Re: [Qemu-devel] [PATCH v3 5/5] qcow2: Catch more unaligned write_zero into zero cluster

["Denis V. Lunev" <den@openvz.org>]

On 05/26/2016 06:48 AM, Eric Blake wrote:
> is_zero_cluster() and is_zero_cluster_top_locked() are used only
> by qcow2_co_write_zeroes().  The former is too broad (we don't
> care if the sectors we are about to overwrite are non-zero, only
> that all other sectors in the cluster are zero), so it needs to
> be called up to twice but with smaller limits - rename it along
> with adding the neeeded parameter.  The latter can be inlined for
> more compact code.
>
> The testsuite change shows that we now have a sparser top file
> when an unaligned write_zeroes overwrites the only portion of
> the backing file with data.
>
> Based on a patch proposal by Denis V. Lunev.
>
> CC: Denis V. Lunev <den@openvz.org>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>   block/qcow2.c              | 47 +++++++++++++++++++++++-----------------------
>   tests/qemu-iotests/154.out |  8 ++++----
>   2 files changed, 27 insertions(+), 28 deletions(-)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 105fd5e..ecac399 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2406,26 +2406,19 @@ finish:
>   }
>
>
> -static bool is_zero_cluster(BlockDriverState *bs, int64_t start)
> +static bool is_zero_sectors(BlockDriverState *bs, int64_t start,
> +                            uint32_t count)
>   {
> -    BDRVQcow2State *s = bs->opaque;
>       int nr;
>       BlockDriverState *file;
> -    int64_t res = bdrv_get_block_status_above(bs, NULL, start,
> -                                              s->cluster_sectors, &nr, &file);
> -    return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == s->cluster_sectors;
> -}
> +    int64_t res;
>
> -static bool is_zero_cluster_top_locked(BlockDriverState *bs, int64_t start)
> -{
> -    BDRVQcow2State *s = bs->opaque;
> -    int nr = s->cluster_sectors;
> -    uint64_t off;
> -    int ret;
> -
> -    ret = qcow2_get_cluster_offset(bs, start << BDRV_SECTOR_BITS, &nr, &off);
> -    assert(nr == s->cluster_sectors);
> -    return ret == QCOW2_CLUSTER_UNALLOCATED || ret == QCOW2_CLUSTER_ZERO;
> +    if (!count) {
> +        return true;
> +    }
> +    res = bdrv_get_block_status_above(bs, NULL, start, count,
> +                                      &nr, &file);
> +    return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == count;
>   }
>
>   static coroutine_fn int qcow2_co_write_zeroes(BlockDriverState *bs,
> @@ -2434,27 +2427,33 @@ static coroutine_fn int qcow2_co_write_zeroes(BlockDriverState *bs,
>       int ret;
>       BDRVQcow2State *s = bs->opaque;
>
> -    int head = sector_num % s->cluster_sectors;
> -    int tail = (sector_num + nb_sectors) % s->cluster_sectors;
> +    uint32_t head = sector_num % s->cluster_sectors;
> +    uint32_t tail = (sector_num + nb_sectors) % s->cluster_sectors;
>
>       trace_qcow2_write_zeroes_start_req(qemu_coroutine_self(), sector_num,
>                                          nb_sectors);
>
> -    if (head != 0 || tail != 0) {
> +    if (head || tail) {
>           int64_t cl_start = sector_num - head;
> +        uint64_t off;
> +        int nr;
>
>           assert(cl_start + s->cluster_sectors >= sector_num + nb_sectors);
>
> -        sector_num = cl_start;
> -        nb_sectors = s->cluster_sectors;
> -
> -        if (!is_zero_cluster(bs, sector_num)) {
> +        /* check whether remainder of cluster already reads as zero */
> +        if (!(is_zero_sectors(bs, cl_start, head) &&
> +              is_zero_sectors(bs, sector_num + nb_sectors,
> +                              -tail & (s->cluster_sectors - 1)))) {

can we have cluster_sectors != 2^n?
In this case this bits logic will be broken.