Re: [Qemu-devel] [PATCH v12 09/17] qapi: Add transaction support to block-dirty-bitmap operations

[Max Reitz <mreitz@redhat.com>]

On 2015-02-09 at 20:35, John Snow wrote:
> This adds four qmp commands to transactions.
>
> Users can stop a dirty bitmap, start backup of it, and start another
> dirty bitmap atomically, so that the dirty bitmap is tracked
> incrementally and we don't miss any write.
>
> For starting a new incremental backup chain, users can also chain
> together a bitmap clear and a full block backup.
>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   blockdev.c       | 155 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>   qapi-schema.json |   6 ++-
>   2 files changed, 160 insertions(+), 1 deletion(-)
>
> diff --git a/blockdev.c b/blockdev.c
> index 83d0608..ed96e72 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1675,6 +1675,138 @@ static void blockdev_backup_clean(BlkTransactionState *common)
>       }
>   }
>   
> +static void block_dirty_bitmap_add_prepare(BlkTransactionState *common,
> +                                           Error **errp)
> +{
> +    BlockDirtyBitmapAdd *action;
> +
> +    action = common->action->block_dirty_bitmap_add;
> +    qmp_block_dirty_bitmap_add(action->node, action->name,
> +                               action->has_granularity, action->granularity,
> +                               errp);
> +}
> +
> +static void block_dirty_bitmap_add_abort(BlkTransactionState *common)
> +{
> +    BlockDirtyBitmapAdd *action;
> +
> +    action = common->action->block_dirty_bitmap_add;
> +    /* Should not fail meaningfully: IF the bitmap was added via .prepare(),
> +     * then the node reference and bitmap name must have been valid.
> +     * THUS: any failure here could only indicate the lack of a bitmap at all.
> +     */
> +    qmp_block_dirty_bitmap_remove(action->node, action->name, NULL);

What if block_dirty_bitmap_add_prepare() failed because a bitmap with 
that name already exists? Wouldn't this silently delete that existing 
bitmap?

I think you should store whether qmp_block_dirty_bitmap_add() was 
successful and only call qmp_block_dirty_bitmap_remove() if it was, and 
since you say it cannot fail (which I agree on), with &error_abort as 
the last argument.

> +}
> +
> +typedef struct BlockDirtyBitmapState {
> +    BlkTransactionState common;
> +    BdrvDirtyBitmap *bitmap;
> +    BlockDriverState *bs;
> +    AioContext *aio_context;
> +} BlockDirtyBitmapState;
> +
> +/**
> + * Enable and Disable re-use the same preparation.
> + */
> +static void block_dirty_bitmap_toggle_prepare(BlkTransactionState *common,
> +                                              Error **errp)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +    BlockDirtyBitmap *action;
> +    BlockDriverState *bs;
> +
> +    /* We may be used by either enable or disable;
> +     * We use the "enable" member of the union here,
> +     * but "disable" should be functionally equivalent: */
> +    action = common->action->block_dirty_bitmap_enable;
> +    assert(action == common->action->block_dirty_bitmap_disable);
> +
> +    state->bitmap = block_dirty_bitmap_lookup(action->node,
> +                                              action->name,
> +                                              &bs,
> +                                              errp);
> +    if (!state->bitmap) {
> +        return;
> +    }
> +
> +    if (bdrv_dirty_bitmap_frozen(state->bitmap)) {
> +        error_setg(errp, "Cannot modify a frozen bitmap.\n");

I'm sorry.\n

> +        return;
> +    }
> +
> +    /* AioContext is released in .clean() */
> +    state->aio_context = bdrv_get_aio_context(bs);
> +    aio_context_acquire(state->aio_context);
> +}
> +
> +static void block_dirty_bitmap_enable_commit(BlkTransactionState *common)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +    bdrv_enable_dirty_bitmap(state->bitmap);
> +}
> +
> +static void block_dirty_bitmap_disable_commit(BlkTransactionState *common)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +    bdrv_disable_dirty_bitmap(state->bitmap);
> +}
> +
> +static void block_dirty_bitmap_toggle_clean(BlkTransactionState *common)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +
> +    if (state->aio_context) {
> +        aio_context_release(state->aio_context);
> +    }
> +}
> +
> +static void block_dirty_bitmap_clear_prepare(BlkTransactionState *common,
> +                                             Error **errp)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +    BlockDirtyBitmap *action;
> +
> +    action = common->action->block_dirty_bitmap_clear;
> +    state->bitmap = block_dirty_bitmap_lookup(action->node,
> +                                              action->name,
> +                                              &state->bs,
> +                                              errp);
> +    if (!state->bitmap) {
> +        return;
> +    }
> +
> +    if (bdrv_dirty_bitmap_frozen(state->bitmap)) {
> +        error_setg(errp, "Cannot modify a frozen bitmap.\n");
> +        return;
> +    }

