Re: [Qemu-devel] [Qemu-block] [PATCH v2] qcow2: do not allocate extra memory

[John Snow <jsnow@redhat.com>]

On 07/14/2016 12:59 PM, Vladimir Sementsov-Ogievskiy wrote:
> There are no needs to allocate more than one cluster, as we set
> avail_out for deflate to one cluster.
> 
> Zlib docs (http://www.zlib.net/manual.html) says:
> "deflate compresses as much data as possible, and stops when the input
> buffer becomes empty or the output buffer becomes full."
> 
> So, deflate will not write more than avail_out to output buffer. If
> there is no enough space in output buffer for compressed data (it may be
> larger than input data) deflate just returns Z_OK. (if all data is
> compressed and written to output buffer deflate returns Z_STREAM_END).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> v2: improve commit message
> 
>  block/qcow.c  | 2 +-
>  block/qcow2.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/qcow.c b/block/qcow.c
> index ac849bd..d8826f3 100644
> --- a/block/qcow.c
> +++ b/block/qcow.c
> @@ -983,7 +983,7 @@ static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
>          return ret;
>      }
>  
> -    out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
> +    out_buf = g_malloc(s->cluster_size);
>  
>      /* best compression, small window, no zlib header */
>      memset(&strm, 0, sizeof(strm));
> diff --git a/block/qcow2.c b/block/qcow2.c
> index a5ea19b..b1c90ae 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2612,7 +2612,7 @@ static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
>          return ret;
>      }
>  
> -    out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
> +    out_buf = g_malloc(s->cluster_size);
>  
>      /* best compression, small window, no zlib header */
>      memset(&strm, 0, sizeof(strm));
> 

Reviewed-by: John Snow <jsnow@redhat.com>