Re: [Qemu-devel] [PATCH v2 03/11] block: Add bdrv_has_zero_init_truncate()

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2019-07-24 at 19:12 +0200, Max Reitz wrote:
> No .bdrv_has_zero_init() implementation returns 1 if growing the file
> would add non-zero areas (at least with PREALLOC_MODE_OFF), so using it
> in lieu of this new function was always safe.
> 
> But on the other hand, it is possible that growing an image that is not
> zero-initialized would still add a zero-initialized area, like when
> using nonpreallocating truncation on a preallocated image.  For callers
> that care only about truncation, not about creation with potential
> preallocation, this new function is useful.
> 
> Alternatively, we could have added a PreallocMode parameter to
> bdrv_has_zero_init().  But the only user would have been qemu-img
> convert, which does not have a plain PreallocMode value right now -- it
> would have to parse the creation option to obtain it.  Therefore, the
> simpler solution is to let bdrv_has_zero_init() inquire the
> preallocation status and add the new bdrv_has_zero_init_truncate() that
> presupposes PREALLOC_MODE_OFF.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  include/block/block.h     |  1 +
>  include/block/block_int.h |  7 +++++++
>  block.c                   | 21 +++++++++++++++++++++
>  3 files changed, 29 insertions(+)
> 
> diff --git a/include/block/block.h b/include/block/block.h
> index 50a07c1c33..5321d8afdf 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -438,6 +438,7 @@ int bdrv_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
>  int bdrv_co_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
>  int bdrv_has_zero_init_1(BlockDriverState *bs);
>  int bdrv_has_zero_init(BlockDriverState *bs);
> +int bdrv_has_zero_init_truncate(BlockDriverState *bs);
>  bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs);
>  bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs);
>  int bdrv_block_status(BlockDriverState *bs, int64_t offset,
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 6a0b1b5008..d7fc6b296b 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -420,9 +420,16 @@ struct BlockDriver {
>      /*
>       * Returns 1 if newly created images are guaranteed to contain only
>       * zeros, 0 otherwise.
> +     * Must return 0 if .bdrv_has_zero_init_truncate() returns 0.
>       */
>      int (*bdrv_has_zero_init)(BlockDriverState *bs);
>  
> +    /*
> +     * Returns 1 if new areas added by growing the image with
> +     * PREALLOC_MODE_OFF contain only zeros, 0 otherwise.
> +     */
> +    int (*bdrv_has_zero_init_truncate)(BlockDriverState *bs);
> +
>      /* Remove fd handlers, timers, and other event loop callbacks so the event
>       * loop is no longer in use.  Called with no in-flight requests and in
>       * depth-first traversal order with parents before child nodes.
> diff --git a/block.c b/block.c
> index cbd8da5f3b..81ae44dcf3 100644
> --- a/block.c
> +++ b/block.c
> @@ -5066,6 +5066,27 @@ int bdrv_has_zero_init(BlockDriverState *bs)
>      return 0;
>  }
>  
> +int bdrv_has_zero_init_truncate(BlockDriverState *bs)
> +{
> +    if (!bs->drv) {
> +        return 0;
> +    }
> +
> +    if (bs->backing) {
> +        /* Depends on the backing image length, but better safe than sorry */
> +        return 0;
> +    }
> +    if (bs->drv->bdrv_has_zero_init_truncate) {
> +        return bs->drv->bdrv_has_zero_init_truncate(bs);
> +    }
> +    if (bs->file && bs->drv->is_filter) {
> +        return bdrv_has_zero_init_truncate(bs->file->bs);
> +    }
> +
> +    /* safe default */
> +    return 0;
> +}
> +
>  bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs)
>  {
>      BlockDriverInfo bdi;


This looks like a very correct change, even for the sake
of clarifying the scope of bdrv_has_zero_init

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Best regards,
	Maxim Levitsky