Re: [Qemu-devel] [PATCH v6 14/42] block: Use CAFs when working with backing chains

[Kevin Wolf <kwolf@redhat.com>]

Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
> Use child access functions when iterating through backing chains so
> filters do not break the chain.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block.c | 40 ++++++++++++++++++++++++++++------------
>  1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 86b84bea21..42abbaf0ba 100644
> --- a/block.c
> +++ b/block.c
> @@ -4376,7 +4376,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>  }
>  
>  /*
> - * Finds the image layer in the chain that has 'bs' as its backing file.
> + * Finds the image layer in the chain that has 'bs' (or a filter on
> + * top of it) as its backing file.
>   *
>   * active is the current topmost image.
>   *
> @@ -4388,11 +4389,18 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>  BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
>                                      BlockDriverState *bs)
>  {
> -    while (active && bs != backing_bs(active)) {
> -        active = backing_bs(active);
> +    bs = bdrv_skip_rw_filters(bs);
> +    active = bdrv_skip_rw_filters(active);

This does more than the commit message says. In addition to iterating
through filters instead of stopping, it also changes the semantics of
the function to return the next non-filter on top of bs instead of the
next node.

The block jobs seem to use it only for bdrv_is_allocated_above(), which
should return the same thing in both cases, so the behaviour stays the
same. qmp_block_commit() will check op blockers on a different node now,
which could be a fix or a bug, I can't tell offhand. Probably the
blocking doesn't really work anyway.

All of this should be mentioned in the commit message at least. Maybe
it's also worth splitting in two patches.

> +    while (active) {
> +        BlockDriverState *next = bdrv_backing_chain_next(active);
> +        if (bs == next) {
> +            return active;
> +        }
> +        active = next;
>      }
>  
> -    return active;
> +    return NULL;
>  }
>  
>  /* Given a BDS, searches for the base layer. */
> @@ -4544,9 +4552,7 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
>       * other intermediate nodes have been dropped.
>       * If 'top' is an implicit node (e.g. "commit_top") we should skip
>       * it because no one inherits from it. We use explicit_top for that. */
> -    while (explicit_top && explicit_top->implicit) {
> -        explicit_top = backing_bs(explicit_top);
> -    }
> +    explicit_top = bdrv_skip_implicit_filters(explicit_top);
>      update_inherits_from = bdrv_inherits_from_recursive(base, explicit_top);
>  
>      /* success - we can delete the intermediate states, and link top->base */
> @@ -5014,7 +5020,7 @@ BlockDriverState *bdrv_lookup_bs(const char *device,
>  bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
>  {
>      while (top && top != base) {
> -        top = backing_bs(top);
> +        top = bdrv_filtered_bs(top);
>      }
>  
>      return top != NULL;
> @@ -5253,7 +5259,17 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>  
>      is_protocol = path_has_protocol(backing_file);
>  
> -    for (curr_bs = bs; curr_bs->backing; curr_bs = curr_bs->backing->bs) {
> +    /*
> +     * Being largely a legacy function, skip any filters here
> +     * (because filters do not have normal filenames, so they cannot
> +     * match anyway; and allowing json:{} filenames is a bit out of
> +     * scope).
> +     */
> +    for (curr_bs = bdrv_skip_rw_filters(bs);
> +         bdrv_filtered_cow_child(curr_bs) != NULL;
> +         curr_bs = bdrv_backing_chain_next(curr_bs))

This could just use bs_below instead of recalculating the node if you
moved the declaration of bs_below to the function scope.

> +    {
> +        BlockDriverState *bs_below = bdrv_backing_chain_next(curr_bs);
>  
>          /* If either of the filename paths is actually a protocol, then
>           * compare unmodified paths; otherwise make paths relative */
> @@ -5261,7 +5277,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>              char *backing_file_full_ret;
>  
>              if (strcmp(backing_file, curr_bs->backing_file) == 0) {
> -                retval = curr_bs->backing->bs;
> +                retval = bs_below;
>                  break;
>              }
>              /* Also check against the full backing filename for the image */
> @@ -5271,7 +5287,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>                  bool equal = strcmp(backing_file, backing_file_full_ret) == 0;
>                  g_free(backing_file_full_ret);
>                  if (equal) {
> -                    retval = curr_bs->backing->bs;
> +                    retval = bs_below;
>                      break;
>                  }
>              }
> @@ -5297,7 +5313,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>              g_free(filename_tmp);
>  
>              if (strcmp(backing_file_full, filename_full) == 0) {
> -                retval = curr_bs->backing->bs;
> +                retval = bs_below;
>                  break;
>              }
>          }

Kevin