Re: [Qemu-devel] [PATCH] util/hbitmap: fix unaligned reset

[Max Reitz <mreitz@redhat.com>]

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On 02.08.19 20:58, Vladimir Sementsov-Ogievskiy wrote:
> hbitmap_reset is broken: it rounds up the requested region. It leads to
> the following bug, which is shown by fixed test:
> 
> assume granularity = 2
> set(0, 3) # count becomes 4
> reset(0, 1) # count becomes 2
> 
> But user of the interface assume that virtual bit 1 should be still
> dirty, so hbitmap should report count to be 4!
> 
> In other words, because of granularity, when we set one "virtual" bit,
> yes, we make all "virtual" bits in same chunk to be dirty. But this
> should not be so for reset.
> 
> Fix this, aligning bound correctly.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> Hi all!
> 
> Hmm, is it a bug or feature? :)
> I don't have a test for mirror yet, but I think that sync mirror may be broken
> because of this, as do_sync_target_write() seems to be using unaligned reset.
> 
>  tests/test-hbitmap.c |  2 +-
>  util/hbitmap.c       | 24 +++++++++++++++++++-----
>  2 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/tests/test-hbitmap.c b/tests/test-hbitmap.c
> index 592d8219db..0008025a9f 100644
> --- a/tests/test-hbitmap.c
> +++ b/tests/test-hbitmap.c
> @@ -424,7 +424,7 @@ static void test_hbitmap_granularity(TestHBitmapData *data,
>      hbitmap_test_set(data, 0, 3);
>      g_assert_cmpint(hbitmap_count(data->hb), ==, 4);
>      hbitmap_test_reset(data, 0, 1);
> -    g_assert_cmpint(hbitmap_count(data->hb), ==, 2);
> +    g_assert_cmpint(hbitmap_count(data->hb), ==, 4);
>  }
>  
>  static void test_hbitmap_iter_granularity(TestHBitmapData *data,
> diff --git a/util/hbitmap.c b/util/hbitmap.c
> index 7905212a8b..61a813994a 100644
> --- a/util/hbitmap.c
> +++ b/util/hbitmap.c
> @@ -473,15 +473,29 @@ void hbitmap_reset(HBitmap *hb, uint64_t start, uint64_t count)
>  {
>      /* Compute range in the last layer.  */
>      uint64_t first;
> -    uint64_t last = start + count - 1;
> +    uint64_t last;
> +    uint64_t end = start + count;
> +    uint64_t gran = UINT64_C(1) << hb->granularity;
>  
> -    trace_hbitmap_reset(hb, start, count,
> -                        start >> hb->granularity, last >> hb->granularity);
> +    /*
> +     * We should clear only bits, fully covered by requested region. Otherwise
> +     * we may clear something that is actually still dirty.
> +     */
> +    first = DIV_ROUND_UP(start, gran);
>  
> -    first = start >> hb->granularity;
> -    last >>= hb->granularity;
> +    if (end == hb->orig_size) {

This should be “>=”.

There are callers that don’t make sure that start + count <=
hb->orig_size (e.g. the backup job just calls it with multiples of
cluster_size, which may or may not end up at the image end; and
hbitmap_truncate() just uses “hb->size << hb->granularity” as the end,
which is arguably not ideal, but that’s how it is).

Max

> +        end = DIV_ROUND_UP(end, gran);
> +    } else {
> +        end = end >> hb->granularity;
> +    }
> +    if (end <= first) {
> +        return;
> +    }
> +    last = end - 1;
>      assert(last < hb->size);
>  
> +    trace_hbitmap_reset(hb, start, count, first, last);
> +
>      hb->count -= hb_count_between(hb, first, last);
>      if (hb_reset_between(hb, HBITMAP_LEVELS - 1, first, last) &&
>          hb->meta) {
> 



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