Re: [Qemu-devel] [PATCH v2] qcow2: Rewrite qcow2_alloc_bytes()

[Kevin Wolf]

Am 05.02.2015 um 16:58 hat Max Reitz geschrieben:
> qcow2_alloc_bytes() is a function with insufficient error handling and
> an unnecessary goto. This patch rewrites it.
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
> v2:
> - s/free_cluster_index/free_byte_index/ [Eric]
> - added an assertion at the start of the function that
>   s->free_byte_offset is either 0 or points to the tail of a cluster
>   (but never to the start)
> - use ROUND_UP() instead of start_of_cluster() + cluster_size [Eric]
> - added an assertion that s->free_byte_offset is set before using it
>   [Eric]
> ---
>  block/qcow2-refcount.c | 77 
> +++++++++++++++++++++++++++++---------------------
>  1 file changed, 45 insertions(+), 32 deletions(-)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 9afdb40..eede60d 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -759,46 +759,51 @@ int qcow2_alloc_clusters_at(BlockDriverState *bs, 
> uint64_t offset,
>  int64_t qcow2_alloc_bytes(BlockDriverState *bs, int size)
>  {
>      BDRVQcowState *s = bs->opaque;
> -    int64_t offset, cluster_offset;
> -    int free_in_cluster;
> +    int64_t offset, new_cluster = 0, cluster_end;
> +    size_t free_in_cluster;
>  
>      BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_BYTES);
>      assert(size > 0 && size <= s->cluster_size);
> -    if (s->free_byte_offset == 0) {
> -        offset = qcow2_alloc_clusters(bs, s->cluster_size);
> -        if (offset < 0) {
> -            return offset;
> +    assert(!s->free_byte_offset || offset_into_cluster(s, 
> s->free_byte_offset));
> +
> +    if (s->free_byte_offset) {
> +        int refcount = qcow2_get_refcount(bs,
> +            s->free_byte_offset >> s->cluster_bits);
> +        if (refcount < 0) {
> +            return refcount;
> +        }
> +
> +        if (refcount == 0xffff) {
> +            s->free_byte_offset = 0;
>          }
> -        s->free_byte_offset = offset;
>      }
> - redo:
> +
>      free_in_cluster = s->cluster_size -
>          offset_into_cluster(s, s->free_byte_offset);
> -    if (size <= free_in_cluster) {
> -        /* enough space in current cluster */
> -        offset = s->free_byte_offset;
> -        s->free_byte_offset += size;
> -        free_in_cluster -= size;
> -        if (free_in_cluster == 0)
> -            s->free_byte_offset = 0;
> -        if (offset_into_cluster(s, offset) != 0)
> -            qcow2_update_cluster_refcount(bs, offset >> s->cluster_bits, 1,
> -                                          QCOW2_DISCARD_NEVER);
> -    } else {
> -        offset = qcow2_alloc_clusters(bs, s->cluster_size);
> -        if (offset < 0) {
> -            return offset;
> +
> +    if (!s->free_byte_offset || free_in_cluster < size) {
> +        new_cluster = qcow2_alloc_clusters(bs, s->cluster_size);

The code could perhaps become a bit nicer if you used
alloc_clusters_noref() here...

> +        if (new_cluster < 0) {
> +            return new_cluster;
> +        }
> +
> +        cluster_end = ROUND_UP(s->free_byte_offset, s->cluster_size);
> +        if (!s->free_byte_offset || cluster_end != new_cluster) {
> +            s->free_byte_offset = new_cluster;
>          }
> -        cluster_offset = start_of_cluster(s, s->free_byte_offset);
> -        if ((cluster_offset + s->cluster_size) == offset) {
> -            /* we are lucky: contiguous data */
> -            offset = s->free_byte_offset;
> -            qcow2_update_cluster_refcount(bs, offset >> s->cluster_bits, 1,
> -                                          QCOW2_DISCARD_NEVER);
> -            s->free_byte_offset += size;
> -        } else {
> -            s->free_byte_offset = offset;
> -            goto redo;
> +    }
> +
> +    assert(s->free_byte_offset);
> +    if (offset_into_cluster(s, s->free_byte_offset)) {

...because this block could become unconditional then, ...

> +        int ret = qcow2_update_cluster_refcount(bs,
> +                s->free_byte_offset >> s->cluster_bits, 1,
> +                QCOW2_DISCARD_NEVER);

...here you could use update_refcount() with the actual byte count
(which also avoids two unnecessary shifts)...

> +        if (ret < 0) {
> +            if (new_cluster > 0) {
> +                qcow2_free_clusters(bs, new_cluster, s->cluster_size,
> +                                    QCOW2_DISCARD_OTHER);
> +            }

...and this part wouldn't be needed because update_refcount() already
tries to fail atomically.

> +            return ret;
>          }
>      }
>  
> @@ -807,6 +812,14 @@ int64_t qcow2_alloc_bytes(BlockDriverState *bs, int size)
>       * be flushed before the caller's L2 table updates.
>       */

It would also simplify the two lines of this comment that aren't in the
patch context any more. ;-)

>      qcow2_cache_set_dependency(bs, s->l2_table_cache, 
> s->refcount_block_cache);
> +
> +    offset = s->free_byte_offset;
> +
> +    s->free_byte_offset += size;
> +    if (!offset_into_cluster(s, s->free_byte_offset)) {
> +        s->free_byte_offset = 0;
> +    }
> +
>      return offset;
>  }

The patch looks correct to me. Let me know if you'd like to address the
point I made above, or if I should apply it as it is.

Kevin