Re: [Qemu-devel] [PATCH v3 05/15] block/mirror: don't install backing chain on abort

[Max Reitz <mreitz@redhat.com>]

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On 2018-09-01 00:28, John Snow wrote:
> In cases where we abort the block/mirror job, there's no point in
> installing the new backing chain before we finish aborting.
> 
> Move this to the "success" portion of mirror_exit.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/mirror.c | 27 ++++++++++++++-------------
>  1 file changed, 14 insertions(+), 13 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index cba555b4ef..c164fee883 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -642,16 +642,6 @@ static void mirror_exit(Job *job)
>       * required before it could become a backing file of target_bs. */
>      bdrv_child_try_set_perm(mirror_top_bs->backing, 0, BLK_PERM_ALL,
>                              &error_abort);
> -    if (s->backing_mode == MIRROR_SOURCE_BACKING_CHAIN) {
> -        BlockDriverState *backing = s->is_none_mode ? src : s->base;
> -        if (backing_bs(target_bs) != backing) {
> -            bdrv_set_backing_hd(target_bs, backing, &local_err);
> -            if (local_err) {
> -                error_report_err(local_err);
> -                ret = -EPERM;
> -            }
> -        }
> -    }
>  
>      if (s->to_replace) {
>          replace_aio_context = bdrv_get_aio_context(s->to_replace);
> @@ -659,9 +649,18 @@ static void mirror_exit(Job *job)
>      }
>  
>      if (s->should_complete && ret == 0) {
> -        BlockDriverState *to_replace = src;
> -        if (s->to_replace) {
> -            to_replace = s->to_replace;
> +        BlockDriverState *to_replace = s->to_replace ? s->to_replace : src;
> +
> +        if (s->backing_mode == MIRROR_SOURCE_BACKING_CHAIN) {
> +            BlockDriverState *backing = s->is_none_mode ? src : s->base;
> +            if (backing_bs(target_bs) != backing) {
> +                bdrv_set_backing_hd(target_bs, backing, &local_err);
> +                if (local_err) {
> +                    error_report_err(local_err);
> +                    ret = -EPERM;
> +                    goto clean;
> +                }
> +            }
>          }
>  
>          if (bdrv_get_flags(target_bs) != bdrv_get_flags(to_replace)) {

Testing shows that on post-READY cancel, s->should_complete is 0 (which
makes sense, because all the rest in this path is about replacing
to_replace by target_bs, which we don't want to do on cancel), so this
would not be executed.

However, I think we do want to give the target the correct backing chain
when the job is canceled this way (as I suppose there are ways to keep
the target around even with drive-mirror).

So we should attach the backing chain whenever ret == 0, not just when
s->should_complete is true.

Max

> @@ -678,6 +677,8 @@ static void mirror_exit(Job *job)
>              ret = -EPERM;
>          }
>      }
> +
> + clean:
>      if (s->to_replace) {
>          bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
>          error_free(s->replace_blocker);
> 



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