Re: [Qemu-devel] [PATCH 3/7] qcow2: Make perform_cow() call do_perform_cow() twice

[Kevin Wolf <kwolf@redhat.com>]

Am 23.05.2017 um 13:22 hat Alberto Garcia geschrieben:
> Instead of calling perform_cow() twice with a different COW region
> each time, call it just once and make perform_cow() handle both
> regions.
> 
> This patch simply moves code around. The next one will do the actual
> reordering of the COW operations.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c | 38 +++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 15 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 6757875ec9..59a0ffba1a 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -414,6 +414,10 @@ static int coroutine_fn do_perform_cow(BlockDriverState *bs,
>      struct iovec iov;
>      int ret;
>  
> +    if (bytes == 0) {
> +        return 0;
> +    }
> +
>      iov.iov_len = bytes;
>      iov.iov_base = qemu_try_blockalign(bs, iov.iov_len);
>      if (iov.iov_base == NULL) {
> @@ -751,31 +755,40 @@ uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
>      return cluster_offset;
>  }
>  
> -static int perform_cow(BlockDriverState *bs, QCowL2Meta *m, Qcow2COWRegion *r)
> +static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
>  {
>      BDRVQcow2State *s = bs->opaque;
> +    Qcow2COWRegion *start = &m->cow_start;
> +    Qcow2COWRegion *end = &m->cow_end;
>      int ret;
>  
> -    if (r->nb_bytes == 0) {
> +    if (start->nb_bytes == 0 && end->nb_bytes == 0) {
>          return 0;
>      }

With this change, it can now happen that we call do_perform_cow() with
bytes == 0. Maybe it would be better for do_perform_cow() to check bytes
first so that it doesn't bdrv_co_preadv/pwritev() for 0 bytes?

Kevin