Re: [Qemu-devel] [PATCH 13/21] qcow2: add .bdrv_store_persistent_dirty_bitmaps()

[Max Reitz <mreitz@redhat.com>]

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On 19.12.2016 16:26, Vladimir Sementsov-Ogievskiy wrote:
> 19.12.2016 18:14, Max Reitz wrote:
>> On 17.12.2016 15:58, Vladimir Sementsov-Ogievskiy wrote:
>>> 09.12.2016 20:05, Max Reitz wrote:
>>>> On 22.11.2016 18:26, Vladimir Sementsov-Ogievskiy wrote:
>>>>> Realize block bitmap storing interface, to allow qcow2 images store
>>>>> persistent bitmaps.
>>>>>
>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>>>> ---
>>>>>   block/qcow2-bitmap.c | 451
>>>>> +++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>   block/qcow2.c        |   1 +
>>> [...]
>>>
>>>>> +
>>>>> +/* store_bitmap_data()
>>>>> + * Store bitmap to image, filling bitmap table accordingly.
>>>>> + */
>>>>> +static uint64_t *store_bitmap_data(BlockDriverState *bs,
>>>>> +                                   BdrvDirtyBitmap *bitmap,
>>>>> +                                   uint32_t *bitmap_table_size,
>>>>> Error **errp)
>>>>> +{
>>>>> +    int ret;
>>>>> +    BDRVQcow2State *s = bs->opaque;
>>>>> +    int64_t sector;
>>>>> +    uint64_t dsc;
>>>>> +    uint64_t bm_size = bdrv_dirty_bitmap_size(bitmap);
>>>>> +    const char *bm_name = bdrv_dirty_bitmap_name(bitmap);
>>>>> +    uint8_t *buf = NULL;
>>>>> +    BdrvDirtyBitmapIter *dbi;
>>>>> +    uint64_t *tb;
>>>>> +    uint64_t tb_size =
>>>>> +            size_to_clusters(s,
>>>>> +                bdrv_dirty_bitmap_serialization_size(bitmap, 0,
>>>>> bm_size));
>>>>> +
>>>>> +    if (tb_size > BME_MAX_TABLE_SIZE ||
>>>>> +            tb_size * s->cluster_size > BME_MAX_PHYS_SIZE) {
>>>> Alignment to the opening parenthesis, please.
>>>>
>>>>> +        error_setg(errp, "Bitmap '%s' is too big", bm_name);
>>>>> +        return NULL;
>>>>> +    }
>>>>> +
>>>>> +    tb = g_try_new0(uint64_t, tb_size);
>>>>> +    if (tb == NULL) {
>>>>> +        error_setg(errp, "No memory");
>>>>> +        return NULL;
>>>>> +    }
>>>>> +
>>>>> +    dbi = bdrv_dirty_iter_new(bitmap, 0);
>>>>> +    buf = g_malloc(s->cluster_size);
>>>>> +    dsc = disk_sectors_in_bitmap_cluster(s, bitmap);
>>>>> +
>>>>> +    while ((sector = bdrv_dirty_iter_next(dbi)) != -1) {
>>>>> +        uint64_t cluster = sector / dsc;
>>>>> +        uint64_t end, write_size;
>>>>> +        int64_t off;
>>>>> +
>>>>> +        sector = cluster * dsc;
>>>>> +        end = MIN(bm_size, sector + dsc);
>>>>> +        write_size =
>>>>> +            bdrv_dirty_bitmap_serialization_size(bitmap, sector,
>>>>> end - sector);
>>>>> +
>>>>> +        off = qcow2_alloc_clusters(bs, s->cluster_size);
>>>>> +        if (off < 0) {
>>>>> +            error_setg_errno(errp, -off,
>>>>> +                             "Failed to allocate clusters for
>>>>> bitmap '%s'",
>>>>> +                             bm_name);
>>>>> +            goto fail;
>>>>> +        }
>>>>> +        tb[cluster] = off;
>>>> Somehow I would feel better with either an assert(cluster < tb_size);
>>>> here or an assert(bdrv_nb_sectors(bs) / dsc == tb_size); (plus the
>>>> error
>>>> handling for bdrv_nb_sectors()) above the loop.
>>> assert((bm_size - 1) / dsc == tb_size - 1) seems ok. and no additional
>>> error handling. Right?
>> Right, bm_size is already equal to bdrv_nb_sectors(bs), and it's not
>> necessarily a multiple of dsc. So that should be good. Alternatively, I
>> think the following would be slightly easier to read:
>>
>> assert(DIV_ROUND_UP(bm_size, dsc) == tb_size);
>>
>>>>> +
>>>>> +        bdrv_dirty_bitmap_serialize_part(bitmap, buf, sector, end
>>>>> - sector);
>>>>> +        if (write_size < s->cluster_size) {
>>>>> +            memset(buf + write_size, 0, s->cluster_size -
>>>>> write_size);
>>>>> +        }
>>>> Should we assert that write_size <= s->cluster_size?
>>> Ok
>>>
>>> [...].
>>>
>>>>> +        const char *name = bdrv_dirty_bitmap_name(bitmap);
>>>>> +        uint32_t granularity = bdrv_dirty_bitmap_granularity(bitmap);
>>>>> +        Qcow2Bitmap *bm;
>>>>> +
>>>>> +        if (!bdrv_dirty_bitmap_get_persistance(bitmap)) {
>>>>> +            continue;
>>>>> +        }
>>>>> +
>>>>> +        if (++new_nb_bitmaps > QCOW2_MAX_BITMAPS) {
>>>>> +            error_setg(errp, "Too many persistent bitmaps");
>>>>> +            goto fail;
>>>>> +        }
>>>>> +
>>>>> +        new_dir_size += calc_dir_entry_size(strlen(name), 0);
>>>>> +        if (new_dir_size > QCOW2_MAX_BITMAP_DIRECTORY_SIZE) {
>>>>> +            error_setg(errp, "Too large bitmap directory");
>>>>> +            goto fail;
>>>>> +        }
>>>> You only need to increment new_nb_bitmaps and increase new_dir_size if
>>>> the bitmap does not already exist in the image (i.e. if
>>>> find_bitmap_by_name() below returns NULL).
>>> Why? No, I need to check the whole sum and the whole size.
>> If the bitmap already exists, you don't create a new directory entry but
>> reuse the existing one. Therefore, the number of bitmaps in the image
>> and the directory size will not grow then.
> 
> new_nb_bitmaps is not number of "newly created bitmaps", but just new
> value of field nb_bitmaps, so, all bitmaps - old and new are calculated
> into new_nb_bitmaps. Anyway, this misunderstanding shows that variable
> name is bad..

Yes. But when you store a bitmap of the same name as an existing one,
you are replacing it. The number of bitmaps does not grow in that case.

Max


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