Re: [Qemu-devel] [PATCH v2 02/15] block: Allow NULL file for bdrv_get_block_status()

[Fam Zheng <famz@redhat.com>]

On Mon, 07/03 17:14, Eric Blake wrote:
> Not all callers care about which BDS owns the mapping for a given
> range of the file.  This patch merely simplifies the callers by
> consolidating the logic in the common call point, while guaranteeing
> a non-NULL file to all the driver callbacks, for no semantic change.
> The only caller that does not care about pnum is bdrv_is_allocated,
> as invoked by vvfat; we can likewise add assertions that the rest
> of the stack does not have to worry about a NULL pnum.
> 
> Furthermore, this will also set the stage for a future cleanup: when
> a caller does not care about which BDS owns an offset, it would be
> nice to allow the driver to optimize things to not have to return
> BDRV_BLOCK_OFFSET_VALID in the first place.  In the case of fragmented
> allocation (for example, it's fairly easy to create a qcow2 image
> where consecutive guest addresses are not at consecutive host
> addresses), the current contract requires bdrv_get_block_status()
> to clamp *pnum to the limit where host addresses are no longer
> consecutive, but allowing a NULL file means that *pnum could be
> set to the full length of known-allocated data.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

Reviewed-by: Fam Zheng <famz@redhat.com>