Re: [RFC PATCH v2 03/26] qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied()

From: Alberto Garcia <berto@igalia.com>
To: Max Reitz <mreitz@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
	Anton Nefedov <anton.nefedov@virtuozzo.com>,
	qemu-block@nongnu.org,
	Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"Denis V . Lunev" <den@openvz.org>
Subject: Re: [RFC PATCH v2 03/26] qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied()
Date: Wed, 13 Nov 2019 15:25:33 +0100	[thread overview]
Message-ID: <w515zjn26qq.fsf@maestria.local.igalia.com> (raw)
In-Reply-To: <7daa553e-b74c-3573-5b67-e140436deb7a@redhat.com>

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