Re: [PATCH v4 19/30] qcow2: Add subcluster support to zero_in_l2_slice()

[Vladimir Sementsov-Ogievskiy]

17.03.2020 21:16, Alberto Garcia wrote:
> Setting the QCOW_OFLAG_ZERO bit of the L2 entry is forbidden if an
> image has subclusters. Instead, the individual 'all zeroes' bits must
> be used.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>

anyway:
Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

still, some comments below

> ---
>   block/qcow2-cluster.c | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 6f2643ba53..746006a117 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1897,7 +1897,7 @@ static int zero_in_l2_slice(BlockDriverState *bs, 
> uint64_t offset,

As I see, function is not prepared to handle unaligned offset. Worth add an 
assertion while being here?

>       assert(nb_clusters <= INT_MAX);
>   
>       for (i = 0; i < nb_clusters; i++) {
> -        uint64_t old_offset;
> +        uint64_t old_offset, l2_entry = 0;

I'd rename s/old_offset/old_l2_entry

>           QCow2ClusterType cluster_type;
>   
>           old_offset = get_l2_entry(s, l2_slice, l2_index + i);

more context:

>         /*
>          * Minimize L2 changes if the cluster already reads back as
>          * zeroes with correct allocation.
>          */
>         cluster_type = qcow2_get_cluster_type(bs, old_offset);
>         if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN ||
>             (cluster_type == QCOW2_CLUSTER_ZERO_ALLOC && !unmap)) {

Worth assert !has_subclusters(s), or mark image corrupted?

>             continue;
>         }


> @@ -1914,12 +1914,18 @@ static int zero_in_l2_slice(BlockDriverState *bs, 
> uint64_t offset,
>   
>           qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
>           if (cluster_type == QCOW2_CLUSTER_COMPRESSED || unmap) {
> -            set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);
>               qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);
>           } else {
> -            uint64_t entry = get_l2_entry(s, l2_slice, l2_index + i);
> -            set_l2_entry(s, l2_slice, l2_index + i, entry | QCOW_OFLAG_ZERO);
> +            l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
>           }
> +
> +        if (has_subclusters(s)) {
> +            set_l2_bitmap(s, l2_slice, l2_index + i, 
> QCOW_L2_BITMAP_ALL_ZEROES);
> +        } else {
> +            l2_entry |= QCOW_OFLAG_ZERO;
> +        }
> +
> +        set_l2_entry(s, l2_slice, l2_index + i, l2_entry);

For subclasters & !unmap case we set the same value.. And we even don't need to 
get it.

may be

          if (cluster_type == QCOW2_CLUSTER_COMPRESSED || unmap) {
              qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);
              set_l2_entry(s, l2_slice, l2_index + i,
                           has_subclusters(s) ? 0 : QCOW_OFLAG_ZERO);
          } else if (!has_subclusters(s)) {
              uint64_t entry = get_l2_entry(s, l2_slice, l2_index + i);
              set_l2_entry(s, l2_slice, l2_index + i, entry | QCOW_OFLAG_ZERO);
          }

          if (has_subclusters(s)) {
              set_l2_bitmap(s, l2_slice, l2_index + i, 
QCOW_L2_BITMAP_ALL_ZEROES);
          }




>       }
>   
>       qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);


--
Best regards,
Vladimir