[v3,5/8] block/mirror: fix and improve do_sync_target_write

Use bdrv_dirty_bitmap_next_dirty_area() instead of
bdrv_dirty_iter_next_area(), because of the following problems of
bdrv_dirty_iter_next_area():

1. Using HBitmap iterators we should carefully handle unaligned offset,
as first call to hbitmap_iter_next() may return a value less than
original offset (actually, it will be original offset rounded down to
bitmap granularity). This handling is not done in
do_sync_target_write().

2. bdrv_dirty_iter_next_area() handles unaligned max_offset
incorrectly:

look at the code:
    if (max_offset == iter->bitmap->size) {
        /* If max_offset points to the image end, round it up by the
         * bitmap granularity */
        gran_max_offset = ROUND_UP(max_offset, granularity);
    } else {
        gran_max_offset = max_offset;
    }

    ret = hbitmap_iter_next(&iter->hbi, false);
    if (ret < 0 || ret + granularity > gran_max_offset) {
        return false;
    }

and assume that max_offset != iter->bitmap->size but still unaligned.
if 0 < ret < max_offset we found dirty area, but the function can
return false in this case (if ret + granularity > max_offset).

3. bdrv_dirty_iter_next_area() uses inefficient loop to find the end of
the dirty area. Let's use more efficient hbitmap_next_zero instead
(bdrv_dirty_bitmap_next_dirty_area() do so)

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/mirror.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

On 08/14/2018 08:14 AM, Vladimir Sementsov-Ogievskiy wrote:
> Use bdrv_dirty_bitmap_next_dirty_area() instead of
> bdrv_dirty_iter_next_area(), because of the following problems of
> bdrv_dirty_iter_next_area():
> 
> 1. Using HBitmap iterators we should carefully handle unaligned offset,
> as first call to hbitmap_iter_next() may return a value less than
> original offset (actually, it will be original offset rounded down to
> bitmap granularity). This handling is not done in
> do_sync_target_write().
> 
> 2. bdrv_dirty_iter_next_area() handles unaligned max_offset
> incorrectly:
> 
> look at the code:
>     if (max_offset == iter->bitmap->size) {
>         /* If max_offset points to the image end, round it up by the
>          * bitmap granularity */
>         gran_max_offset = ROUND_UP(max_offset, granularity);
>     } else {
>         gran_max_offset = max_offset;
>     }
> 
>     ret = hbitmap_iter_next(&iter->hbi, false);
>     if (ret < 0 || ret + granularity > gran_max_offset) {
>         return false;
>     }
> 
> and assume that max_offset != iter->bitmap->size but still unaligned.
> if 0 < ret < max_offset we found dirty area, but the function can
> return false in this case (if ret + granularity > max_offset).
> 
> 3. bdrv_dirty_iter_next_area() uses inefficient loop to find the end of
> the dirty area. Let's use more efficient hbitmap_next_zero instead
> (bdrv_dirty_bitmap_next_dirty_area() do so)
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/mirror.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index b48c3f8cf5..d2806812c8 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -1167,25 +1167,22 @@ static void do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
>                                   uint64_t offset, uint64_t bytes,
>                                   QEMUIOVector *qiov, int flags)
>  {
> -    BdrvDirtyBitmapIter *iter;
>      QEMUIOVector target_qiov;
> -    uint64_t dirty_offset;
> -    int dirty_bytes;
> +    uint64_t dirty_offset = offset;
> +    uint64_t dirty_bytes;
>  
>      if (qiov) {
>          qemu_iovec_init(&target_qiov, qiov->niov);
>      }
>  
> -    iter = bdrv_dirty_iter_new(job->dirty_bitmap);
> -    bdrv_set_dirty_iter(iter, offset);
> -
>      while (true) {
>          bool valid_area;
>          int ret;
>  
>          bdrv_dirty_bitmap_lock(job->dirty_bitmap);
> -        valid_area = bdrv_dirty_iter_next_area(iter, offset + bytes,
> -                                               &dirty_offset, &dirty_bytes);
> +        valid_area = bdrv_dirty_bitmap_next_dirty_area(
> +            job->dirty_bitmap, &dirty_offset,
> +            MIN(offset + bytes, dirty_offset + INT_MAX), &dirty_bytes);

Looks correct, though the invocation looks harder to read now. Might be
a sign that changing the signature is a good idea.... which I think
you're planning to do.

ACK.

>          if (!valid_area) {
>              bdrv_dirty_bitmap_unlock(job->dirty_bitmap);
>              break;
> @@ -1241,9 +1238,10 @@ static void do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
>                  break;
>              }
>          }
> +
> +        dirty_offset += dirty_bytes;
>      }
>  
> -    bdrv_dirty_iter_free(iter);
>      if (qiov) {
>          qemu_iovec_destroy(&target_qiov);
>      }
>