Re: [PATCH 2/5] block: Introduce BDRV_O_NO_DATA_{WRITE,RESIZE}

[Kevin Wolf <kwolf@redhat.com>]

Am 05.02.2026 um 15:47 hat Hanna Czenczek geschrieben:
> Add BDS flags that prevent taking WRITE and/or RESIZE permissions on
> pure data (no metadata) children.  These are going to be used by qcow2
> during formatting, when we need write access to format the metadata
> file, but no write access to an external data file.  This will allow
> creating a qcow2 image for a raw image while the latter is currently in
> use by the VM.
> 
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  include/block/block-common.h | 11 +++++++++++
>  block.c                      | 15 +++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/include/block/block-common.h b/include/block/block-common.h
> index c8c626daea..504f6aa113 100644
> --- a/include/block/block-common.h
> +++ b/include/block/block-common.h
> @@ -245,6 +245,17 @@ typedef enum {
>  
>  #define BDRV_O_CBW_DISCARD_SOURCE 0x80000 /* for copy-before-write filter */
>  
> +/*
> + * Promise not to write any data to pure (non-metadata-bearing) data storage
> + * children, so we don't need the WRITE permission for them.
> + * For image creation, formatting requires write access to the image, but not
> + * necessarily to its pure storage children.  This allows creating an image on
> + * top of an existing raw storage image that is already attached to the VM.
> + */
> +#define BDRV_O_NO_DATA_WRITE  0x100000

Can't we just use BDRV_O_NO_IO for this one? It is stricter because it
doesn't allow reading either, but I don't think image creation ever
requires reading from the image?

> +/* Same as O_NO_DATA_WRITE, but for resizing */
> +#define BDRV_O_NO_DATA_RESIZE 0x200000

How does this differ from BDRV_O_RESIZE (apart from being negative)?
Doesn't resizing always refer to the data part?

Kevin