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

[Vladimir Sementsov-Ogievskiy]

03.10.2018 02:33, 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>

Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Hm, I rememberd:
commit 3260cdfffbf00f33923f5f9f6bef45932d7ac28b
Author: Liang Li <address@hidden>
Date:   Wed Feb 7 11:35:49 2018 -0500

     hbitmap: fix missing restore count when finish deserialization

     The .count of HBitmap is forgot to set in function
     hbitmap_deserialize_finish, let's set it to the right value.


  - I always forget to update this field.. We definitely should add some 
generic check on it somewhere, at least in tests.

> ---
>
> 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;
>   }
>   


-- 
Best regards,
Vladimir