Re: [PATCH v4 03/30] qcow2: Add calculate_l2_meta()

[Vladimir Sementsov-Ogievskiy]

09.04.2020 18:12, Alberto Garcia wrote:
> On Thu 09 Apr 2020 10:30:13 AM CEST, Vladimir Sementsov-Ogievskiy wrote:
> > > +static void calculate_l2_meta(BlockDriverState *bs,
> > > +                              uint64_t host_cluster_offset,
> > > +                              uint64_t guest_offset, unsigned bytes,
> > > +                              QCowL2Meta **m, bool keep_old)
> > > +{
> > > +    BDRVQcow2State *s = bs->opaque;
> > > +    unsigned cow_start_from = 0;
> > > +    unsigned cow_start_to = offset_into_cluster(s, guest_offset);
> > > +    unsigned cow_end_from = cow_start_to + bytes;
> > > +    unsigned cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
> > > +    unsigned nb_clusters = size_to_clusters(s, cow_end_from);
> > > +    QCowL2Meta *old_m = *m;
> > > +
> > > +    *m = g_malloc0(sizeof(**m));
> > > +    **m = (QCowL2Meta) {
> > > +        .next           = old_m,
> > > +
> > > +        .alloc_offset   = host_cluster_offset,
> > > +        .offset         = start_of_cluster(s, guest_offset),
> > > +        .nb_clusters    = nb_clusters,
> > > +
> > > +        .keep_old_clusters = keep_old,
> > > +
> > > +        .cow_start = {
> > > +            .offset     = cow_start_from,
> > > +            .nb_bytes   = cow_start_to - cow_start_from,
> > > +        },
> > > +        .cow_end = {
> > > +            .offset     = cow_end_from,
> >
> > Hmm. So, you make it equal to requested_bytes from handle_alloc().
>
> No, requested_bytes from handle_alloc is:
>
>     requested_bytes = *bytes + offset_into_cluster(s, guest_offset);
>
> But *bytes is later modified before calling calculate_l2_meta():
>
>     *bytes = MIN(*bytes, nb_bytes - offset_into_cluster(s, guest_offset));
>
> More details here:
>
>     https://lists.gnu.org/archive/html/qemu-block/2019-10/msg01808.html

Ahah, me again, sorry :)



--
Best regards,
Vladimir