Re: [Qemu-devel] [PATCH v6 3/3] block/stream: introduce a bottom node

[Max Reitz]

On 06.05.19 17:34, Vladimir Sementsov-Ogievskiy wrote:
> From: Andrey Shinkevich <address@hidden>
> 
> The bottom node is the intermediate block device that has the base as its
> backing image. It is used instead of the base node while a block stream
> job is running to avoid dependency on the base that may change due to the
> parallel jobs. The change may take place due to a filter node as well that
> is inserted between the base and the intermediate bottom node. It occurs
> when the base node is the top one for another commit or stream job.
> After the introduction of the bottom node, don't freeze its backing child,
> that's the base, anymore.
> 
> Suggested-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Signed-off-by: Andrey Shinkevich <address@hidden>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Reviewed-by: Alberto Garcia <address@hidden>
> ---
>  block/stream.c         | 49 +++++++++++++++++++++---------------------
>  tests/qemu-iotests/245 |  4 ++--
>  2 files changed, 27 insertions(+), 26 deletions(-)
> 
> diff --git a/block/stream.c b/block/stream.c
> index 65b13b27e0..fc97c89f81 100644
> --- a/block/stream.c
> +++ b/block/stream.c

[...]

> @@ -248,26 +250,25 @@ void stream_start(const char *job_id, BlockDriverState 
> *bs,
>       * already have our own plans. Also don't allow resize as the image size 
> is
>       * queried only at the job start and then cached. */
>      s = block_job_create(job_id, &stream_job_driver, NULL, bs,
> -                         BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED 
> |
> -                         BLK_PERM_GRAPH_MOD,
> -                         BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED 
> |
> -                         BLK_PERM_WRITE,
> +                         basic_flags | BLK_PERM_GRAPH_MOD,
> +                         basic_flags | BLK_PERM_WRITE,
>                           speed, creation_flags, NULL, NULL, errp);
>      if (!s) {
>          goto fail;
>      }
>  
> -    /* Block all intermediate nodes between bs and base, because they will
> -     * disappear from the chain after this operation. The streaming job reads
> -     * every block only once, assuming that it doesn't change, so block 
> writes
> -     * and resizes. */
> -    for (iter = backing_bs(bs); iter && iter != base; iter = 
> backing_bs(iter)) {
> -        block_job_add_bdrv(&s->common, "intermediate node", iter, 0,
> -                           BLK_PERM_CONSISTENT_READ | 
> BLK_PERM_WRITE_UNCHANGED,
> -                           &error_abort);
> +    /*
> +     * Block all intermediate nodes between bs and bottom (inclusive), 
> because
> +     * they will disappear from the chain after this operation. The streaming
> +     * job reads every block only once, assuming that it doesn't change, so
> +     * forbid writes and resizes.
> +     */
> +    for (iter = bs; iter != bottom; iter = backing_bs(iter)) {
> +        block_job_add_bdrv(&s->common, "intermediate node", backing_bs(iter),
> +                           0, basic_flags, &error_abort);

I don’t understand this change.  Isn’t it doing exactly the same as before?

(If so, I just find it harder to read because every iteration isn’t
about @iter but backing_bs(iter).)

The rest looks good to me.

Max

>      }
>  
> -    s->base = base;
> +    s->bottom = bottom;
>      s->backing_file_str = g_strdup(backing_file_str);
>      s->bs_read_only = bs_read_only;
>      s->chain_frozen = true;

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