Re: [Qemu-devel] [PATCH v2 04/21] block: Allow references for backing files

[Max Reitz <mreitz@redhat.com>]

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On 23.11.2015 16:59, Kevin Wolf wrote:
> For bs->file, using references to existing BDSes has been possible for a
> while already. This patch enables the same for bs->backing_hd.

It's just "backing" now (instead of "backing_hd").

> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c               | 47 +++++++++++++++++++++++++++++------------------
>  block/mirror.c        |  2 +-
>  include/block/block.h |  3 ++-
>  3 files changed, 32 insertions(+), 20 deletions(-)

An additional thing I believe this patch can do (or a follow-up) is to
remove the part of bdrv_open_inherit() which sets BDRV_O_NO_BACKING if
the @backing option is the empty string.

With the changes done to bdrv_open_backing_file() here, we can easily
put that case into that function (where I think it belongs). If
!reference[0], we can just return success without having actually opened
a backing file.

We may have to think about the case of
{'backing': '', 'backing.driver': 'null-co'}, though. Right now, that is
an error (with a rather obscure error message, probably), and if we want
to keep it an error, we'd have to check whether options is empty in
bdrv_open_backing_file() if !reference[0], and emit a nice error if it
is not ("cannot specify backing options for suppressed backing file" or
something like that).

> diff --git a/block.c b/block.c
> index 675e5a8..ca6c4e9 100644
> --- a/block.c
> +++ b/block.c
> @@ -1182,30 +1182,43 @@ out:
>  /*
>   * Opens the backing file for a BlockDriverState if not yet open
>   *
> - * options is a QDict of options to pass to the block drivers, or NULL for an
> - * empty set of options. The reference to the QDict is transferred to this
> - * function (even on failure), so if the caller intends to reuse the dictionary,
> - * it needs to use QINCREF() before calling bdrv_file_open.
> + * bdref_key specifies the key for the image's BlockdevRef in the options QDict.
> + * That QDict has to be flattened; therefore, if the BlockdevRef is a QDict
> + * itself, all options starting with "${bdref_key}." are considered part of the
> + * BlockdevRef.
> + *
> + * TODO Can this be unified with bdrv_open_image()?
>   */
> -int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
> +int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
> +                           const char *bdref_key, Error **errp)
>  {
>      char *backing_filename = g_malloc0(PATH_MAX);
> +    char *bdref_key_dot;
> +    const char *reference = NULL;
>      int ret = 0;
>      BlockDriverState *backing_hd;
> +    QDict *options;
> +    QDict *tmp_parent_options = NULL;
>      Error *local_err = NULL;
>  
>      if (bs->backing != NULL) {
> -        QDECREF(options);
>          goto free_exit;
>      }
>  
>      /* NULL means an empty set of options */
> -    if (options == NULL) {
> -        options = qdict_new();
> +    if (parent_options == NULL) {
> +        tmp_parent_options = qdict_new();
> +        parent_options = tmp_parent_options;
>      }
>  
>      bs->open_flags &= ~BDRV_O_NO_BACKING;
> -    if (qdict_haskey(options, "file.filename")) {
> +
> +    bdref_key_dot = g_strdup_printf("%s.", bdref_key);
> +    qdict_extract_subqdict(parent_options, &options, bdref_key_dot);
> +    g_free(bdref_key_dot);
> +
> +    reference = qdict_get_try_str(parent_options, bdref_key);
> +    if (reference || qdict_haskey(options, "file.filename")) {
>          backing_filename[0] = '\0';
>      } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
>          QDECREF(options);
> @@ -1228,19 +1241,17 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
>          goto free_exit;
>      }
>  
> -    backing_hd = bdrv_new();
> -
>      if (bs->backing_format[0] != '\0' && !qdict_haskey(options, "driver")) {
>          qdict_put(options, "driver", qstring_from_str(bs->backing_format));
>      }
>  
>      assert(bs->backing == NULL);

This assert() can be dropped along with the bdrv_new() call (they are
related: originally, the bdrv_open_inherit() call below took
&bs->backing_hd as its first parameter, and when that was changed, this
assert() was apparently overlooked).

(http://lists.nongnu.org/archive/html/qemu-block/2015-11/msg00344.html)


Anyway, with the commit message fixed:

Reviewed-by: Max Reitz <mreitz@redhat.com>

> +    backing_hd = NULL;
>      ret = bdrv_open_inherit(&backing_hd,
>                              *backing_filename ? backing_filename : NULL,
> -                            NULL, options, 0, bs, &child_backing, &local_err);
> +                            reference, options, 0, bs, &child_backing,
> +                            &local_err);
>      if (ret < 0) {
> -        bdrv_unref(backing_hd);
> -        backing_hd = NULL;
>          bs->open_flags |= BDRV_O_NO_BACKING;
>          error_setg(errp, "Could not open backing file: %s",
>                     error_get_pretty(local_err));
> @@ -1253,8 +1264,11 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
>      bdrv_set_backing_hd(bs, backing_hd);
>      bdrv_unref(backing_hd);
>  
> +    qdict_del(parent_options, bdref_key);
> +
>  free_exit:
>      g_free(backing_filename);
> +    QDECREF(tmp_parent_options);
>      return ret;
>  }
>  
> @@ -1537,10 +1551,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, const char *filename,
>  
>      /* If there is a backing file, use it */
>      if ((flags & BDRV_O_NO_BACKING) == 0) {
> -        QDict *backing_options;
> -
> -        qdict_extract_subqdict(options, &backing_options, "backing.");
> -        ret = bdrv_open_backing_file(bs, backing_options, &local_err);
> +        ret = bdrv_open_backing_file(bs, options, "backing", &local_err);
>          if (ret < 0) {
>              goto close_and_fail;
>          }
> diff --git a/block/mirror.c b/block/mirror.c
> index 0620068..223b7aa 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -648,7 +648,7 @@ static void mirror_complete(BlockJob *job, Error **errp)
>      Error *local_err = NULL;
>      int ret;
>  
> -    ret = bdrv_open_backing_file(s->target, NULL, &local_err);
> +    ret = bdrv_open_backing_file(s->target, NULL, "backing", &local_err);
>      if (ret < 0) {
>          error_propagate(errp, local_err);
>          return;
> diff --git a/include/block/block.h b/include/block/block.h
> index 73edb1a..6d8ce86 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -210,7 +210,8 @@ BdrvChild *bdrv_open_child(const char *filename,
>                             const BdrvChildRole *child_role,
>                             bool allow_none, Error **errp);
>  void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd);
> -int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp);
> +int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
> +                           const char *bdref_key, Error **errp);
>  int bdrv_append_temp_snapshot(BlockDriverState *bs, int flags, Error **errp);
>  int bdrv_open(BlockDriverState **pbs, const char *filename,
>                const char *reference, QDict *options, int flags, Error **errp);
> 



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