Re: [Qemu-devel] [PATCH v2] bitmap: Update count after a merge

[John Snow]

On 10/02/2018 07:33 PM, John Snow wrote:
> From: Eric Blake <address@hidden>
> 
> We need an accurate count of the number of bits set in a bitmap
> after a merge. In particular, since the merge operation short-circuits
> a merge from an empty source, if you have bitmaps A, B, and C where
> B started empty, then merge C into B, and B into A, an inaccurate
> count meant that A did not get the contents of C.
> 
> In the worst case, we may falsely regard the bitmap as empty when
> it has had new writes merged into it.
> 
> Fixes: be58721db
> CC: address@hidden
> Signed-off-by: Eric Blake <address@hidden>
> Signed-off-by: John Snow <address@hidden>
> ---
> 
> v2: based off of Eric's cover letter, now rebased properly
>     on top of the jsnow/bitmaps staging branch to use the
>     correct bitmap target (result).
> 
> 
>  util/hbitmap.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/util/hbitmap.c b/util/hbitmap.c
> index d5aca5159f..8d402c59d9 100644
> --- a/util/hbitmap.c
> +++ b/util/hbitmap.c
> @@ -759,6 +759,9 @@ bool hbitmap_merge(const HBitmap *a, const HBitmap *b, 
> HBitmap *result)
>          }
>      }
>  
> +    /* Recompute the dirty count */
> +    result->count = hb_count_between(result, 0, result->size - 1);
> +
>      return true;
>  }
>  
> 

Tests on the way, but for now...

Thanks, applied to my bitmaps tree:

https://github.com/jnsnow/qemu/commits/bitmaps
https://github.com/jnsnow/qemu.git

--js