Re: [Qemu-devel] [PATCH 11/34] block: Allow references for backing files

[Wen Congyang]

On 05/09/2015 01:21 AM, 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.

1. We reference to an existing BDSes, and some disk uses this blk. Do
we allow this?
2. bs->backing_hd->blk can be not NULL now? If we do an active commit
to this backing file(use mirror job), we will call bdrv_swap() in
mirror_exit(), and the function bdrv_swap() doesn't allow that
new_bs->blk(here is bs->backing_hd) is not NULL.

Thanks
Wen Congyang

> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block.c               | 42 ++++++++++++++++++++++++------------------
>  block/mirror.c        |  2 +-
>  include/block/block.h |  3 ++-
>  3 files changed, 27 insertions(+), 20 deletions(-)
> 
> diff --git a/block.c b/block.c
> index e93bf63..95dc51e 100644
> --- a/block.c
> +++ b/block.c
> @@ -1109,30 +1109,41 @@ 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.
> + * bdrev_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;
>      Error *local_err = NULL;
>  
>      if (bs->backing_hd != NULL) {
> -        QDECREF(options);
>          goto free_exit;
>      }
>  
>      /* NULL means an empty set of options */
> -    if (options == NULL) {
> -        options = qdict_new();
> +    if (parent_options == NULL) {
> +        parent_options = qdict_new();
>      }
>  
>      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);
> @@ -1155,20 +1166,17 @@ 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_hd == NULL);
> +    backing_hd = NULL;
>      ret = bdrv_open_inherit(&backing_hd,
>                              *backing_filename ? backing_filename : NULL,
> -                            NULL, options, 0, bs, &child_backing,
> +                            reference, options, 0, bs, &child_backing,
>                              NULL, &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));
> @@ -1176,6 +1184,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
> *options, Error **errp)
>          goto free_exit;
>      }
>      bdrv_set_backing_hd(bs, backing_hd);
> +    qdict_del(parent_options, bdref_key);
>  
>  free_exit:
>      g_free(backing_filename);
> @@ -1463,10 +1472,7 @@ 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 58f391a..f1bc342 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -592,7 +592,7 @@ 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 27ab2c8..341a551 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -208,7 +208,8 @@ int bdrv_open_image(BlockDriverState **pbs, const char 
> *filename,
>                      BlockDriverState* parent, 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,
>