Re: [Qemu-devel] [PATCH v3 10/20] mirror: Switch mirror_cow_align() to byte-based

[Jeff Cody]

On Tue, Jun 27, 2017 at 02:24:48PM -0500, Eric Blake wrote:
> We are gradually converting to byte-based interfaces, as they are
> easier to reason about than sector-based.  Convert another internal
> function (no semantic change), and add mirror_clip_bytes() as a
> counterpart to mirror_clip_sectors().  Some of the conversion is
> a bit tricky, requiring temporaries to convert between units; it
> will be cleared up in a following patch.
> 
> Signed-off-by: Eric Blake <address@hidden>
> Reviewed-by: John Snow <address@hidden>
> 

Reviewed-by: Jeff Cody <address@hidden>

> ---
> v2: tweak mirror_clip_bytes() signature to match previous patch
> ---
>  block/mirror.c | 64 
> ++++++++++++++++++++++++++++++++++------------------------
>  1 file changed, 38 insertions(+), 26 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 1a43304..1a4602a 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -176,6 +176,15 @@ static void mirror_read_complete(void *opaque, int ret)
>      aio_context_release(blk_get_aio_context(s->common.blk));
>  }
> 
> +/* Clip bytes relative to offset to not exceed end-of-file */
> +static inline int64_t mirror_clip_bytes(MirrorBlockJob *s,
> +                                        int64_t offset,
> +                                        int64_t bytes)
> +{
> +    return MIN(bytes, s->bdev_length - offset);
> +}
> +
> +/* Clip nb_sectors relative to sector_num to not exceed end-of-file */
>  static inline int mirror_clip_sectors(MirrorBlockJob *s,
>                                        int64_t sector_num,
>                                        int nb_sectors)
> @@ -184,44 +193,39 @@ static inline int mirror_clip_sectors(MirrorBlockJob *s,
>                 s->bdev_length / BDRV_SECTOR_SIZE - sector_num);
>  }
> 
> -/* Round sector_num and/or nb_sectors to target cluster if COW is needed, and
> - * return the offset of the adjusted tail sector against original. */
> -static int mirror_cow_align(MirrorBlockJob *s,
> -                            int64_t *sector_num,
> -                            int *nb_sectors)
> +/* Round offset and/or bytes to target cluster if COW is needed, and
> + * return the offset of the adjusted tail against original. */
> +static int mirror_cow_align(MirrorBlockJob *s, int64_t *offset,
> +                            unsigned int *bytes)
>  {
>      bool need_cow;
>      int ret = 0;
> -    int chunk_sectors = s->granularity >> BDRV_SECTOR_BITS;
> -    int64_t align_sector_num = *sector_num;
> -    int align_nb_sectors = *nb_sectors;
> -    int max_sectors = chunk_sectors * s->max_iov;
> +    int64_t align_offset = *offset;
> +    unsigned int align_bytes = *bytes;
> +    int max_bytes = s->granularity * s->max_iov;
> 
> -    need_cow = !test_bit(*sector_num / chunk_sectors, s->cow_bitmap);
> -    need_cow |= !test_bit((*sector_num + *nb_sectors - 1) / chunk_sectors,
> +    need_cow = !test_bit(*offset / s->granularity, s->cow_bitmap);
> +    need_cow |= !test_bit((*offset + *bytes - 1) / s->granularity,
>                            s->cow_bitmap);
>      if (need_cow) {
> -        bdrv_round_sectors_to_clusters(blk_bs(s->target), *sector_num,
> -                                       *nb_sectors, &align_sector_num,
> -                                       &align_nb_sectors);
> +        bdrv_round_to_clusters(blk_bs(s->target), *offset, *bytes,
> +                               &align_offset, &align_bytes);
>      }
> 
> -    if (align_nb_sectors > max_sectors) {
> -        align_nb_sectors = max_sectors;
> +    if (align_bytes > max_bytes) {
> +        align_bytes = max_bytes;
>          if (need_cow) {
> -            align_nb_sectors = QEMU_ALIGN_DOWN(align_nb_sectors,
> -                                               s->target_cluster_size >>
> -                                               BDRV_SECTOR_BITS);
> +            align_bytes = QEMU_ALIGN_DOWN(align_bytes,
> +                                          s->target_cluster_size);
>          }
>      }
> -    /* Clipping may result in align_nb_sectors unaligned to chunk boundary, 
> but
> +    /* Clipping may result in align_bytes unaligned to chunk boundary, but
>       * that doesn't matter because it's already the end of source image. */
> -    align_nb_sectors = mirror_clip_sectors(s, align_sector_num,
> -                                           align_nb_sectors);
> +    align_bytes = mirror_clip_bytes(s, align_offset, align_bytes);
> 
> -    ret = align_sector_num + align_nb_sectors - (*sector_num + *nb_sectors);
> -    *sector_num = align_sector_num;
> -    *nb_sectors = align_nb_sectors;
> +    ret = align_offset + align_bytes - (*offset + *bytes);
> +    *offset = align_offset;
> +    *bytes = align_bytes;
>      assert(ret >= 0);
>      return ret;
>  }
> @@ -257,10 +261,18 @@ static int mirror_do_read(MirrorBlockJob *s, int64_t 
> sector_num,
>      nb_sectors = MIN(s->buf_size >> BDRV_SECTOR_BITS, nb_sectors);
>      nb_sectors = MIN(max_sectors, nb_sectors);
>      assert(nb_sectors);
> +    assert(nb_sectors < BDRV_REQUEST_MAX_SECTORS);
>      ret = nb_sectors;
> 
>      if (s->cow_bitmap) {
> -        ret += mirror_cow_align(s, &sector_num, &nb_sectors);
> +        int64_t offset = sector_num * BDRV_SECTOR_SIZE;
> +        unsigned int bytes = nb_sectors * BDRV_SECTOR_SIZE;
> +        int gap;
> +
> +        gap = mirror_cow_align(s, &offset, &bytes);
> +        sector_num = offset / BDRV_SECTOR_SIZE;
> +        nb_sectors = bytes / BDRV_SECTOR_SIZE;
> +        ret += gap / BDRV_SECTOR_SIZE;
>      }
>      assert(nb_sectors << BDRV_SECTOR_BITS <= s->buf_size);
>      /* The sector range must meet granularity because:
> -- 
> 2.9.4
>