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

[John Snow <jsnow@redhat.com>]

From: Eric Blake <eblake@redhat.com>

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: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
---

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;
 }
 
-- 
2.14.4