Following your reply regarding patch 8, this needs a check whether the 
bitmap is disabled, too.\n

> +
> +    /* AioContext is released in .clean() */
> +    state->aio_context = bdrv_get_aio_context(state->bs);
> +    aio_context_acquire(state->aio_context);
> +}
> +
> +static void block_dirty_bitmap_clear_commit(BlkTransactionState *common)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +    bdrv_clear_dirty_bitmap(state->bitmap);
> +}
> +
> +static void block_dirty_bitmap_clear_clean(BlkTransactionState *common)
> +{
> +    BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
> +                                             common, common);
> +
> +    if (state->aio_context) {
> +        aio_context_release(state->aio_context);
> +    }
> +}
> +
>   static void abort_prepare(BlkTransactionState *common, Error **errp)
>   {
>       error_setg(errp, "Transaction aborted using Abort action");
> @@ -1715,6 +1847,29 @@ static const BdrvActionOps actions[] = {
>           .abort = internal_snapshot_abort,
>           .clean = internal_snapshot_clean,
>       },
> +    [TRANSACTION_ACTION_KIND_BLOCK_DIRTY_BITMAP_ADD] = {
> +        .instance_size = sizeof(BlkTransactionState),
> +        .prepare = block_dirty_bitmap_add_prepare,
> +        .abort = block_dirty_bitmap_add_abort,
> +    },
> +    [TRANSACTION_ACTION_KIND_BLOCK_DIRTY_BITMAP_ENABLE] = {
> +        .instance_size = sizeof(BlockDirtyBitmapState),
> +        .prepare = block_dirty_bitmap_toggle_prepare,
> +        .commit = block_dirty_bitmap_enable_commit,
> +        .clean = block_dirty_bitmap_toggle_clean,
> +    },
> +    [TRANSACTION_ACTION_KIND_BLOCK_DIRTY_BITMAP_DISABLE] = {
> +        .instance_size = sizeof(BlockDirtyBitmapState),
> +        .prepare = block_dirty_bitmap_toggle_prepare,
> +        .commit = block_dirty_bitmap_disable_commit,
> +        .clean = block_dirty_bitmap_toggle_clean,
> +    },
> +    [TRANSACTION_ACTION_KIND_BLOCK_DIRTY_BITMAP_CLEAR] = {
> +        .instance_size = sizeof(BlockDirtyBitmapState),
> +        .prepare = block_dirty_bitmap_clear_prepare,
> +        .commit = block_dirty_bitmap_clear_commit,
> +        .clean = block_dirty_bitmap_clear_clean,
> +    }
>   };
>   
>   /*
> diff --git a/qapi-schema.json b/qapi-schema.json
> index e16f8eb..cf4aa12 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -1339,7 +1339,11 @@
>          'drive-backup': 'DriveBackup',
>          'blockdev-backup': 'BlockdevBackup',
>          'abort': 'Abort',
> -       'blockdev-snapshot-internal-sync': 'BlockdevSnapshotInternal'
> +       'blockdev-snapshot-internal-sync': 'BlockdevSnapshotInternal',
> +       'block-dirty-bitmap-add': 'BlockDirtyBitmapAdd',
> +       'block-dirty-bitmap-enable': 'BlockDirtyBitmap',
> +       'block-dirty-bitmap-disable': 'BlockDirtyBitmap',
> +       'block-dirty-bitmap-clear': 'BlockDirtyBitmap'

Maybe add version information to the comment above this union?

Max

>      } }
>   
>   ##