Re: [Qemu-devel] [PATCH 1/3] mirror: clarify mirror_do_read return code

[Fam Zheng <famz@redhat.com>]

On Wed, 06/22 15:51, John Snow wrote:
> mirror_do_read intends to return the number of sectors processed after
> the starting sector, without regard to how many sectors were processed
> before the starting sector due to alignment.
> 
> Clean up the comments and code to hopefully illustrate this more clearly.
> 
> This also fixes an issue in initialization where if the mirror buffer size
> is initialized to smaller than the number of sectors being requested for
> transfer, we report back an incorrectly large number to the caller.

Good catch!

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

> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/mirror.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index a04ed9c..2ba9642 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -218,7 +218,9 @@ static inline void mirror_wait_for_io(MirrorBlockJob *s)
>  }
>  
>  /* Submit async read while handling COW.
> - * Returns: nb_sectors if no alignment is necessary, or
> + * Returns: The number of sectors copied after and including sector_num,
> + *          excluding any sectors copied prior to sector_num due to alignment.
> + *          This will be nb_sectors if no alignment is necessary, or
>   *          (new_end - sector_num) if tail is rounded up or down due to
>   *          alignment or buffer limit.
>   */
> @@ -227,7 +229,7 @@ static int mirror_do_read(MirrorBlockJob *s, int64_t sector_num,
>  {
>      BlockBackend *source = s->common.blk;
>      int sectors_per_chunk, nb_chunks;
> -    int ret = nb_sectors;
> +    int ret;
>      MirrorOp *op;
>  
>      sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
> @@ -235,6 +237,7 @@ static int mirror_do_read(MirrorBlockJob *s, int64_t sector_num,
>      /* We can only handle as much as buf_size at a time. */
>      nb_sectors = MIN(s->buf_size >> BDRV_SECTOR_BITS, nb_sectors);
>      assert(nb_sectors);
> +    ret = nb_sectors;
>  
>      if (s->cow_bitmap) {
>          ret += mirror_cow_align(s, &sector_num, &nb_sectors);
> -- 
> 2.4.11
>