Re: [Qemu-devel] [PATCH v6 20/42] block/snapshot: Fix fallback

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

09.08.2019 19:13, Max Reitz wrote:
> If the top node's driver does not provide snapshot functionality and we
> want to fall back to a node down the chain, we need to snapshot all
> non-COW children.  For simplicity's sake, just do not fall back if there
> is more than one such child.
> 
> bdrv_snapshot_goto() becomes a bit weird because we may have to redirect
> the actual child pointer, so it only works if the fallback child is
> bs->file or bs->backing (and then we have to find out which it is).
> 
> Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   block/snapshot.c | 100 +++++++++++++++++++++++++++++++++++++----------
>   1 file changed, 79 insertions(+), 21 deletions(-)
> 
> diff --git a/block/snapshot.c b/block/snapshot.c
> index f2f48f926a..35403c167f 100644
> --- a/block/snapshot.c
> +++ b/block/snapshot.c
> @@ -146,6 +146,32 @@ bool bdrv_snapshot_find_by_id_and_name(BlockDriverState *bs,
>       return ret;
>   }
>   
> +/**
> + * Return the child BDS to which we can fall back if the given BDS
> + * does not support snapshots.
> + * Return NULL if there is no BDS to (safely) fall back to.
> + */
> +static BlockDriverState *bdrv_snapshot_fallback(BlockDriverState *bs)
> +{
> +    BlockDriverState *child_bs = NULL;
> +    BdrvChild *child;
> +
> +    QLIST_FOREACH(child, &bs->children, next) {
> +        if (child == bdrv_filtered_cow_child(bs)) {
> +            /* Ignore: COW children need not be included in snapshots */
> +            continue;
> +        }
> +
> +        if (child_bs) {
> +            /* Cannot fall back to a single child if there are multiple */
> +            return NULL;
> +        }
> +        child_bs = child->bs;
> +    }
> +
> +    return child_bs;
> +}
> +
>   int bdrv_can_snapshot(BlockDriverState *bs)
>   {
>       BlockDriver *drv = bs->drv;
> @@ -154,8 +180,9 @@ int bdrv_can_snapshot(BlockDriverState *bs)
>       }
>   
>       if (!drv->bdrv_snapshot_create) {
> -        if (bs->file != NULL) {
> -            return bdrv_can_snapshot(bs->file->bs);
> +        BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs);
> +        if (fallback_bs) {
> +            return bdrv_can_snapshot(fallback_bs);
>           }
>           return 0;
>       }
> @@ -167,14 +194,15 @@ int bdrv_snapshot_create(BlockDriverState *bs,
>                            QEMUSnapshotInfo *sn_info)
>   {
>       BlockDriver *drv = bs->drv;
> +    BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs);
>       if (!drv) {
>           return -ENOMEDIUM;
>       }
>       if (drv->bdrv_snapshot_create) {
>           return drv->bdrv_snapshot_create(bs, sn_info);
>       }
> -    if (bs->file) {
> -        return bdrv_snapshot_create(bs->file->bs, sn_info);
> +    if (fallback_bs) {
> +        return bdrv_snapshot_create(fallback_bs, sn_info);
>       }
>       return -ENOTSUP;
>   }
> @@ -184,6 +212,7 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
>                          Error **errp)
>   {
>       BlockDriver *drv = bs->drv;
> +    BlockDriverState *fallback_bs;
>       int ret, open_ret;
>   
>       if (!drv) {
> @@ -204,39 +233,66 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
>           return ret;
>       }
>   
> -    if (bs->file) {
> -        BlockDriverState *file;
> -        QDict *options = qdict_clone_shallow(bs->options);
> +    fallback_bs = bdrv_snapshot_fallback(bs);
> +    if (fallback_bs) {
> +        QDict *options;
>           QDict *file_options;
>           Error *local_err = NULL;
> +        bool is_backing_child;
> +        BdrvChild **child_pointer;
> +
> +        /*
> +         * We need a pointer to the fallback child pointer, so let us
> +         * see whether the child is referenced by a field in the BDS
> +         * object.
> +         */
> +        if (fallback_bs == bs->file->bs) {
> +            is_backing_child = false;
> +            child_pointer = &bs->file;
> +        } else if (fallback_bs == bs->backing->bs) {
> +            is_backing_child = true;
> +            child_pointer = &bs->backing;
> +        } else {
> +            /*
> +             * The fallback child is not referenced by a field in the
> +             * BDS object.  We cannot go on then.
> +             */
> +            error_setg(errp, "Block driver does not support snapshots");
> +            return -ENOTSUP;
> +        }
> +

