Re: [Qemu-devel] [Qemu-block] [PATCH v3 19/20] block: Minimize raw use of bds->total_sectors

[Jeff Cody <jcody@redhat.com>]

On Tue, Jun 27, 2017 at 02:24:57PM -0500, Eric Blake wrote:
> bdrv_is_allocated_above() was relying on intermediate->total_sectors,
> which is a field that can have stale contents depending on the value
> of intermediate->has_variable_length.  An audit shows that we are safe
> (we were first calling through bdrv_co_get_block_status() which in
> turn calls bdrv_nb_sectors() and therefore just refreshed the current
> length), but it's nicer to favor our accessor functions to avoid having
> to repeat such an audit, even if it means refresh_total_sectors() is
> called more frequently.
> 
> Suggested-by: John Snow <jsnow@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 

Reviewed-by: Jeff Cody <jcody@redhat.com>

> ---
> v2: new patch
> ---
>  block/io.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 0545180..5bbf153 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1924,6 +1924,7 @@ int bdrv_is_allocated_above(BlockDriverState *top,
>      intermediate = top;
>      while (intermediate && intermediate != base) {
>          int64_t pnum_inter;
> +        int64_t size_inter;
>          int psectors_inter;
> 
>          ret = bdrv_is_allocated(intermediate, sector_num * BDRV_SECTOR_SIZE,
> @@ -1941,13 +1942,14 @@ int bdrv_is_allocated_above(BlockDriverState *top,
> 
>          /*
>           * [sector_num, nb_sectors] is unallocated on top but intermediate
> -         * might have
> -         *
> -         * [sector_num+x, nr_sectors] allocated.
> +         * might have [sector_num+x, nb_sectors-x] allocated.
>           */
> +        size_inter = bdrv_nb_sectors(intermediate);
> +        if (size_inter < 0) {
> +            return size_inter;
> +        }
>          if (n > psectors_inter &&
> -            (intermediate == top ||
> -             sector_num + psectors_inter < intermediate->total_sectors)) {
> +            (intermediate == top || sector_num + psectors_inter < size_inter)) {
>              n = psectors_inter;
>          }
> 
> -- 
> 2.9.4
> 
>