Re: [Qemu-devel] [PATCH 4/5] block/qcow2-bitmap: Count queued bitmaps towards nb_bitmaps

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

06.06.2019 21:41, John Snow wrote:
> When we check to see if we can store a bitmap, we don't check how many
> we've queued up. This can cause a problem saving bitmaps on close
> instead of when we request them to be added. With the stricter add
> interface, prohibit these bitmaps specifically.
> 
> To match, make the remove interface more strict as well; now rejecting
> any requests to remove bitmaps that were never queued for storage.
> 
> We don't need to "find" the bitmap when the interface has been given the
> bitmap explicitly, but this is done to make sure that the bitmap given
> actually does belong to the bs we were passed as a paranoia check to
> enforce consistency.

If you want to check it, I'd really prefer to do it explictly,
by adding "bool bdrv_has_dirty_bitmap(bs, bitmap) handler, or bitmap.bs field",
instead of hiding it under inconvenient interface of helper, so we actually do

name = bdrv_dirty_bitmap_name(bitmap);
bitmap = bdrv_find_dirty_bitmap(bs, name);

it really looks strange.

Hmmm, when I read series cover letter and this commit message, I thought
that you'll just calculate current number of persistent bitmaps on bs..
Do we really need to introduce additional counters on qcow2 state?

So, you want to check nb_queued + s->nb_bitmaps instead of just s->nb_bitmaps.

I think we can just count persistent dirty bitmaps and take that number
(as, there should not be in-image bitmaps, for which we don't have in-ram
version, but we can check it too and return an error on mismatch (or count
in-image-not-in-ram bitmaps and this count to number of in-ram persistent
bitmaps it seems an extra thing)..

> 
> ---
> 
> "What about directory size?" Please see the following patch.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   block/qcow2.h        |  1 +
>   block/dirty-bitmap.c |  8 +++-----
>   block/qcow2-bitmap.c | 31 ++++++++++++++++++++++++++-----
>   3 files changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/block/qcow2.h b/block/qcow2.h
> index ce07f003f7..ebf60ac236 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -317,6 +317,7 @@ typedef struct BDRVQcow2State {
>       QCowSnapshot *snapshots;
>   
>       uint32_t nb_bitmaps;
> +    uint32_t nb_queued_bitmaps;
>       uint64_t bitmap_directory_size;
>       uint64_t bitmap_directory_offset;
>   
> diff --git a/block/dirty-bitmap.c b/block/dirty-bitmap.c
> index 4667f9e65a..084c42af57 100644
> --- a/block/dirty-bitmap.c
> +++ b/block/dirty-bitmap.c
> @@ -450,11 +450,9 @@ void bdrv_release_named_dirty_bitmaps(BlockDriverState *bs)
>   }
>   
>   /**
> - * Remove persistent dirty bitmap from the storage if it exists.
> - * Absence of bitmap is not an error, because we have the following scenario:
> - * BdrvDirtyBitmap can have .persistent = true but not yet saved and have no
> - * stored version. For such bitmap bdrv_remove_persistent_dirty_bitmap() should
> - * not fail.
> + * Remove a persistent dirty bitmap from storage,
> + * or dequeue it from being stored if it is enqueued.
> + *
>    * This function doesn't release the corresponding BdrvDirtyBitmap.
>    */
>   int bdrv_remove_persistent_dirty_bitmap(BlockDriverState *bs,
> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
> index 930a6c91ff..7193c66787 100644
> --- a/block/qcow2-bitmap.c
> +++ b/block/qcow2-bitmap.c
> @@ -1402,6 +1402,23 @@ static Qcow2Bitmap *find_bitmap_by_name(Qcow2BitmapList *bm_list,
>       return NULL;
>   }
>   
> +static int qcow2_remove_queued_dirty_bitmap(
> +    BlockDriverState *bs, const char *name, Error **errp)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    BdrvDirtyBitmap *bitmap = bdrv_find_dirty_bitmap(bs, name);
> +    if (!bitmap) {
> +        error_setg(errp, "Node '%s' has no stored or enqueued bitmap '%s'",
> +                   bdrv_get_node_name(bs), name);
> +        return -ENOENT;
> +    }
> +    assert(s->nb_queued_bitmaps > 0);
> +    assert(bdrv_dirty_bitmap_get_persistence(bitmap));
> +    s->nb_queued_bitmaps -= 1;
> +
> +    return 0;
> +}
> +
>   int qcow2_remove_persistent_dirty_bitmap(BlockDriverState *bs,
>                                            BdrvDirtyBitmap *bitmap,
>                                            Error **errp)
> @@ -1413,9 +1430,7 @@ int qcow2_remove_persistent_dirty_bitmap(BlockDriverState *bs,
>       const char *name = bdrv_dirty_bitmap_name(bitmap);
>   
>       if (s->nb_bitmaps == 0) {
> -        /* Absence of the bitmap is not an error: see explanation above
> -         * bdrv_remove_persistent_dirty_bitmap() definition. */
> -        return 0;
> +        return qcow2_remove_queued_dirty_bitmap(bs, name, errp);
>       }
>   
>       if ((ret = bitmap_list_load(bs, &bm_list, errp))) {
> @@ -1424,6 +1439,7 @@ int qcow2_remove_persistent_dirty_bitmap(BlockDriverState *bs,
>   
>       bm = find_bitmap_by_name(bm_list, name);
>       if (bm == NULL) {
> +        ret = qcow2_remove_queued_dirty_bitmap(bs, name, errp);
>           goto fail;
>       }
>   
> @@ -1544,6 +1560,7 @@ void qcow2_store_persistent_dirty_bitmaps(BlockDriverState *bs, Error **errp)
>           error_setg_errno(errp, -ret, "Failed to update bitmap extension");
>           goto fail;
>       }
> +    s->nb_queued_bitmaps = 0;
>   
>       /* Bitmap directory was successfully updated, so, old data can be dropped.
>        * TODO it is better to reuse these clusters */
> @@ -1618,6 +1635,7 @@ int qcow2_add_persistent_dirty_bitmap(BlockDriverState *bs,
>       Qcow2BitmapList *bm_list;
>       const char *name = bdrv_dirty_bitmap_name(bitmap);
>       uint32_t granularity = bdrv_dirty_bitmap_granularity(bitmap);
> +    uint32_t nb_bitmaps;
>       int ret = 0;
>   
>       if (s->qcow_version < 3) {
> @@ -1636,11 +1654,12 @@ int qcow2_add_persistent_dirty_bitmap(BlockDriverState *bs,
>           goto fail;
>       }
>   
> -    if (s->nb_bitmaps == 0) {
> +    nb_bitmaps = s->nb_bitmaps + s->nb_queued_bitmaps;
> +    if (nb_bitmaps == 0) {
>           return 0;
>       }
>   
> -    if (s->nb_bitmaps >= QCOW2_MAX_BITMAPS) {
> +    if (nb_bitmaps >= QCOW2_MAX_BITMAPS) {
>           error_setg(errp,
>                      "Maximum number of persistent bitmaps is already reached");
>           ret = -EOVERFLOW;
> @@ -1667,6 +1686,8 @@ int qcow2_add_persistent_dirty_bitmap(BlockDriverState *bs,
>           goto fail;
>       }
>   
> +    s->nb_queued_bitmaps += 1;
> +
>       return 0;
>   fail:
>       error_prepend(errp, "Can't make bitmap '%s' persistent in '%s': ",
> 


-- 
Best regards,
Vladimir