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

[Eric Blake]

On 10/3/18 9:32 AM, Vladimir Sementsov-Ogievskiy wrote:
> 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.

My suggestion (in another thread) was to enhance 
x-debug-block-dirty-bitmap-sha256 to include 'count' alongside the 
checksum, to make it easier to write such tests.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org