On Wed 30 Oct 2019 03:24:08 PM CET, Max Reitz wrote:
>> static void calculate_l2_meta(BlockDriverState *bs, uint64_t host_offset,
>> uint64_t guest_offset, uint64_t bytes,
>> - QCowL2Meta **m, bool keep_old)
>> + uint64_t *l2_slice, QCowL2Meta **m, bool
>> keep_old)
>> {
>> BDRVQcow2State *s = bs->opaque;
>> - unsigned cow_start_from = 0;
>> + int l2_index = offset_to_l2_slice_index(s, guest_offset);
>> + uint64_t l2_entry;
>> + unsigned cow_start_from, cow_end_to;
>> 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;
>> + QCow2ClusterType type;
>> +
>> + /* Return if there's no COW (all clusters are normal and we keep them)
>> */
>> + if (keep_old) {
>> + int i;
>> + for (i = 0; i < nb_clusters; i++) {
>> + l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
>
> I’d assert somewhere that l2_index + nb_clusters - 1 won’t overflow.
>
>> + if (qcow2_get_cluster_type(bs, l2_entry) !=
>> QCOW2_CLUSTER_NORMAL) {
>
> Wouldn’t cluster_needs_cow() be better?
The semantics of cluster_needs_cow() change in this patch (which also
updates the documentation). But I should maybe change the name instead.
>> + break;
>> + }
>> + }
>> + if (i == nb_clusters) {
>> + return;
>> + }
>> + }
>
> So I understand we always need to create an L2Meta structure in all
> other cases because we at least need to turn those clusters into
> normal clusters? (Even if they’re already allocated, as in the case
> of allocated zero clusters.)
That's correct.
>> -/* Returns true if writing to a cluster requires COW */
>> +/* Returns true if the cluster is unallocated or has refcount > 1 */
>> static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
>> {
>> switch (qcow2_get_cluster_type(bs, l2_entry)) {
>> case QCOW2_CLUSTER_NORMAL:
>> + case QCOW2_CLUSTER_ZERO_ALLOC:
>> if (l2_entry & QCOW_OFLAG_COPIED) {
>> return false;
>
> Don’t zero-allocated clusters need COW always? (Because the at the
> given host offset isn’t guaranteed to be zero.)
Yeah, hence the semantics change I described earlier. I should probably
call it cluster_needs_new_allocation() or something like that, which is
what this means now ("true if unallocated or refcount > 1").
>> - * Returns the number of contiguous clusters that can be used for an
>> allocating
>> - * write, but require COW to be performed (this includes yet unallocated
>> space,
>> - * which must copy from the backing file)
>> + * Returns the number of contiguous clusters that can be written to
>> + * using one single write request, starting from @l2_index.
>> + * At most @nb_clusters are checked.
>> + *
>> + * If @want_cow is true this counts clusters that are either
>> + * unallocated, or allocated but with refcount > 1.
>
> +(So they need to be newly allocated and COWed)?
Yes. Which in this context is the same as "newly allocated" I guess,
because every newly allocated cluster requires COW.
> (Or is the past participle of COW COWn? Or maybe CedOW?)
:-))
>> + * If @want_cow is false this counts clusters that are already
>> + * allocated and can be written to using their current locations
>
> s/using their current locations/in-place/?
Ok.
>> @@ -1475,13 +1489,14 @@ static int handle_alloc(BlockDriverState *bs,
>> uint64_t guest_offset,
>> *bytes = MIN(*bytes, nb_bytes - offset_into_cluster(s, guest_offset));
>> assert(*bytes != 0);
>>
>> - calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes,
>> - m, keep_old_clusters);
>> + calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes,
>> l2_slice,
>> + m, false);
>>
>> - return 1;
>> + ret = 1;
>>
>> -fail:
>> - if (*m && (*m)->nb_clusters > 0) {
>> +out:
>> + qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
>
> Is this a bug fix?
No, we call qcow2_cache_put() later because calculate_l2_meta() needs
l2_slice.
Berto
