Re: [PATCH v2 1/2] copy-before-write: allow specifying minimum cluster size

[Markus Armbruster]

Fiona Ebner <f.ebner@proxmox.com> writes:

> In the context of backup fleecing, discarding the source will not work
> when the fleecing image has a larger granularity than the one used for
> block-copy operations (can happen if the backup target has smaller
> cluster size), because cbw_co_pdiscard_snapshot() will align down the
> discard requests and thus effectively ignore then.
>
> To make @discard-source work in such a scenario, allow specifying the
> minimum cluster size used for block-copy operations and thus in
> particular also the granularity for discard requests to the source.
>
> The type 'size' (corresponding to uint64_t in C) is used in QAPI to
> rule out negative inputs and for consistency with already existing
> @cluster-size parameters. Since block_copy_calculate_cluster_size()
> uses int64_t for its result, a check that the input is not too large
> is added in block_copy_state_new() before calling it. The calculation
> in block_copy_calculate_cluster_size() is done in the target int64_t
> type.
>
> Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
>
> Changes in v2:
> * Use 'size' type in QAPI.
> * Remove option in cbw_parse_options(), i.e. before parsing generic
>   blockdev options.
>
>  block/block-copy.c         | 22 ++++++++++++++++++----
>  block/copy-before-write.c  | 10 +++++++++-
>  include/block/block-copy.h |  1 +
>  qapi/block-core.json       |  8 +++++++-
>  4 files changed, 35 insertions(+), 6 deletions(-)
>
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 7e3b378528..36eaecaaf4 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c
> @@ -310,6 +310,7 @@ void block_copy_set_copy_opts(BlockCopyState *s, bool 
> use_copy_range,
>  }
>  
>  static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
> +                                                 int64_t min_cluster_size,
>                                                   Error **errp)
>  {
>      int ret;
> @@ -335,7 +336,7 @@ static int64_t 
> block_copy_calculate_cluster_size(BlockDriverState *target,
>                      "used. If the actual block size of the target exceeds "
>                      "this default, the backup may be unusable",
>                      BLOCK_COPY_CLUSTER_SIZE_DEFAULT);
> -        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
> +        return MAX(min_cluster_size, 
> (int64_t)BLOCK_COPY_CLUSTER_SIZE_DEFAULT);

BLOCK_COPY_CLUSTER_SIZE_DEFAULT gets promoted from int to
min_cluster_size's type int64_t even without the cast.  The cast makes
it explicit.  Matter of taste, and I'm not the maintainer here :)

>      } else if (ret < 0 && !target_does_cow) {
>          error_setg_errno(errp, -ret,
>              "Couldn't determine the cluster size of the target image, "
> @@ -345,16 +346,18 @@ static int64_t 
> block_copy_calculate_cluster_size(BlockDriverState *target,
>          return ret;
>      } else if (ret < 0 && target_does_cow) {
>          /* Not fatal; just trudge on ahead. */
> -        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
> +        return MAX(min_cluster_size, 
> (int64_t)BLOCK_COPY_CLUSTER_SIZE_DEFAULT);

Likewise.

>      }
>  
> -    return MAX(BLOCK_COPY_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
> +    return MAX(min_cluster_size,
> +               (int64_t)MAX(BLOCK_COPY_CLUSTER_SIZE_DEFAULT, 
> bdi.cluster_size));

Likewise.

>  }
>  
>  BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
>                                       BlockDriverState *copy_bitmap_bs,
>                                       const BdrvDirtyBitmap *bitmap,
>                                       bool discard_source,
> +                                     uint64_t min_cluster_size,
>                                       Error **errp)
>  {
>      ERRP_GUARD();
> @@ -365,7 +368,18 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, 
> BdrvChild *target,
>  
>      GLOBAL_STATE_CODE();
>  
> -    cluster_size = block_copy_calculate_cluster_size(target->bs, errp);
> +    if (min_cluster_size > INT64_MAX) {
> +        error_setg(errp, "min-cluster-size too large: %lu > %ld",
> +                   min_cluster_size, INT64_MAX);
> +        return NULL;
> +    } else if (min_cluster_size && !is_power_of_2(min_cluster_size)) {
> +        error_setg(errp, "min-cluster-size needs to be a power of 2");
> +        return NULL;
> +    }
> +
> +    cluster_size = block_copy_calculate_cluster_size(target->bs,
> +                                                     
> (int64_t)min_cluster_size,
> +                                                     errp);

This is now more obviously safe.  Thanks!

