Re: [Qemu-devel] [PATCH] qcow2-refcount: Snapshot update for zero clusters

[Kevin Wolf]

Am 29.08.2013 um 12:16 hat Max Reitz geschrieben:
> Do not try to update the refcount for zero clusters in
> qcow2_update_snapshot_refcount.
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/qcow2-refcount.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

Please don't forget to add a test case for v2.

> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 1244693..7555242 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -863,9 +863,13 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>              for(j = 0; j < s->l2_size; j++) {
>                  offset = be64_to_cpu(l2_table[j]);
>                  if (offset != 0) {
> +                    uint64_t cluster_index;
> +
>                      old_offset = offset;
>                      offset &= ~QCOW_OFLAG_COPIED;
> -                    if (offset & QCOW_OFLAG_COMPRESSED) {
> +
> +                    switch (qcow2_get_cluster_type(offset)) {
> +                    case QCOW2_CLUSTER_COMPRESSED:
>                          nb_csectors = ((offset >> s->csize_shift) &
>                                         s->csize_mask) + 1;
>                          if (addend != 0) {
> @@ -880,8 +884,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>                          }
>                          /* compressed clusters are never modified */
>                          refcount = 2;
> -                    } else {
> -                        uint64_t cluster_index = (offset & L2E_OFFSET_MASK) 
> >> s->cluster_bits;
> +                        break;
> +
> +                    case QCOW2_CLUSTER_NORMAL:
> +                        cluster_index = (offset & L2E_OFFSET_MASK) >> 
> s->cluster_bits;
>                          if (addend != 0) {
>                              refcount = update_cluster_refcount(bs, 
> cluster_index, addend,
>                                                                 
> QCOW2_DISCARD_SNAPSHOT);
> @@ -893,6 +899,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>                              ret = refcount;
>                              goto fail;
>                          }
> +                        break;

I think this part isn't quite right. Even zero clusters can have an
allocated offset, so I think what's really needed is that the same code
runs from QCOW2_CLUSTER_ZERO. However, add an if (cluster_index == 0)
case for the unallocated case.

> +
> +                    default:
> +                        refcount = 0;

Other places enumerate the enum values explicitly and have a default
label that simply abort()s. This will help catching forgotten places
when a new value is added.

>                      }
>  
>                      if (refcount == 1) {

Kevin