Hmm.. Should not this check be included into bdrv_snapshot_fallback(), to
work only with file and backing?

And could we allow fallback only for filters? Is there real usecase except filters?
Or may be, drop fallback at all?


>   
> -        file = bs->file->bs;
>           /* Prevent it from getting deleted when detached from bs */
> -        bdrv_ref(file);
> +        bdrv_ref(fallback_bs);
>   
> -        qdict_extract_subqdict(options, &file_options, "file.");
> +        qdict_extract_subqdict(options, &file_options,
> +                               is_backing_child ? "backing." : "file.");
>           qobject_unref(file_options);
> -        qdict_put_str(options, "file", bdrv_get_node_name(file));
> +        qdict_put_str(options, is_backing_child ? "backing" : "file",
> +                      bdrv_get_node_name(fallback_bs));
>   
>           if (drv->bdrv_close) {
>               drv->bdrv_close(bs);
>           }
> -        bdrv_unref_child(bs, bs->file);
> -        bs->file = NULL;
>   
> -        ret = bdrv_snapshot_goto(file, snapshot_id, errp);
> +        assert(fallback_bs == (*child_pointer)->bs);
> +        bdrv_unref_child(bs, *child_pointer);
> +        *child_pointer = NULL;
> +
> +        ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp);
>           open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err);
>           qobject_unref(options);
>           if (open_ret < 0) {
> -            bdrv_unref(file);
> +            bdrv_unref(fallback_bs);
>               bs->drv = NULL;
>               /* A bdrv_snapshot_goto() error takes precedence */
>               error_propagate(errp, local_err);
>               return ret < 0 ? ret : open_ret;
>           }
>   
> -        assert(bs->file->bs == file);
> -        bdrv_unref(file);
> +        assert(fallback_bs == (*child_pointer)->bs);
> +        bdrv_unref(fallback_bs);
>           return ret;
>       }
>   
> @@ -272,6 +328,7 @@ int bdrv_snapshot_delete(BlockDriverState *bs,
>                            Error **errp)
>   {
>       BlockDriver *drv = bs->drv;
> +    BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs);
>       int ret;
>   
>       if (!drv) {
> @@ -288,8 +345,8 @@ int bdrv_snapshot_delete(BlockDriverState *bs,
>   
>       if (drv->bdrv_snapshot_delete) {
>           ret = drv->bdrv_snapshot_delete(bs, snapshot_id, name, errp);
> -    } else if (bs->file) {
> -        ret = bdrv_snapshot_delete(bs->file->bs, snapshot_id, name, errp);
> +    } else if (fallback_bs) {
> +        ret = bdrv_snapshot_delete(fallback_bs, snapshot_id, name, errp);
>       } else {
>           error_setg(errp, "Block format '%s' used by device '%s' "
>                      "does not support internal snapshot deletion",
> @@ -305,14 +362,15 @@ int bdrv_snapshot_list(BlockDriverState *bs,
>                          QEMUSnapshotInfo **psn_info)
>   {
>       BlockDriver *drv = bs->drv;
> +    BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs);
>       if (!drv) {
>           return -ENOMEDIUM;
>       }
>       if (drv->bdrv_snapshot_list) {
>           return drv->bdrv_snapshot_list(bs, psn_info);
>       }
> -    if (bs->file) {
> -        return bdrv_snapshot_list(bs->file->bs, psn_info);
> +    if (fallback_bs) {
> +        return bdrv_snapshot_list(fallback_bs, psn_info);
>       }
>       return -ENOTSUP;
>   }
> 


-- 
Best regards,
Vladimir