>      if (cluster_size < 0) {
>          return NULL;
>      }
> diff --git a/block/copy-before-write.c b/block/copy-before-write.c
> index cd65524e26..ef0bc4dcfe 100644
> --- a/block/copy-before-write.c
> +++ b/block/copy-before-write.c
> @@ -417,6 +417,7 @@ static BlockdevOptions *cbw_parse_options(QDict *options, 
> Error **errp)
>      qdict_extract_subqdict(options, NULL, "bitmap");
>      qdict_del(options, "on-cbw-error");
>      qdict_del(options, "cbw-timeout");
> +    qdict_del(options, "min-cluster-size");
>  
>  out:
>      visit_free(v);
> @@ -432,6 +433,7 @@ static int cbw_open(BlockDriverState *bs, QDict *options, 
> int flags,
>      BDRVCopyBeforeWriteState *s = bs->opaque;
>      BdrvDirtyBitmap *bitmap = NULL;
>      int64_t cluster_size;
> +    uint64_t min_cluster_size = 0;
>      g_autoptr(BlockdevOptions) full_opts = NULL;
>      BlockdevOptionsCbw *opts;
>      int ret;
> @@ -476,8 +478,14 @@ static int cbw_open(BlockDriverState *bs, QDict 
> *options, int flags,
>               bs->file->bs->supported_zero_flags);
>  
>      s->discard_source = flags & BDRV_O_CBW_DISCARD_SOURCE;
> +
> +    if (opts->has_min_cluster_size) {
> +        min_cluster_size = opts->min_cluster_size;
> +    }
> +
>      s->bcs = block_copy_state_new(bs->file, s->target, bs, bitmap,
> -                                  flags & BDRV_O_CBW_DISCARD_SOURCE, errp);
> +                                  flags & BDRV_O_CBW_DISCARD_SOURCE,
> +                                  min_cluster_size, errp);
>      if (!s->bcs) {
>          error_prepend(errp, "Cannot create block-copy-state: ");
>          return -EINVAL;

You can pass opts->min_cluster_size directly, as in v1.  When
!opts->has_min_cluster_size, then opts->min_cluster_size is zero.

> diff --git a/include/block/block-copy.h b/include/block/block-copy.h
> index bdc703bacd..dd5cc82f3b 100644
> --- a/include/block/block-copy.h
> +++ b/include/block/block-copy.h
> @@ -28,6 +28,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, 
> BdrvChild *target,
>                                       BlockDriverState *copy_bitmap_bs,
>                                       const BdrvDirtyBitmap *bitmap,
>                                       bool discard_source,
> +                                     uint64_t min_cluster_size,
>                                       Error **errp);
>  
>  /* Function should be called prior any actual copy request */
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index df5e07debd..8fc0a4b234 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -4642,12 +4642,18 @@
>  #     @on-cbw-error parameter will decide how this failure is handled.
>  #     Default 0.  (Since 7.1)
>  #
> +# @min-cluster-size: Minimum size of blocks used by copy-before-write
> +#     operations.  Has to be a power of 2.  No effect if smaller than
> +#     the maximum of the target's cluster size and 64 KiB.  Default 0.
> +#     (Since 9.1)
> +#
>  # Since: 6.2
>  ##
>  { 'struct': 'BlockdevOptionsCbw',
>    'base': 'BlockdevOptionsGenericFormat',
>    'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap',
> -            '*on-cbw-error': 'OnCbwError', '*cbw-timeout': 'uint32' } }
> +            '*on-cbw-error': 'OnCbwError', '*cbw-timeout': 'uint32',
> +            '*min-cluster-size': 'size' } }
>  
>  ##
>  # @BlockdevOptions:

QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>