Re: [Qemu-devel] [PATCH v10 18/20] qcow2: Switch store_bitmap_data() to byte-based iteration

[John Snow <jsnow@redhat.com>]

On 09/25/2017 10:55 AM, Eric Blake wrote:
> Now that we have adjusted the majority of the calls this function
> makes to be byte-based, it is easier to read the code if it makes
> passes over the image using bytes rather than sectors.
> 
> iotests 165 was rather weak - on a default 64k-cluster image, where
> bitmap granularity also defaults to 64k bytes, a single cluster of
> the bitmap table thus covers (64*1024*8) bits which each cover 64k
> bytes, or 32G of image space.  But the test only uses a 1G image,
> so it cannot trigger any more than one loop of the code in
> store_bitmap_data(); and it was writing to the first cluster.  In
> order to test that we are properly aligning which portions of the
> bitmap are being written to the file, we really want to test a case
> where the first dirty bit returned by bdrv_dirty_iter_next() is not
> aligned to the start of a cluster, which we can do by modifying the
> test to write data that doesn't happen to fall in the first cluster
> of the image.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Vladimir Sementsov-Ogievskiy<vsementsov@virtuozzo.com>
> 
> ---
> v10: no change, add R-b
> v9: update iotests to show why aligning down is needed [Kevin], R-b dropped
> v8: no change
> v7: rebase to earlier change, make rounding of offset obvious (no semantic
> change, so R-b kept) [Kevin]
> v5-v6: no change
> v4: new patch
> ---
>  block/qcow2-bitmap.c   | 31 ++++++++++++++++---------------
>  tests/qemu-iotests/165 |  2 +-
>  2 files changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
> index 692ce0de88..df957c66d5 100644
> --- a/block/qcow2-bitmap.c
> +++ b/block/qcow2-bitmap.c
> @@ -1072,10 +1072,9 @@ static uint64_t *store_bitmap_data(BlockDriverState *bs,
>  {
>      int ret;
>      BDRVQcow2State *s = bs->opaque;
> -    int64_t sector;
> -    uint64_t limit, sbc;
> +    int64_t offset;
> +    uint64_t limit;
>      uint64_t bm_size = bdrv_dirty_bitmap_size(bitmap);
> -    uint64_t bm_sectors = DIV_ROUND_UP(bm_size, BDRV_SECTOR_SIZE);
>      const char *bm_name = bdrv_dirty_bitmap_name(bitmap);
>      uint8_t *buf = NULL;
>      BdrvDirtyBitmapIter *dbi;
> @@ -1100,18 +1099,22 @@ static uint64_t *store_bitmap_data(BlockDriverState *bs,
>      dbi = bdrv_dirty_iter_new(bitmap);
>      buf = g_malloc(s->cluster_size);
>      limit = bytes_covered_by_bitmap_cluster(s, bitmap);
> -    sbc = limit >> BDRV_SECTOR_BITS;
>      assert(DIV_ROUND_UP(bm_size, limit) == tb_size);
> 
> -    while ((sector = bdrv_dirty_iter_next(dbi) >> BDRV_SECTOR_BITS) >= 0) {
> -        uint64_t cluster = sector / sbc;
> +    while ((offset = bdrv_dirty_iter_next(dbi)) >= 0) {
> +        uint64_t cluster = offset / limit;
>          uint64_t end, write_size;
>          int64_t off;
> 
> -        sector = cluster * sbc;
> -        end = MIN(bm_sectors, sector + sbc);
> -        write_size = bdrv_dirty_bitmap_serialization_size(bitmap,
> -            sector * BDRV_SECTOR_SIZE, (end - sector) * BDRV_SECTOR_SIZE);
> +        /*
> +         * We found the first dirty offset, but want to write out the
> +         * entire cluster of the bitmap that includes that offset,
> +         * including any leading zero bits.
> +         */
> +        offset = QEMU_ALIGN_DOWN(offset, limit);
> +        end = MIN(bm_size, offset + limit);
> +        write_size = bdrv_dirty_bitmap_serialization_size(bitmap, offset,
> +                                                          end - offset);
>          assert(write_size <= s->cluster_size);
> 
>          off = qcow2_alloc_clusters(bs, s->cluster_size);
> @@ -1123,9 +1126,7 @@ static uint64_t *store_bitmap_data(BlockDriverState *bs,
>          }
>          tb[cluster] = off;
> 
> -        bdrv_dirty_bitmap_serialize_part(bitmap, buf,
> -                                         sector * BDRV_SECTOR_SIZE,
> -                                         (end - sector) * BDRV_SECTOR_SIZE);
> +        bdrv_dirty_bitmap_serialize_part(bitmap, buf, offset, end - offset);
>          if (write_size < s->cluster_size) {
>              memset(buf + write_size, 0, s->cluster_size - write_size);
>          }
> @@ -1143,11 +1144,11 @@ static uint64_t *store_bitmap_data(BlockDriverState *bs,
>              goto fail;
>          }
> 
> -        if (end >= bm_sectors) {
> +        if (end >= bm_size) {
>              break;
>          }
> 
> -        bdrv_set_dirty_iter(dbi, end * BDRV_SECTOR_SIZE);
> +        bdrv_set_dirty_iter(dbi, end);
>      }
> 
>      *bitmap_table_size = tb_size;
> diff --git a/tests/qemu-iotests/165 b/tests/qemu-iotests/165
> index 74d7b79a0b..a3932db3de 100755
> --- a/tests/qemu-iotests/165
> +++ b/tests/qemu-iotests/165
> @@ -27,7 +27,7 @@ disk = os.path.join(iotests.test_dir, 'disk')
>  disk_size = 0x40000000 # 1G
> 
>  # regions for qemu_io: (start, count) in bytes
> -regions1 = ((0,        0x100000),
> +regions1 = ((0x0fff00, 0x10000),
>              (0x200000, 0x100000))
> 
>  regions2 = ((0x10000000, 0x20000),
> 

Reviewed-by: John Snow <jsnow@redhat.com>

Just need to figure out who stages it. Either Fam, Kevin or myself. I'll
just assume Kevin has higher precedence here with his qcow2 maintainership.

If otherwise, let me know and I'll do it.

--js