a
proof listed in a comment is correct, so I'm also fine leaving things as is.

> +            s->free_byte_offset = new_cluster;

At this point, s->free_byte_offset violates the precondition if we just
allocated a cluster, so we have to make sure that we restore the
precondition before exiting...

> +    if (offset_into_cluster(s, s->free_byte_offset)) {
> +        int ret = qcow2_update_cluster_refcount(bs,
> +                s->free_byte_offset >> s->cluster_bits, 1,
> +                QCOW2_DISCARD_NEVER);
> +        if (ret < 0) {
> +            if (new_cluster > 0) {
> +                qcow2_free_clusters(bs, new_cluster, s->cluster_size,
> +                                    QCOW2_DISCARD_OTHER);
> +            }
> +            return ret;

...this early exit only happens if s->free_byte_offset was pointing to a
tail, and there are no other early exits, with the final exit properly
restoring the precondition.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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